structure of brain essay

Parts of the Brain: Anatomy, Structure & Functions

Olivia Guy-Evans, MSc

Associate Editor for Simply Psychology

BSc (Hons) Psychology, MSc Psychology of Education

Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.

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Saul Mcleod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

On This Page:

The brain controls all functions of the body, interprets information from the outside world, and defines who we are as individuals and how we experience the world.

The brain receives information through our senses: sight, touch, taste, smell, and hearing. This information is processed in the brain, allowing us to give meaning to the input it receives.

The brain is part of the central nervous system ( CNS ) along with the spinal cord. There is also a peripheral nervous system (PNS) comprised of 31 pairs of spinal nerves that branch from the spinal cord and cranial nerves that branch from the brain.

Brain Parts

The brain is composed of the cerebrum, cerebellum, and brainstem (Fig. 1).

Figure 1. The brain has three main parts: the cerebrum, cerebellum, and brainstem.

The cerebrum is the largest and most recognizable part of the brain. It consists of grey matter (the cerebral cortex ) and white matter at the center. The cerebrum is divided into two hemispheres, the left and right, and contains the lobes of the brain (frontal, temporal, parietal, and occipital lobes).

The cerebrum produces higher functioning roles such as thinking, learning, memory, language, emotion, movement, and perception.

The Cerebellum

The cerebellum is located under the cerebrum and monitors and regulates motor behaviors, especially automatic movements.

This structure is also important for regulating posture and balance and has recently been suggested for being involved in learning and attention.

Although the cerebellum only accounts for roughly 10% of the brain’s total weight, this area is thought to contain more neurons (nerve cells) than the rest of the brain combined.

The brainstem is located at the base of the brain. This area connects the cerebrum and the cerebellum to the spinal cord, acting as a relay station for these areas.

The brainstem regulates automatic functions such as sleep cycles, breathing, body temperature, digestion, coughing, and sneezing.

Right Brain vs. Left Brain

The cerebrum is divided into two halves: the right and left hemispheres (Fig. 2). The left hemisphere controls the right half of the body, and the right hemisphere controls the left half.

The two hemispheres are connected by a thick band of neural fibers known as the corpus callosum, consisting of about 200 million axons.

The corpus callosum allows the two hemispheres to communicate and allows information being processed on one side of the brain to be shared with the other.

Figure 2. The cerebrum is divided into left and right hemispheres. The nerve fibers corpus callosum connects the two sides.

Hemispheric lateralization is the idea that each hemisphere is responsible for different functions. Each of these functions is localized to either the right or left side.

The left hemisphere is associated with language functions, such as formulating grammar and vocabulary and containing different language centers (Broca’s and Wernicke’s area).

The right hemisphere is associated with more visuospatial functions such as visualization, depth perception, and spatial navigation. These left and right functions are the case in most people, especially those who are right-handed.

Lobes of the Brain

Each cerebral hemisphere can be subdivided into four lobes, each associated with different functions.

The four lobes of the brain are the frontal, parietal, temporal, and occipital lobes (Figure 3).

Figure 3. The cerebrum is divided into four lobes: frontal, parietal, occipital, and temporal.

Frontal lobes

The frontal lobes are located at the front of the brain, behind the forehead (Figure 4).

Their main functions are associated with higher cognitive functions, including problem-solving, decision-making, attention, intelligence, and voluntary behaviors.

The frontal lobes contain the motor cortex  responsible for planning and coordinating movements.

It also contains the prefrontal cortex, which is responsible for initiating higher-lever cognitive functioning, and Broca’s Area, which is essential for language production.

Figure 4. Frontal lobe structure.

Temporal lobes

The temporal lobes are located on both sides of the brain, near the temples of the head, hence the name temporal lobes (Figure 5).

The main functions of these lobes include understanding, language, memory acquisition, face recognition, object recognition, perception, and auditory information processing.

There is a temporal lobe in both the left and right hemispheres. The left temporal lobe, which is usually the most dominant in people, is associated with language, learning, memorizing, forming words, and remembering verbal information.

The left lobe also contains a vital language center known as Wernicke’s area, which is essential for language development. The right temporal lobe is usually associated with learning and memorizing non-verbal information and determining facial expressions.

Figure 5. Temporal lobe structure.

Parietal lobes

The parietal lobe is located at the top of the brain, between the frontal and occipital lobes, and above the temporal lobes (Figure 6).

The parietal lobe is essential for integrating information from the body’s senses to allow us to build a coherent picture of the world around us.

These lobes allow us to perceive our bodies through somatosensory information (e.g., through touch, pressure, and temperature). It can also help with visuospatial processing, reading, and number representations (mathematics).

The parietal lobes also contain the somatosensory cortex, which receives and processes sensory information, integrating this into a representational map of the body.

This means it can pinpoint the exact area of the body where a sensation is felt, as well as perceive the weight of objects, shape, and texture.

Figure 6. Parietal lobe structure.

Occipital lobes

The occipital lobes are located at the back of the brain behind the temporal and parietal lobes and below the occipital bone of the skull (Figure 7).

The occipital lobes receive sensory information from the eyes’ retinas, which is then encoded into different visual data. Some of the functions of the occipital lobes include being able to assess the size, depth, and distance, determine color information, object and facial recognition, and mapping the visual world.

The occipital lobes also contain the primary visual cortex, which receives sensory information from the retinas, transmitting this information relating to location, spatial data, motion, and the colors of objects in the field of vision.

Figure 7. Occipital lobe structure.

Cerebral Cortex

The surface of the cerebrum is called the cerebral cortex  and has a wrinkled appearance, consisting of bulges, also known as gyri, and deep furrows, known as sulci (Figure 8).

A gyrus (plural: gyri) is the name given to the bumps and ridges on the cerebral cortex (the outermost layer of the brain). A sulcus (plural: sulci) is another name for a groove in the cerebral cortex.

Figure 8. The cortex contains neurons (grey matter) interconnected to other brain areas by axons (white matter). The cortex has a folded appearance. A fold is called a gyrus, and the valley between is a sulcus.

The cerebral cortex is primarily constructed of grey matter (neural tissue made up of neurons), with between 14 and 16 billion neurons found here.

The many folds and wrinkles of the cerebral cortex allow a wider surface area for an increased number of neurons to live there, permitting large amounts of information to be processed.

Deep Structures

The amygdala is a structure deep in the brain that is involved in the processing of emotions and fear learning. The amygdala is a part of the limbic system, a neural network that mediates emotion and memory (Figure 9).

This structure also ties emotional meaning to memories, processes rewards, and helps us make decisions. This structure has also been linked with the fight-or-flight response.

Figure 9. The amygdala in the limbic system plays a key role in how animals assess and respond to environmental threats and challenges by evaluating the emotional importance of sensory information and prompting an appropriate response.

Thalamus and Hypothalamus

The thalamus relays information between the cerebral cortex, brain stem, and other cortical structures (Figure 10).

Because of its interactive role in relaying sensory and motor information, the thalamus contributes to many processes, including attention, perception, timing, and movement. The hypothalamus modulates a range of behavioral and physiological functions.

It controls autonomic functions such as hunger, thirst, body temperature, and sexual activity. To do this, the hypothalamus integrates information from different brain parts and responds to various stimuli such as light, odor, and stress.

Figure 10. The thalamus is often described as the brain’s relay station as a great deal of information that reaches the cerebral cortex first stops in the thalamus before being sent to its destination.

Hippocampus

The hippocampus is a curved-shaped structure in the limbic system associated with learning and memory (Figure 11).

This structure is most strongly associated with the formation of memories, is an early storage system for new long-term memories, and plays a role in the transition of these long-term memories to more permanent memories.

Figure 11. Hippocampus location in the brain

Basal Ganglia

The basal ganglia are a group of structures that regulate the coordination of fine motor movements, balance, and posture alongside the cerebellum.

These structures are connected to other motor areas and link the thalamus with the motor cortex. The basal ganglia are also involved in cognitive and emotional behaviors, as well as playing a role in reward and addiction.

Figure 12. The Basal Ganglia Illustration

Ventricles and Cerebrospinal Fluid

Within the brain, there are fluid-filled interconnected cavities called ventricles , which are extensions of the spinal cord. These are filled with a substance called cerebrospinal fluid, which is a clear and colorless liquid.

The ventricles produce cerebrospinal fluid and transport and remove this fluid. The ventricles do not have a unique function, but they provide cushioning to the brain and are useful for determining the locations of other brain structures.

Cerebrospinal fluid circulates the brain and spinal cord and functions to cushion the brain within the skull. If damage occurs to the skull, the cerebrospinal fluid will act as a shock absorber to help protect the brain from injury.

As well as providing cushioning, the cerebrospinal fluid circulates nutrients and chemicals filtered from the blood and removes waste products from the brain. Cerebrospinal fluid is constantly absorbed and replenished by the ventricles.

If there were a disruption or blockage, this can cause a build-up of cerebrospinal fluid and can cause enlarged ventricles.

Neurons are the nerve cells of the central nervous system that transmit information through electrochemical signals throughout the body. Neurons contain a soma, a cell body from which the axon extends.

Axons are nerve fibers that are the longest part of the neuron, which conduct electrical impulses away from the soma.

There are dendrites at the end of the neuron, which are branch-like structures that send and receive information from other neurons.

A myelin sheath, a fatty insulating layer, forms around the axon, allowing nerve impulses to travel down the axon quickly.

There are different types of neurons. Sensory neurons transmit sensory information, motor neurons transmit motor information, and relay neurons allow sensory and motor neurons to communicate.

The communication between neurons is called synapses. Neurons communicate with each other via synaptic clefts, which are gaps between the endings of neurons.

During synaptic transmission, chemicals, such as neurotransmitters, are released from the endings of the previous neuron (also known as the presynaptic neuron).

These chemicals enter the synaptic cleft to then be transported to receptors on the next neuron (also known as the postsynaptic neuron).

Once transported to the next neuron, the chemical messengers continue traveling down neurons to influence many functions, such as behavior and movement.

Glial Cells

Glial cells are non-neuronal cells in the central nervous system which work to provide the neurons with nourishment, support, and protection.

These are star-shaped cells that function to maintain the environment for neuronal signaling by controlling the levels of neurotransmitters surrounding the synapses.

They also work to clean up what is left behind after synaptic transmission, either recycling any leftover neurotransmitters or cleaning up when a neuron dies.

Oligodendrocytes

These types of glial have the appearance of balls with spikes all around them. They function by wrapping around the axons of neurons to form a protective layer called the myelin sheath.

This is a substance that is rich in fat and provides insulation to the neurons to aid neuronal signaling.

Microglial cells have oval bodies and many branches projecting out of them. The primary function of these cells is to respond to injuries or diseases in the central nervous system.

They respond by clearing away any dead cells or removing any harmful toxins or pathogens that may be present, so they are, therefore, important to the brain’s health.

Ependymal cells

These cells are column-shaped and usually line up together to form a membrane called the ependyma. The ependyma is a thin membrane lining the spinal cord and ventricles of the brain .

In the ventricles, these cells have small hairlike structures called cilia, which help encourage the flow of cerebrospinal fluid.

Cranial Nerves

There are 12 types of cranial nerves which are linked directly to the brain without having to pass through the spinal cord. These allow sensory information to pass from the organs of the face to the brain:

Mnemonic for Order of Cranial Nerves:

S ome S ay M arry M oney B ut M y B rother S ays B ig B rains M atter M ore

• Cranial I: Sensory
• Cranial II: Sensory
• Cranial III: Motor
• Cranial IV: Motor
• Cranial V: Both (sensory & motor)
• Cranial VI: Motor
• Cranial VII: Both (sensory & motor)
• Cranial VIII: Sensory
• Cranial IX: Both (sensory & motor)
• Cranial X: Both (sensory & motor)
• Cranial XI: Motor
• Cranial XII: Motor

Purves, D., Augustine, G., Fitzpatrick, D., Katz, L., LaMantia, A., McNamara, J., & Williams, S. (2001). Neuroscience 2nd edition . sunderland (ma) sinauer associates. Types of Eye Movements and Their Functions.

Mayfield Brain and Spine (n.d.). Anatomy of the Brain. Retrieved July 28, 2021, from: https://mayfieldclinic.com/pe-anatbrain.htm

Robertson, S. (2018, August 23). What is Grey Matter? News Medical Life Sciences. https://www.news-medical.net/health/What-is-Grey-Matter.aspx

Guy-Evans, O. (2021, April 13). Temporal lobe: definition, functions, and location. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/temporal-lobe.html

Guy-Evans, O. (2021, April 15). Parietal lobe: definition, functions, and location. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/parietal-lobe.html

Guy-Evans, O. (2021, April 19). Occipital lobe: definition, functions, and location. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/occipital-lobe.html

Guy-Evans, O. (2021, May 08). Frontal lobe function, location in brain, damage, more. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/frontal-lobe.html

Guy-Evans, O. (2021, June 09). Gyri and sulci of the brain. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/gyri-and-sulci-of-the-brain.html

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Brain Anatomy and How the Brain Works

What is the brain.

The brain is a complex organ that controls thought, memory, emotion, touch, motor skills, vision, breathing, temperature, hunger and every process that regulates our body. Together, the brain and spinal cord that extends from it make up the central nervous system, or CNS.

What is the brain made of?

Weighing about 3 pounds in the average adult, the brain is about 60% fat. The remaining 40% is a combination of water, protein, carbohydrates and salts. The brain itself is a not a muscle. It contains blood vessels and nerves, including neurons and glial cells.

What is the gray matter and white matter?

Gray and white matter are two different regions of the central nervous system. In the brain, gray matter refers to the darker, outer portion, while white matter describes the lighter, inner section underneath. In the spinal cord, this order is reversed: The white matter is on the outside, and the gray matter sits within.

Gray matter is primarily composed of neuron somas (the round central cell bodies), and white matter is mostly made of axons (the long stems that connects neurons together) wrapped in myelin (a protective coating). The different composition of neuron parts is why the two appear as separate shades on certain scans.

Each region serves a different role. Gray matter is primarily responsible for processing and interpreting information, while white matter transmits that information to other parts of the nervous system.

How does the brain work?

The brain sends and receives chemical and electrical signals throughout the body. Different signals control different processes, and your brain interprets each. Some make you feel tired, for example, while others make you feel pain.

Some messages are kept within the brain, while others are relayed through the spine and across the body’s vast network of nerves to distant extremities. To do this, the central nervous system relies on billions of neurons (nerve cells).

Main Parts of the Brain and Their Functions

At a high level, the brain can be divided into the cerebrum, brainstem and cerebellum.

The cerebrum (front of brain) comprises gray matter (the cerebral cortex) and white matter at its center. The largest part of the brain, the cerebrum initiates and coordinates movement and regulates temperature. Other areas of the cerebrum enable speech, judgment, thinking and reasoning, problem-solving, emotions and learning. Other functions relate to vision, hearing, touch and other senses.

Cerebral Cortex

Cortex is Latin for “bark,” and describes the outer gray matter covering of the cerebrum. The cortex has a large surface area due to its folds, and comprises about half of the brain’s weight.

The cerebral cortex is divided into two halves, or hemispheres. It is covered with ridges (gyri) and folds (sulci). The two halves join at a large, deep sulcus (the interhemispheric fissure, AKA the medial longitudinal fissure) that runs from the front of the head to the back. The right hemisphere controls the left side of the body, and the left half controls the right side of the body. The two halves communicate with one another through a large, C-shaped structure of white matter and nerve pathways called the corpus callosum. The corpus callosum is in the center of the cerebrum.

The brainstem (middle of brain) connects the cerebrum with the spinal cord. The brainstem includes the midbrain, the pons and the medulla.

• Midbrain. The midbrain (or mesencephalon) is a very complex structure with a range of different neuron clusters (nuclei and colliculi), neural pathways and other structures. These features facilitate various functions, from hearing and movement to calculating responses and environmental changes. The midbrain also contains the substantia nigra, an area affected by Parkinson’s disease that is rich in dopamine neurons and part of the basal ganglia, which enables movement and coordination.
• Pons. The pons is the origin for four of the 12 cranial nerves, which enable a range of activities such as tear production, chewing, blinking, focusing vision, balance, hearing and facial expression. Named for the Latin word for “bridge,” the pons is the connection between the midbrain and the medulla.
• Medulla. At the bottom of the brainstem, the medulla is where the brain meets the spinal cord. The medulla is essential to survival. Functions of the medulla regulate many bodily activities, including heart rhythm, breathing, blood flow, and oxygen and carbon dioxide levels. The medulla produces reflexive activities such as sneezing, vomiting, coughing and swallowing.

The spinal cord extends from the bottom of the medulla and through a large opening in the bottom of the skull. Supported by the vertebrae, the spinal cord carries messages to and from the brain and the rest of the body.

The cerebellum (“little brain”) is a fist-sized portion of the brain located at the back of the head, below the temporal and occipital lobes and above the brainstem. Like the cerebral cortex, it has two hemispheres. The outer portion contains neurons, and the inner area communicates with the cerebral cortex. Its function is to coordinate voluntary muscle movements and to maintain posture, balance and equilibrium. New studies are exploring the cerebellum’s roles in thought, emotions and social behavior, as well as its possible involvement in addiction, autism and schizophrenia.

Brain Coverings: Meninges

Three layers of protective covering called meninges surround the brain and the spinal cord.

• The outermost layer, the dura mater , is thick and tough. It includes two layers: The periosteal layer of the dura mater lines the inner dome of the skull (cranium) and the meningeal layer is below that. Spaces between the layers allow for the passage of veins and arteries that supply blood flow to the brain.
• The arachnoid mater is a thin, weblike layer of connective tissue that does not contain nerves or blood vessels. Below the arachnoid mater is the cerebrospinal fluid, or CSF. This fluid cushions the entire central nervous system (brain and spinal cord) and continually circulates around these structures to remove impurities.
• The pia mater is a thin membrane that hugs the surface of the brain and follows its contours. The pia mater is rich with veins and arteries.

Lobes of the Brain and What They Control

Each brain hemisphere (parts of the cerebrum) has four sections, called lobes: frontal, parietal, temporal and occipital. Each lobe controls specific functions.

• Frontal lobe. The largest lobe of the brain, located in the front of the head, the frontal lobe is involved in personality characteristics, decision-making and movement. Recognition of smell usually involves parts of the frontal lobe. The frontal lobe contains Broca’s area, which is associated with speech ability.
• Parietal lobe. The middle part of the brain, the parietal lobe helps a person identify objects and understand spatial relationships (where one’s body is compared with objects around the person). The parietal lobe is also involved in interpreting pain and touch in the body. The parietal lobe houses Wernicke’s area, which helps the brain understand spoken language.
• Occipital lobe. The occipital lobe is the back part of the brain that is involved with vision.
• Temporal lobe. The sides of the brain, temporal lobes are involved in short-term memory, speech, musical rhythm and some degree of smell recognition.

Deeper Structures Within the Brain

Pituitary gland.

Sometimes called the “master gland,” the pituitary gland is a pea-sized structure found deep in the brain behind the bridge of the nose. The pituitary gland governs the function of other glands in the body, regulating the flow of hormones from the thyroid, adrenals, ovaries and testicles. It receives chemical signals from the hypothalamus through its stalk and blood supply.

Hypothalamus

The hypothalamus is located above the pituitary gland and sends it chemical messages that control its function. It regulates body temperature, synchronizes sleep patterns, controls hunger and thirst and also plays a role in some aspects of memory and emotion.

Small, almond-shaped structures, an amygdala is located under each half (hemisphere) of the brain. Included in the limbic system, the amygdalae regulate emotion and memory and are associated with the brain’s reward system, stress, and the “fight or flight” response when someone perceives a threat.

Hippocampus

A curved seahorse-shaped organ on the underside of each temporal lobe, the hippocampus is part of a larger structure called the hippocampal formation. It supports memory, learning, navigation and perception of space. It receives information from the cerebral cortex and may play a role in Alzheimer’s disease.

Pineal Gland

The pineal gland is located deep in the brain and attached by a stalk to the top of the third ventricle. The pineal gland responds to light and dark and secretes melatonin, which regulates circadian rhythms and the sleep-wake cycle.

Ventricles and Cerebrospinal Fluid

Deep in the brain are four open areas with passageways between them. They also open into the central spinal canal and the area beneath arachnoid layer of the meninges.

The ventricles manufacture cerebrospinal fluid , or CSF, a watery fluid that circulates in and around the ventricles and the spinal cord, and between the meninges. CSF surrounds and cushions the spinal cord and brain, washes out waste and impurities, and delivers nutrients.

Blood Supply to the Brain

Two sets of blood vessels supply blood and oxygen to the brain: the vertebral arteries and the carotid arteries.

The external carotid arteries extend up the sides of your neck, and are where you can feel your pulse when you touch the area with your fingertips. The internal carotid arteries branch into the skull and circulate blood to the front part of the brain.

The vertebral arteries follow the spinal column into the skull, where they join together at the brainstem and form the basilar artery , which supplies blood to the rear portions of the brain.

The circle of Willis , a loop of blood vessels near the bottom of the brain that connects major arteries, circulates blood from the front of the brain to the back and helps the arterial systems communicate with one another.

Cranial Nerves

Inside the cranium (the dome of the skull), there are 12 nerves, called cranial nerves:

• Cranial nerve 1: The first is the olfactory nerve, which allows for your sense of smell.
• Cranial nerve 2: The optic nerve governs eyesight.
• Cranial nerve 3: The oculomotor nerve controls pupil response and other motions of the eye, and branches out from the area in the brainstem where the midbrain meets the pons.
• Cranial nerve 4: The trochlear nerve controls muscles in the eye. It emerges from the back of the midbrain part of the brainstem.
• Cranial nerve 5: The trigeminal nerve is the largest and most complex of the cranial nerves, with both sensory and motor function. It originates from the pons and conveys sensation from the scalp, teeth, jaw, sinuses, parts of the mouth and face to the brain, allows the function of chewing muscles, and much more.
• Cranial nerve 6: The abducens nerve innervates some of the muscles in the eye.
• Cranial nerve 7: The facial nerve supports face movement, taste, glandular and other functions.
• Cranial nerve 8: The vestibulocochlear nerve facilitates balance and hearing.
• Cranial nerve 9: The glossopharyngeal nerve allows taste, ear and throat movement, and has many more functions.
• Cranial nerve 10: The vagus nerve allows sensation around the ear and the digestive system and controls motor activity in the heart, throat and digestive system.
• Cranial nerve 11: The accessory nerve innervates specific muscles in the head, neck and shoulder.
• Cranial nerve 12: The hypoglossal nerve supplies motor activity to the tongue.

The first two nerves originate in the cerebrum, and the remaining 10 cranial nerves emerge from the brainstem, which has three parts: the midbrain, the pons and the medulla.

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Parts of the Brain

Anatomy, Functions, and Conditions

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Steven Gans, MD is board-certified in psychiatry and is an active supervisor, teacher, and mentor at Massachusetts General Hospital.

The Cerebral Cortex

The four lobes, the brain stem, the cerebellum, the limbic system, other parts of the brain, brain conditions, protecting your brain.

The human brain  is not only one of the most important organs in the human body; it is also the most complex. The brain is made up of billions of neurons and it also has a number of specialized parts that are each involved in important functions.

While there is still a great deal that researchers do not yet know about the brain, they have learned a great deal about the anatomy and function of the brain. Understanding these parts can help give people a better idea of how disease and damage may affect the brain and its ability to function.

The cerebral cortex is the part of the brain that makes human beings unique. Functions that originate in the cerebral cortex include:

• Consciousness
• Higher-order thinking
• Imagination
• Information processing
• Voluntary physical action

The cerebral cortex is what we see when we look at the brain. It is the outermost portion that can be divided into four lobes. Each bump on the surface of the brain is known as a gyrus, while each groove is known as a sulcus.

The cerebral cortex is the part of the brain that is responsible for a number of complex functions including information processing, language, and memory.

The cerebral cortex can be divided into four sections, which are known as lobes. The frontal lobe, parietal lobe, occipital lobe, and temporal lobe have been associated with different functions ranging from reasoning to auditory perception.

Frontal Lobe

This lobe is located at the front of the brain and is associated with reasoning, motor skills, higher level cognition, and expressive language. At the back of the frontal lobe, near the central sulcus, lies the motor cortex.

The motor cortex receives information from various lobes of the brain and uses this information to carry out body movements. Damage to the frontal lobe can lead to changes in sexual habits, socialization, and attention as well as increased risk-taking .

Parietal Lobe

The parietal lobe is located in the middle section of the brain and is associated with processing tactile sensory information such as pressure, touch, and pain . A portion of the brain known as the somatosensory cortex is located in this lobe and is essential to the processing of the body's senses. 

Temporal Lobe

The temporal lobe is located on the bottom section of the brain. This lobe is also the location of the primary auditory cortex, which is important for interpreting sounds and the language we hear.

The hippocampus is also located in the temporal lobe, which is why this portion of the brain is also heavily associated with the formation of memories . Damage to the temporal lobe can lead to problems with memory, speech perception, and language skills.

Occipital Lobe

The occipital lobe is located at the back portion of the brain and is associated with interpreting visual stimuli and information. The primary visual cortex, which receives and interprets information from the retinas of the eyes, is located in the occipital lobe.

Damage to this lobe can cause visual problems such as difficulty recognizing objects, an inability to identify colors, and trouble recognizing words.

The brain comprises four lobes, each associated with different functions. The frontal lobe is found at the front of the brain; the parietal lobe is behind the frontal lobe; the temporal lobe is located at the sides of the head; and the occipital lobe is found at the back of the head.

The brainstem is an area located at the base of the brain that contains structures vital for involuntary functions such as the heartbeat and breathing. The brain stem is comprised of the midbrain, pons, and medulla.

The midbrain is often considered the smallest region of the brain. It acts as a sort of relay station for auditory and visual information. The midbrain controls many important functions such as the visual and auditory systems as well as eye movement.

Portions of the midbrain called the red nucleus and the substantia nigra are involved in the control of body movement. The darkly pigmented substantia nigra contains a large number of dopamine-producing neurons.

The degeneration of neurons in the substantia nigra is associated with Parkinson’s disease.

The medulla is located directly above the spinal cord in the lower part of the brain stem and controls many vital autonomic functions such as heart rate, breathing, and blood pressure.

The pons connects the cerebral cortex to the medulla and to the cerebellum and serves a number of important functions. It plays a role in several autonomic processes, such as stimulating breathing and controlling sleep cycles.

The brainstem, which includes the midbrain, medulla, and pons, is responsible for involuntary processes, including breathing, heartbeat, and blood pressure.

Sometimes referred to as the ​"little brain," the cerebellum lies on top of the pons behind the brain stem. The cerebellum makes up approximately 10% of the brain's total size , but it accounts for more than 50% of the total number of neurons located in the entire brain .

The cerebellum is comprised of small lobes and serves several functions.

• It receives information from the inner ear's balance system, sensory nerves, and auditory and visual systems. It is involved in the coordination of movements as well as motor learning.
• It is also associated with motor movement and control, but this is not because the motor commands originate here. Instead, the cerebellum modifies these signals and makes motor movements accurate and useful.
• The cerebellum helps control posture, balance, and the coordination of voluntary movements. This allows different muscle groups to act together and produce coordinated fluid movement.
• In addition to playing an essential role in motor control, the cerebellum is also important in certain cognitive functions, including speech.

The cerebrum is the largest part of the brain and is responsible for managing conscious thought, the coordination of movement, learning, speech, behavior, and personality.

Although there is no totally agreed-upon list of the structures that make up the limbic system, four of the main regions include:

The Hypothalamus

The hypothalamus is a grouping of nuclei that lie along the base of the brain near the pituitary gland. The hypothalamus connects with many other regions of the brain and is responsible for controlling hunger, thirst, emotions , body temperature regulation, and circadian rhythms.

The hypothalamus also controls the pituitary gland by secreting hormones. This gives the hypothalamus a great deal of control over many body functions.

The Amygdala

The amygdala is a cluster of nuclei located close to the base of the brain. It is primarily involved in functions including memory, emotion, and the body's fight-or-flight response . The structure processes external stimuli and then relays that information to the hippocampus, which can then prompt a response to deal with outside threats.

The Thalamus

Located above the brainstem, the thalamus processes and transmits movement and sensory information . It is essentially a relay station, taking in sensory information and then passing it on to the cerebral cortex. The cerebral cortex also sends information to the thalamus, which then sends this information to other systems.

The Hippocampus

The hippocampus is a structure located in the temporal lobe. It is important in memory and learning and is sometimes considered to be part of the limbic system because it plays an important part in the control of emotional responses . It plays a role in the body's fight-or-flight response and in the recall and regulation of emotional memories.

The limbic system controls behaviors essential for survival, including the fight or flight response, feeding behavior, and reproduction.

Other important structures play an essential role in supporting the structure and function of the brain. Some of these parts of the brain include:

The meninges are the layers that surround the brain and spinal cord and provide protection. There are three layers of meninges:

• The dura mater : This is the thick, outmost layer located directly under the skull and vertebral column.
• The arachnoid mater : This is a thin layer of web-like connective tissue. Under this layer is cerebrospinal fluid that helps cushion the brain and spinal cord.
• The pia mater : This layer contains veins and arteries and is found directly atop the brain and spinal cord.

The brain also contains 12 cranial nerves. Each nerve plays a vital role in relaying essential information to the brain. These nerves include:

• The olfactory nerve : Essential for the sense of smell
• The optic nerve : Controls eyesight
• The oculomotor nerve : Controls the motions of the eye and the response of the pupil
• The trochlea nerve : Controls the muscles of the ey
• The trigeminal nerve : Carries sensory and motor information to and from the face, jaw, teeth, and scalp
• Abducens nerve : Associated with specific movements of the eye
• Facial nerve : Responsible for sensory and motor functions controlling the face, tongue, tear glands, and parts of the ear
• The vestibulocochlear nerve , which regulates hearing and balance
• The glossopharyngeal nerve : Important for sensory information from parts of the tongue and stimulating specific throat muscles
• The vagus nerve : Plays many important roles, including carrying sensory information from the ear, heart, intestines
• The accessory nerve : Controls the muscles of the neck
• The hypoglossal nerve : Responsible for the muscle movements of the tongue

In addition to the main parts of the brain, there are also other important structures that are important for normal functioning. This includes the protective meninges and the cranial nerves that transmit signals to and from the brain.

The brain can also be affected by a number of conditions and by damage. According to the National Institute of Neurological Disorders and Stroke, there are more than 600 types of neurological diseases. Some conditions that can affect the brain and its function include:

• Brain tumors
• Cerebrovascular diseases such as stroke and vascular dementia
• Convulsive disorders such as epilepsy
• Degenerative diseases such as Alzheimer's disease and Parkinson's disease
• Developmental disorders such as cerebral palsy
• Infectious diseases such as AIDS dementia
• Metabolic diseases such as Gaucher's disease
• Neurogenetic diseases including Huntington's disease and muscular dystrophy
• Trauma such as head injury and spinal cord injury

By studying the brain and learning more about its anatomy and function, researchers are able to develop new treatments and preventative strategies for conditions that affect the brain.

Disease and damage can affect the brain's ability to function. Tumors, strokes, degenerative conditions, trauma, and infectious diseases are just a few of the conditions that can damage the brain.

You can't change your genetics or some other risk factors. But it's important to take steps to help protect the health of your brain.

Diet and Exercise

Research suggests that regular physical activity is essential for brain health. For example, that exercise can help delay brain aging as well as degenerative diseases such as Alzheimer's, diabetes, and multiple sclerosis. It is also associated with improvements in cognitive abilities and memory.

Similarly, a nutritious, balanced diet that includes omega-3 fatty acids, vitamins, and antioxidants is important for brain function (as well as overall health).

It's also essential to protect your brain from injury by, for example, wearing a helmet when participating in physical activities that pose a risk for collision or falls, and always wearing a seatbelt when driving or riding in a car.

Sleep can also play a pivotal role in brain health and mental well-being . Studies have found that sleep can actually play a role in the development and maintenance of some psychiatric conditions including anxiety, depression, and bipolar disorder.

Mental Activity

Evidence also suggests that staying mentally engaged can also play an important role in protecting your brain from some degenerative conditions. Activities that may help include learning new things and staying socially active.

A Word From Verywell

The human brain is remarkably complex and researchers are still discovering many of the mysteries of how the mind works. By better understanding how different parts of the brain function, you can also better appreciate how disease or injury may impact it. If you think that you are experiencing symptoms of a brain condition, talk to your doctor for further evaluation.

Boly M, Massimini M, Tsuchiya N, Postle BR, Koch C, Tononi G. Are the neural correlates of consciousness in the front or in the back of the cerebral cortex? Clinical and neuroimaging evidence . J Neurosci . 2017;37(40):9603-9613. doi:10.1523/JNEUROSCI.3218-16.2017

Jawabri KH, Sharma S. Physiology, cerebral cortex functions . In: StatPearls [Internet]. StatPearls Publishing.

Hurley RA, Flashman LA, Chow TW, Taber KH. The brainstem: anatomy, assessment, and clinical syndromes . J Neuropsychiatry Clin Neurosci . 2010;22(1):iv-7. doi:10.1176/jnp.2010.22.1.iv

Basinger H, Hogg JP. Neuroanatomy, brainstem . In: StatPearls [Internet]. StatPearls Publishing.

Wagner MJ, Kim TH, Savall J, Schnitzer MJ, Luo L. Cerebellar granule cells encode the expectation of reward . Nature . 2017;544(7648):96-100. doi:10.1038/nature21726

Biran J, Tahor M, Wircer E, Levkowitz G. Role of developmental factors in hypothalamic function . Front Neuroanat . 2015;9:47. doi:10.3389/fnana.2015.00047

Baxter MG, Croxson PL. Facing the role of the amygdala in emotional information processing . Proc Nat Acad Sci . 2012;109(52):21180-21181. doi:10.1073/pnas.1219167110

Fama R, Sullivan EV. Thalamic structures and associated cognitive functions: Relations with age and aging . Neurosci Biobehav Rev . 2015;54:29-37. doi:10.1016/j.neubiorev.2015.03.008

Anand KS, Dhikav V. Hippocampus in health and disease: An overview . Ann Indian Acad Neurol . 2012;15(4):239-46. doi:10.4103/0972-2327.104323

Zhu Y, Gao H, Tong L, et al. Emotion regulation of hippocampus using real-time fmri neurofeedback in healthy human . Front Hum Neurosci . 2019;13:242. doi:10.3389/fnhum.2019.00242 

National Institute of Neurological Disorders and Stroke. Brain basics: know your brain .

Di Liegro CM, Schiera G, Proia P, Di Liegro I. Physical activity and brain health .  Genes (Basel) . 2019;10(9):720. doi:10.3390/genes10090720

Scott AJ, Webb TL, Rowse G.  Does improving sleep lead to better mental health?. A protocol for a meta-analytic review of randomised controlled trials .  BMJ Open . 2017;7(9):e016873. doi:10.1136/bmjopen-2017-016873

Sommerlad A, Sabia S, Singh-manoux A, Lewis G, Livingston G.  Association of social contact with dementia and cognition: 28-year follow-up of the Whitehall II cohort study .  PLoS Med . 2019;16(8):e1002862. doi:10.1371/journal.pmed.1002862

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Kalat JW. Biological Psychology . Cengage Learning; 2016. 

By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Introductory essay

Written by the educators who created Mapping and Manipulating the Brain, a brief look at the key facts, tough questions and big ideas in their field. Begin this TED Study with a fascinating read that gives context and clarity to the material.

Here is this mass of jelly, three-pound mass of jelly you can hold in the palm of your hand, and it can contemplate the vastness of interstellar space. It can contemplate the meaning of infinity and it can contemplate itself contemplating on the meaning of infinity. VS Ramachandran

The brain may well be our body's most mysterious organ. Unbelievably complex, utterly fascinating, and notoriously difficult to study, we're left wondering: What exactly does the brain do and how does it do it?

Despite two centuries of intensive research, supported in recent decades by impressive technological advances, answers to many of our questions about the brain are still distant. The reason is easy to appreciate: the brain contains more than ten billion cells — a number equivalent to the total human population on Earth — interacting with each other through about 1,000 times as many connections. Imagine that what's going on in your brain is like a shrunk-down version of the global human population interacting through the Internet. The Internet is hard enough to understand even though we created it; now imagine trying to understand a process of similar complexity without the benefit of knowing how it was generated!

As you listen to these TEDTalks and expand your study of neuroscience through other sources, remember that although we might now know a great deal more about the brain than we did at the start of the 19th century, it's a tiny fraction of what there is to know. Bear in mind that many current ideas may prove wrong. Indeed, it's the excitement of generating and testing, and trying to prove or disprove ideas that might explain the great unknown inside our heads that motivates many research neuroscientists around the world.

A brief history of brain science

The Egyptians wrote the first known descriptions of the brain and its anatomy about 3700 years ago, but another 1200 years elapsed before Greek philosophers of the Hippocratic School identified the brain as the organ responsible for our cognitive functions. Around 400 B.C., Hippocrates declared, "Men ought to know that from the brain, and from the brain only, arise our pleasures, joy, laughter and jests, as well as our sorrows, pains, griefs, and tears." However, not everyone agreed: although Plato and Hippocrates thought that the brain was responsible for sensation, intelligence and mental processes, Aristotle believed it was the heart.

Over the next 2500 years, the work of great European intellectuals including Galen of Bergama, Leonardo da Vinci and Rene Descartes improved our understanding of the brain. By the start of the 19th century, the brain's importance as the organ of perception and higher mental function was beyond doubt.

In the early 1800s, scientists made an important conceptual breakthrough when they hypothesized that different brain functions are carried out in specific and distinct brain regions. Brain regionalization, a concept central to several of the TEDTalks we'll watch, remains an important though controversial component of modern neuroscience.

Some of the initial models of brain regionalization were severely misguided, mainly because they were built on little or no evidence. For example, the Viennese physician Franz Joseph Gall (1758-1828) became convinced for the flimsiest of reasons that each of mankind's mental faculties, including our moral and intellectual capabilities, are each controlled by a separate "organ" within the cerebral hemispheres of the brain. The pseudo-science of phrenology that grew out of Gall's claims gained an enormous popular following in the 19th century; advocates believed that skilled practitioners could feel the lumps and bumps on an individual's skull to gain information about the underlying "organs" and thus fully describe the individual's personality and mental abilities.

Although phrenology is now discredited, the fundamental idea that different functions are localized to different areas of the brain turned out to have merit — even if Gall got the details wrong. The story of phrenology also provides a salutary lesson on the dangers of accepting popular beliefs about aspects of brain function and dysfunction that are difficult to critically evaluate through scientific experimentation. Even today, it's common to find that people think they know more than it's currently possible to know about how and why brains work or go wrong; for example, the causes and cures for various types of mental illness, which may contribute to the social stigma that surrounds these conditions.

Through the late 19th and early 20th centuries, scientists including Pierre Paul Broca, Carl Wernicke, Korbinian Brodmann and Wilder Penfield found credible scientific evidence supporting the subdivision of the brain into discrete areas with different specific functions. Their work was based on studies of patients with localized lesions of the brain, of the anatomical differences between different parts of the brain and of the effects of stimulating discrete brain regions on bodily actions. Together, scientists such as these laid the foundations of modern neuroscience. As you watch the TEDTalks in Mapping and Manipulating the Brain , notice how the speakers reference some of the same approaches used by Broca, Wernicke, Brodmann and Penfield, and how they apply the concepts of brain regionalization and localization of function . Bear in mind, however, that although these concepts are useful, they're also controversial -- more on this below.

How brains are built

Spanish scientist Santiago Ramón Y Cajal (1852-1934) is often thought of as the father of modern neuroscience. Through his extensive and beautiful studies of the microscopic structure of the brain, he discovered that the neuron is the fundamental unit of the nervous system. Since Ramón Y Cajal's breakthrough, scientists have sought to understand how the billions of neurons in the brain are organized to support so many complex functions.

This daunting task would likely be easier if we could follow the process by which the brain is generated, but following brain development is very difficult to do in humans. Thus, we often have to infer how the human brain develops by studying the developing brains of other species, so-called "model organisms" selected for their particular advantages in certain experimental procedures. Aside from helping us to work out how the adult brain functions, research on brain development is a major area in neuroscience for other reasons as well. For example, many conditions like schizophrenia and autism can be traced back to abnormalities in earlier brain development.

The great molecular, structural and functional diversity of brain cells, along with their specializations and precise interactions, are acquired in an organized way through processes that build on differences between the relatively small numbers of cells in the early embryo. As more and more cells are generated in a growing organism, new cells diversify in specific ways as a result of interactions with pre-existing cells, continually adding to the organism's complexity in a highly regulated manner. To understand how brains develop we need to know how their cells develop in specific and reproducible ways as a result of their own internal mechanisms interacting with an expanding array of stimuli from outside the cell.

Since, as discussed above, regionalization is a prominent organizing feature in mature brains, when and how is it established during brain development? Some of the most exciting research on brain development in recent years has focused on this question.

For neurons to develop regional identities, they must possess or acquire information on where they are located within the brain so that they can take on the appropriate specializations. How neurons gain positional information has been one of the most prominent themes in developmental neuroscience in the last 50 years or so, as indeed it has in the broader field of developmental biology (positional identity is required not only by brain cells).

The model that has dominated current thinking was famously elaborated in the 1960s by Lewis Wolpert in his French flag analogy. Here, a signal produced by a group of organizer cells diffuses from its source through a surrounding field of cells. In so doing, it forms a concentration gradient with more of the signal present in areas closer to the source. Cells respond to the concentration of this signal. In Wolpert's French Flag analogy, they become blue, white or red (in reality, they would become cells of different types, not different colors). Close to the source, cells receive signals above the highest threshold (to become blue, or type 1). Beyond this, cells respond to a lower dose (to become white, or type 2) while farther still cells do not receive enough of the signal to respond (and become red, or type 3). Here the model is expressed in terms of three outcomes, but there might be a different number of outcomes depending on the locations and/ or stages of development. The important point is that cells can work out where they are based on the level of signal they receive and they respond accordingly by developing different attributes.

Beyond Wolpert's basic model, the issue of how brain regionalization develops is an important question and we have relatively few answers. Regional specification is a prerequisite for the development of the connections that must link each region of the brain in a stereotypical and highly precise way (but allowing room for plasticity at a fine level). How these trillions of connections are made is another of life's great mysteries.

The connectome and connectionism

Since Ramón Y Cajal's first description of the neuron, scientists have vastly expanded our understanding of the structure and function of these individual building blocks of the brain. However, as Tim Berners-Lee comments, this is just the first step in understanding how our brains really work: "There are billions of neurons in our brains, but what are neurons? Just cells. The brain has no knowledge until connections are made between neurons. All that we know, all that we are, comes from the way our neurons are connected."

You'll hear about the "connectome" in Sebastian Seung's TEDTalk. The suffix "–ome" is used with increasing frequency to indicate a complete collection of whatever units are specified in the first part of the word, such as genes (hence genome), proteins (proteome) or connections (connectome). The connectome of the human brain is bewildering in its complexity, but the development of new brain imaging methods has catalyzed the first serious attempts to map it in living brains. At present, the resolution of imaging methods that can be applied to living brains isn't sufficient to follow individual connections (called axons). In these TEDTalks you'll hear about an attempt to come at the problem from the other direction, using very high resolution imaging of non-living brain tissue to reconstruct the ultramicroscopic anatomy of connections around individual cells. The extent to which these approaches are likely to succeed remains controversial.

The theory known as connectionism addresses a somewhat different matter within the field of brain organization: the relationship between connectivity and function. Essentially, the idea is that higher mental processes such as object recognition, memory and language result from the activity of the connections between areas of the brain rather than the activity of specific discrete regions. Whereas connectionists would agree that primary sensory and motor functions (i.e. responses to sensory stimuli and the activation of movements) are strongly localized to defined areas within the brain, they argue that this applies less clearly at higher cognitive levels. The theory emphasizes the relationship between connected brain areas and the function of the brain as a whole, with all parts having the potential to contribute to cognitive function. You should appreciate, therefore, that there is as yet no accepted view of the extent to which our higher mental functions are localized to particular parts of the brain. It is worth remembering this as you listen to the TEDTalks; keep an open mind on these truly fascinating issues.

Ways of studying brain function

In these TEDTalks, you're going to hear about some of the ways in which we can work out what the human brain does and how it does it. One longstanding approach is to examine what happens when people suffer brain lesions. Phineas Gage, a Vermont railroad worker, provides one spectacular historical example from 1848. Gage was packing gunpowder into a hole when it exploded, blowing the tamping rod through the front of his brain. Astonishingly, he survived and recovered, but those closest to him claimed that he had a very different personality. From this example, scientists hypothesized that elements of human personality are localized to the frontal lobes.

In Jill Bolte Taylor's TEDTalk, you'll hear how Taylor's own stroke provides further evidence for localization of brain function. A few words of caution, however: when we study the effects of a lesion on the brain, we're really learning about what the rest of the brain does without the damaged part, which is not quite the same as what the damaged structure itself does. Maybe this seems rather subtle, but in some cases it becomes important, for example if a lesion causes other parts of the brain to alter what they do.

You'll also hear about powerful techniques for observing the activity of living brains, for example using functional magnetic resonance imaging (FMRI; see the TEDTalk by Oliver Sacks). And you'll hear about methods for looking at the fine structure of neurons in post-mortem material, as in Sebastian Seung's TEDTalk. All have advantages and limitations, but together they give ever- increasing insight into the workings of the human mind.

Let's begin the TEDTalks with neuroanatomist Jill Bolte Taylor, who provides a basic overview of the brain and describes what she learned firsthand about its structure and function when at age 37 she suffered a massive hemorrhage in the left hemisphere of her brain.

Jill Bolte Taylor

My stroke of insight, relevant talks.

VS Ramachandran

3 clues to understanding your brain.

Oliver Sacks

What hallucination reveals about our minds.

Sebastian Seung

I am my connectome.

Christopher deCharms

A look inside the brain in real time.

A light switch for neurons

Rebecca Saxe

How we read each other's minds.

Advertisement

Introduction: The Human Brain

By Helen Phillips

4 September 2006

A false-colour Magnetic Resonance Image (MRI) of a mid-sagittal section through the head of a normal 42 year-old woman, showing structures of the brain, spine and facial tissues

(Image: Mehau Kulyk / Science Photo Library)

The brain is the most complex organ in the human body. It produces our every thought, action , memory , feeling and experience of the world. This jelly-like mass of tissue, weighing in at around 1.4 kilograms, contains a staggering one hundred billion nerve cells, or neurons .

The complexity of the connectivity between these cells is mind-boggling. Each neuron can make contact with thousands or even tens of thousands of others, via tiny structures called synapses . Our brains form a million new connections for every second of our lives. The pattern and strength of the connections is constantly changing and no two brains are alike.

It is in these changing connections that memories are stored, habits learned and personalities shaped , by reinforcing certain patterns of brain activity, and losing others.

Grey matter

While people often speak of their “ grey matter “, the brain also contains white matter . The grey matter is the cell bodies of the neurons, while the white matter is the branching network of thread-like tendrils – called dendrites and axons – that spread out from the cell bodies to connect to other neurons.

But the brain also has another, even more numerous type of cell, called glial cells . These outnumber neurons ten times over. Once thought to be support cells, they are now known to amplify neural signals and to be as important as neurons in mental calculations. There are many different types of neuron, only one of which is unique to humans and the other great apes, the so called spindle cells .

Brain structure is shaped partly by genes , but largely by experience . Only relatively recently it was discovered that new brain cells are being born throughout our lives – a process called neurogenesis . The brain has bursts of growth and then periods of consolidation , when excess connections are pruned. The most notable bursts are in the first two or three years of life, during puberty , and also a final burst in young adulthood.

How a brain ages also depends on genes and lifestyle too. Exercising the brain and giving it the right diet can be just as important as it is for the rest of the body.

Chemical messengers

The neurons in our brains communicate in a variety of ways. Signals pass between them by the release and capture of neurotransmitter and neuromodulator chemicals, such as glutamate , dopamine , acetylcholine , noradrenalin , serotonin and endorphins .

Some neurochemicals work in the synapse , passing specific messages from release sites to collection sites, called receptors. Others also spread their influence more widely, like a radio signal , making whole brain regions more or less sensitive.

These neurochemicals are so important that deficiencies in them are linked to certain diseases. For example, a loss of dopamine in the basal ganglia, which control movements, leads to Parkinson’s disease . It can also increase susceptibility to addiction because it mediates our sensations of reward and pleasure.

Similarly, a deficiency in serotonin , used by regions involved in emotion, can be linked to depression or mood disorders, and the loss of acetylcholine in the cerebral cortex is characteristic of Alzheimer’s disease .

Brain scanning

Within individual neurons, signals are formed by electrochemical pulses. Collectively, this electrical activity can be detected outside the scalp by an electroencephalogram (EEG).

These signals have wave-like patterns , which scientists classify from alpha (common while we are relaxing or sleeping ), through to gamma (active thought). When this activity goes awry, it is called a seizure . Some researchers think that synchronising the activity in different brain regions is important in perception .

Other ways of imaging brain activity are indirect. Functional magnetic resonance imaging ( fMRI ) or positron emission tomography ( PET ) monitor blood flow. MRI scans, computed tomography ( CT ) scans and diffusion tensor images (DTI) use the magnetic signatures of different tissues, X-ray absorption, or the movement of water molecules in those tissues, to image the brain.

These scanning techniques have revealed which parts of the brain are associated with which functions . Examples include activity related to sensations , movement, libido , choices , regrets , motivations and even racism . However, some experts argue that we put too much trust in these results and that they raise privacy issues .

Before scanning techniques were common, researchers relied on patients with brain damage caused by strokes , head injuries or illnesses, to determine which brain areas are required for certain functions . This approach exposed the regions connected to emotions , dreams , memory , language and perception and to even more enigmatic events, such as religious or “ paranormal ” experiences.

One famous example was the case of Phineas Gage , a 19 th century railroad worker who lost part of the front of his brain when a 1-metre-long iron pole was blasted through his head during an explosion. He recovered physically, but was left with permanent changes to his personality , showing for the first time that specific brain regions are linked to different processes.

Structure in mind

The most obvious anatomical feature of our brains is the undulating surfac of the cerebrum – the deep clefts are known as sulci and its folds are gyri. The cerebrum is the largest part of our brain and is largely made up of the two cerebral hemispheres . It is the most evolutionarily recent brain structure, dealing with more complex cognitive brain activities.

It is often said that the right hemisphere is more creative and emotional and the left deals with logic, but the reality is more complex . Nonetheless, the sides do have some specialisations , with the left dealing with speech and language , the right with spatial and body awareness.

See our Interactive Graphic for more on brain structure

Further anatomical divisions of the cerebral hemispheres are the occipital lobe at the back, devoted to vision , and the parietal lobe above that, dealing with movement , position, orientation and calculation .

Behind the ears and temples lie the temporal lobes , dealing with sound and speech comprehension and some aspects of memory . And to the fore are the frontal and prefrontal lobes , often considered the most highly developed and most “human” of regions, dealing with the most complex thought, decision making , planning, conceptualising, attention control and working memory. They also deal with complex social emotions such as regret , morality and empathy .

Another way to classify the regions is as sensory cortex and motor cortex , controlling incoming information, and outgoing behaviour respectively.

Below the cerebral hemispheres, but still referred to as part of the forebrain, is the cingulate cortex , which deals with directing behaviour and pain . And beneath this lies the corpus callosum , which connects the two sides of the brain. Other important areas of the forebrain are the basal ganglia , responsible for movement , motivation and reward.

Urges and appetites

Beneath the forebrain lie more primitive brain regions. The limbic system , common to all mammals, deals with urges and appetites. Emotions are most closely linked with structures called the amygdala , caudate nucleus and putamen . Also in the limbic brain are the hippocampus – vital for forming new memories; the thalamus – a kind of sensory relay station; and the hypothalamus , which regulates bodily functions via hormone release from the pituitary gland .

The back of the brain has a highly convoluted and folded swelling called the cerebellum , which stores patterns of movement, habits and repeated tasks – things we can do without thinking about them.

The most primitive parts, the midbrain and brain stem , control the bodily functions we have no conscious control of, such as breathing , heart rate, blood pressure, sleep patterns , and so on. They also control signals that pass between the brain and the rest of the body, through the spinal cord.

Though we have discovered an enormous amount about the brain, huge and crucial mysteries remain. One of the most important is how does the brain produces our conscious experiences ?

The vast majority of the brain’s activity is subconscious . But our conscious thoughts, sensations and perceptions – what define us as humans – cannot yet be explained in terms of brain activity.

• psychology /

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Brain Basics: Know Your Brain

The brain is the most complex part of the human body. This three-pound organ is the seat of intelligence, interpreter of the senses, initiator of body movement, and controller of behavior. Lying in its bony shell and washed by protective fluid, the brain is the source of all the qualities that define our humanity. It is the crown jewel of the human body.

This fact sheet is a basic introduction to the human brain. It can help you understand how the healthy brain works, how to keep your brain healthy, and what happens when the brain doesn't work like it should.

The Structure of the Brain

The brain is like a group of experts. All the parts of the brain work together, but each part has its own special responsibilities. The brain can be divided into three basic units: the forebrain , the midbrain , and the hindbrain .

The hindbrain includes the upper part of the spinal cord, the brain stem, and a wrinkled ball of tissue called the cerebellum . The hindbrain controls the body’s vital functions such as respiration and heart rate.

The cerebellum coordinates movement and is involved in learned movements. When you play the piano or hit a tennis ball, you are activating the cerebellum.

The uppermost part of the brainstem is the midbrain, which controls some reflex actions and is part of the circuit involved in the control of eye movements and other voluntary movements. The forebrain is the largest and most highly developed part of the human brain: it consists primarily of the  cerebrum and the structures hidden beneath it ( see " The Inner Brain").

When people see pictures of the brain it is usually the cerebrum that they notice. The cerebrum sits at the topmost part of the brain and is the source of conscious thoughts and actions. It holds your memories and allows you to plan, imagine, and think. It allows you to recognize friends, read, and play games.

The cerebrum is split into two halves (hemispheres) by a deep fissure. The two cerebral hemispheres communicate with each other through a thick tract of nerve fibers that lies at the base of this fissure, called the corpus callosum. Although the two hemispheres seem to be mirror images of each other, they are different. For instance, the ability to form words seems to lie primarily in the left hemisphere, while the right hemisphere seems to control many abstract reasoning skills.

For some as-yet-unknown reason, nearly all of the signals from the brain to the body and vice versa cross over on their way to and from the brain. This means that the right cerebral hemisphere primarily controls the left side of the body, and the left hemisphere primarily controls the right side. When one side of the brain is damaged, the opposite side of the body is affected. For example, a stroke in the right hemisphere of the brain can leave the left arm and leg paralyzed.

The Cerebral Cortex

Coating the surface of the cerebrum and the cerebellum is a vital layer of tissue the thickness of a stack of two or three dimes. It is called the cortex, from the Latin word for bark. Most of the actual information processing in the brain takes place in the cerebral cortex. When people talk about "gray matter" in the brain, they are talking about the cortex. The cortex is gray because nerves in this area lack the insulation that makes most other parts of the brain appear to be white. The folds in the brain add to its surface area and therefore increase the amount of gray matter and the volume of information that can be processed.

The Geography of Thought

Each cerebral hemisphere can be divided into sections, or lobes, each of which specializes in different functions. To understand each lobe and its specialty, we will take a tour of the cerebral hemispheres.

Frontal lobes

The two  frontal lobes lie directly behind the forehead. When you plan a schedule, imagine the future, or use reasoned arguments, these two lobes do much of the work. One of the ways the frontal lobes seem to do these things is by acting as short-term storage sites, allowing one idea to be kept in mind while other ideas are considered.

Motor cortex

In the back portion of each frontal lobe is a  motor cortex , which helps plan, control, and execute voluntary movement, like moving your arm or kicking a ball.

Parietal lobes

When you enjoy a good meal—the taste, smell, and texture of the food—two sections behind the frontal lobes called the  parietal lobes are at work. The parietal lobes also support reading and arithmetic.

Somatosensory cortex

The forward parts of these lobes, just behind the motor areas, is the somatosensory cortex . These areas receive information about temperature, taste, touch, and movement from the rest of the body.

Occipital lobes

As you look at the words and pictures on this page, two areas at the back of the brain are at work. These lobes, called the  occipital lobes , process images from the eyes and link that information with images stored in memory. Damage to the occipital lobes can cause blindness.

Temporal lobes

The last lobes on our tour of the cerebral hemispheres are the  temporal lobes , which lie in front of the visual areas and nest under the parietal and frontal lobes. Whether you appreciate symphonies or rock music, your brain responds through the activity of these lobes. At the top of each temporal lobe is an area responsible for receiving information from the ears. The underside of each temporal lobe plays a crucial role in forming and retrieving memories, including those associated with music. Other parts of this lobe integrate memories and sensations of taste, sound, sight, and touch.

The Inner Brain

Deep within the brain, hidden from view, lie structures that are the gatekeepers between the spinal cord and the cerebral hemispheres. These structures not only determine our emotional state, but they also modify our perceptions and responses and allow us to initiate movements that without thinking about them. Like the lobes in the cerebral hemispheres, the structures described below come in pairs: each is duplicated in the opposite half of the brain.

The  hypothalamus , about the size of a pearl, directs a multitude of important functions. It wakes you up in the morning and gets the adrenaline flowing during a test or job interview. The hypothalamus is also an important emotional center, controlling the chemicals that make you feel exhilarated, angry, or unhappy. Near the hypothalamus lies the  thalamus , a major clearinghouse for information going to and from the spinal cord and the cerebrum.

An arching tract of nerve cells leads from the hypothalamus and the thalamus to the  hippocampus . This tiny nub acts as a memory indexer—sending memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrieving them when necessary. The  basal ganglia  (not shown) are clusters of nerve cells surrounding the thalamus. They are responsible for initiating and integrating movements. Parkinson’s disease, which results in tremors, rigidity, and a stiff, shuffling walk, affects the nerve cells in the basal ganglia.

The brain and the rest of the nervous system are composed of many different types of cells, but the primary functional unit is a cell called the neuron. All sensations, movements, thoughts, memories, and feelings are the result of signals that pass through neurons. Neurons consist of three parts: the cell body , dendrites , and the axon .

The cell body contains the nucleus, where most of the molecules that the neuron needs to survive and function are manufactured. Dendrites extend out from the cell body like the branches of a tree and receive messages from other nerve cells. Signals then pass from the dendrites through the cell body and travel away from the cell body down an axon to another neuron, a muscle cell, or cells in some other organ.

The neuron is usually surrounded by many support cells. Some types of cells wrap around the axon to form an insulating  myelin sheath . Myelin is a fatty molecule which provides insulation for the axon and helps nerve signals travel faster and farther. Axons may be very short, such as those that carry signals from one cell in the cortex to another cell less than a hair’s width away. Other axons may be very long, such as those that carry messages from the brain all the way down the spinal cord.

The Synapse

Scientists have learned a great deal about neurons by studying the synapse—the place where a signal passes from the neuron to another cell. When the signal reaches the end of the axon it stimulates the release of tiny sacs called  vesicles. These vesicles release chemicals known as  neurotransmitters  into the  synaptic cleft. The neurotransmitters cross the synapse and attach to  receptors on the neighboring cell. These receptors can change the properties of the receiving cell. If the receiving cell is also a neuron, the signal can continue the transmission to the next cell.

Some Key Neurotransmitters At Work

Neurotransmitters are chemicals that brain cells use to talk to each other. Some neurotransmitters make cells more active (called  excitatory ) while others block or dampen a cell's activity (called  inhibitory ).

• Acetylcholine is an excitatory neurotransmitter. It governs muscle contractions and causes glands to secrete hormones. Alzheimer’s disease , which initially affects memory formation, is associated with a shortage of acetylcholine.
• Glutamate is a major excitatory neurotransmitter. Too much glutamate can kill or damage neurons and has been linked to disorders including Parkinson's disease , stroke , seizures, and increased sensitivity to pain .
• GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that helps control muscle activity and is an important part of the visual system. Drugs that increase GABA levels in the brain are used to treat epileptic seizures and tremors in patients with Huntington’s disease .
• Serotonin is a neurotransmitter that constricts blood vessels and brings on sleep. It is also involved in temperature regulation. Low levels of serotonin may cause sleep problems and depression, while too much serotonin can lead to seizures.
• Dopamine  can be excitatory or inhibitory and is involved in mood and the control of complex movements. The loss of dopamine activity in some portions of the brain leads to the muscular rigidity of Parkinson’s disease . Many medications used to treat mental health disorders and conditions work by modifying the action of dopamine in the brain.

Neurological Disorders

The brain is one of the hardest working organs in the body. When the brain is healthy it functions quickly and automatically. But when problems occur, the results can be devastating. NINDS supports research on hundreds of neurological disorders. Knowing more about the brain can lead to the development of new treatments for diseases and disorders of the nervous system and improve many areas of human health.

Unlocking the Brain, Earth's Most Complex Biological Structure (Essay)

James Olds is head of the U.S. National Science Foundation's Directorate for Biological Sciences and is a named professor of molecular neuroscience at George Mason University. Olds contributed this article to Live Science's Expert Voices: Op-Ed & Insights .

Your brain is essentially what makes you … you . It controls your thinking, problem solving and voluntary behaviors. At the same time, it continuously helps regulate critical aspects of your physiology, such as your heart rate and breathing. And yet your brain — a nonstop multitasking marvel — runs on only about 20 watts of energy, the same wattage as an energy-saving light bulb.

Still, for the most part, the brain is an unknown frontier: Neuroscientists don't yet fully understand how information is processed by the brain. That's the case even in the brain of a worm , which has just several hundred neurons — nothing compared to the human brain, which has 80 billion to 100 billion neurons. The chain of events that generates a thought, behavior or physiological response remains mysterious.

Why? The brain is the most complex biological structure known to scientists. When researchers do figure out how it works, they will accomplish perhaps the greatest scientific achievement in recorded human history. [ Unlocking the Secrets of the Brain (Gallery) ]

A Big Bang theory for the brain

Neuroscientists all over the world are working to develop an overarching theory of how a healthy brain works . Similar to the way the Big Bang theory offers one possible explanation for the cosmos and helps guide research on the origins of the universe, a theory of healthy brain function would offer a possible explanation of how the brain and the entire nervous system work, and would help guide neuroscience research.

A theory of healthy brain function may also help to explain how injuries and diseases disrupt brain function. Such a theory could help researchers identify new directions for research on traumatic brain injuries and brain diseases. 

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Knowledge about healthy brain function could also help inspire the development of smart technologies that mimic some of the human brain's unparalleled capabilities. If supercomputers — which each can consume millions of dollars' worth of electricity annually, as well as huge amounts of cooling water — could match the brain's energy efficiency and processing power, their massive energy consumption would plummet, and science and innovation would leap forward.

Neuroscientists have made some progress toward understanding the brain. They have identified brain regions that regulate particular functions, including speech and motor function, and they can recognize structural and functional changes that occur in the brain throughout an animal's life span.

And recently, neuroscientists have developed advanced tools for visualizing and analyzing parts of the brain in unprecedented detail. These tools provide the first detailed glimpses of the brain, and are thrusting neuroscience forward, much as the first powerful telescopes provided the deep glimpses into the universe and thrust astronomy forward.

The BRAIN Initiative

Building on these and other recent innovations, President Barack Obama launched the Brain Research through Advancing Innovative Neurotechnologies Initiative ( BRAIN Initiative ) in April 2013. Federally funded in 2015 at $200 million, the initiative is a public-private research effort to revolutionize scientists' understanding of the brain.

As part of the initiative, NSF is working to reveal  how a healthy brain works . Magnetic resonance imaging (MRI) technology, bionic limbs and laser eye surgery were all grounded in early NSF-funded fundamental research, and this new research on the healthy brain could lead to equally profound advances. As a U.S. federal science agency, NSF will spend about $48.48 million on grants supporting the BRAIN Initiative, part of approximately $106.44 million in grants we will provide for all "Understanding the Brain" research across a range of neuroscience topics. 

With that support, our research teams are tackling the mysteries of the brain from varied angles. They are inventing new probes to monitor and manipulate the brain; building computer models to help reveal the activities of neurons that drive thoughts and behavior; improving brain imaging technologies; and studying the nervous systems of a wide range of species.

Those researchers are also are creating the cyberinfrastructure to store and manage the "big data" generated by brain studies. This is critically important: If nanoscale images of one brain were stored in a stack of 1TB hard drives, the stack would reach to the moon, or beyond.

Pushing the brain boundaries

In addition, NSF provided 36 interdisciplinary teams with a total of  $10.8 million in early concept grants , each addressing this vexing question: How do circuits of neurons generate behaviors and enable learning and perception? 

One of those teams, from the University of North Carolina School of Medicine, is improving a new kind of microscope to simultaneously view individual neurons firing in two or more different regions of a brain at the same time. This microscope will enable researchers to see in detail, for the first time, how different areas of the brain team up to process information.

Taking an entirely different tack, researchers at the new $25 million NSF Center for Brains, Minds & Machines at MIT are investigating human intelligence and the potential for creating intelligent machines. As researchers learn how to build those machines, they will likely also advance humanity's understanding of human intelligence.

If  history is any guide, these and other fundamental brain-research projects will have important applications. For example, fundamental research on light-sensitive organisms  led to the development, in 2005, of a breakthrough technology called optogenetics for selectively turning individual neurons on and off by exposing them to light. ( See an animation explaining optogenetics here.  )

Today, optogenetics, is used to study the potential roles of faulty neurons seen in Parkinson's disease, schizophrenia , depression , strokes, PTSD , addictions and some forms of blindness. 

And most recently, viewers of the 2014 World Cup recently saw another important application of fundamental brain research: The first kick of the games was performed by a person with paraplegia wearing an exoskeleton which built upon NSF-funded research into how neurons are involved in motor learning — research that began nearly twenty years ago.

Across government, and across the nation, hopes are high that additional, fundamental, neuroscience research will lay the groundwork for continued advances that will help society take additional strides forward.

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook , Twitter and Google+ . The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Live Science.

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Anatomy of the Brain

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Brain Divisions

Anatomy of the brain: structures, brain ventricles, more about the brain.

• B.A., Biology, Emory University
• A.S., Nursing, Chattahoochee Technical College

The anatomy of the brain is complex due its intricate structure and function. This amazing organ acts as a control center by receiving, interpreting, and directing sensory information throughout the body. The brain and spinal cord are the two main structures of the central nervous system . There are three major divisions of the brain. They are the forebrain, the midbrain, and the hindbrain.

Key Takeaways

• The forebrain, the midbrain, and the hindbrain are the three main parts of the brain.
• The forebrain has two major parts called the diencephalon and the telencephalon. The forebrain is responsible for a number of functions related to thinking, perceiving, and evaluating sensory information.
• The midbrain, also called the mesencephalon, connects the hindbrain and the forebrain. It is associated with motor functions and auditory and visual responses.
• The hindbrain contains both the metencephalon and the myelencephalon. The hindbrain is associated with balance and equilibrium and the coordination of movement along with autonomic functions like our breathing and our heart rate.
• Both the midbrain and the hindbrain make up the brainstem.

The forebrain is the division of the brain that is responsible for a variety of functions including receiving and processing sensory information, thinking, perceiving, producing and understanding language, and controlling motor function. There are two major divisions of forebrain: the diencephalon and the telencephalon. The diencephalon contains structures such as the thalamus and hypothalamus which are responsible for such functions as motor control, relaying sensory information, and controlling autonomic functions. The telencephalon contains the largest part of the brain, the cerebrum . Most of the actual information processing in the brain takes place in the cerebral cortex .

The midbrain and the hindbrain together make up the brainstem . The midbrain or mesencephalon , is the portion of the brainstem that connects the hindbrain and the forebrain. This region of the brain is involved in auditory and visual responses as well as motor function.

The hindbrain extends from the spinal cord and is composed of the metencephalon and myelencephalon. The metencephalon contains structures such as the pons and cerebellum . These regions assists in maintaining balance and equilibrium, movement coordination, and the conduction of sensory information. The myelencephalon is composed of the medulla oblongata which is responsible for controlling such autonomic functions as breathing, heart rate, and digestion.

The brain contains various structures that have a multitude of functions. Below is a list of major structures of the brain and some of their functions. Basal Ganglia

• Involved in cognition and voluntary movement
• Diseases related to damages of this area are Parkinson's and Huntington's
• Relays information between the peripheral nerves and spinal cord to the upper parts of the brain
• Consists of the midbrain, medulla oblongata, and the pons

Broca's Area

• Speech production
• Understanding language

Central Sulcus (Fissure of Rolando)

• Deep grove that separates the parietal and frontal lobes
• Controls movement coordination
• Maintains balance and equilibrium

Cerebral Cortex

• Outer portion (1.5mm to 5mm) of the cerebrum
• Receives and processes sensory information
• Divided into cerebral cortex lobes

Cerebral Cortex Lobes

• Frontal Lobes -involved with decision-making, problem solving, and planning
• Occipital Lobes -involved with vision and color recognition
• Parietal Lobes - receives and processes sensory information
• Temporal Lobes - involved with emotional responses, memory, and speech
• Largest portion of the brain
• Consists of folded bulges called gyri that create deep furrows

Corpus Callosum

• Thick band of fibers that connects the left and right brain hemispheres

Cranial Nerves

• Twelve pairs of nerves that originate in the brain, exit the skull, and lead to the head, neck and torso

Fissure of Sylvius (Lateral Sulcus)

• Deep grove that separates the parietal and temporal lobes

Limbic System Structures

• Amygdala - involved in emotional responses, hormonal secretions, and memory
• Cingulate Gyrus - a fold in the brain involved with sensory input concerning emotions and the regulation of aggressive behavior
• Fornix - an arching, fibrous band of white matter axons (nerve fibers) that connect the hippocampus to the hypothalamus
• Hippocampus - sends memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrieves them when necessary
• Hypothalamus - directs a multitude of important functions such as body temperature, hunger, and homeostasis
• Olfactory Cortex - receives sensory information from the olfactory bulb and is involved in the identification of odors
• Thalamus - mass of gray matter cells that relay sensory signals to and from the spinal cord and the cerebrum

Medulla Oblongata

• Lower part of the brainstem that helps to control autonomic functions
• Membranes that cover and protect the brain and spinal cord

Olfactory Bulb

• Bulb-shaped end of the olfactory lobe
• Involved in the sense of smell

Pineal Gland

• Endocrine gland involved in biological rhythms
• Secretes the hormone melatonin

Pituitary Gland

• Endocrine gland involved in homeostasis
• Regulates other endocrine glands
• Relays sensory information between the cerebrum and cerebellum

Wernicke's Area

• Region of the brain where spoken language is understood

Cerebral Peduncle

• anterior portion of the midbrain consisting of large bundles of nerve fiber tracts that connect the forebrain to the hindbrain

Reticular Formation

• Nerve fibers located inside the brainstem and a component of the tegmentum ( midbrain )
• Regulates awareness and sleep

Substantia Nigra

• Helps to control voluntary movement and regulates mood ( midbrain )
• The dorsal region of the mesencephalon ( midbrain )
• Assists in visual and auditory reflexes
• The ventral region of the mesencephalon ( midbrain )
• Includes the reticular formation and the red nucleus

Ventricular System - connecting system of internal brain cavities filled with cerebrospinal fluid

• Aqueduct of Sylvius - canal that is located between the third ventricle and the fourth ventricle
• Choroid Plexus - produces cerebrospinal fluid
• Fourth Ventricle - canal that runs between the pons, medulla oblongata, and the cerebellum
• Lateral Ventricle - largest of the ventricles and located in both brain hemispheres
• Third Ventricle - provides a pathway for cerebrospinal fluid to flow

For additional information about the brain, see Divisions of the Brain . Would you like to test your knowledge of the human brain? Take the Human Brain Quiz!

• Divisions of the Brain: Forebrain, Midbrain, Hindbrain
• Brainstem: Function and Location
• Functions of the Central Nervous System
• Anatomy of the Cerebellum and its Function
• Where in the Brain Is the Pons
• Anatomy of the Brain: Your Cerebrum
• Overview of the Medulla Oblongata
• Learn About the Mesencephalon (Midbrain) Function and Structures
• Basic Parts of the Brain and Their Responsibilities
• Get a Description and Diagram of Thalamus Gray Matter
• Third Ventricle
• Hypothalamus Activity and Hormone Production
• Diencephalon Section of the Brain
• The Limbic System of the Brain
• White Matter and Your Brain
• Amygdala's Location and Function

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Ackerman S. Discovering the Brain. Washington (DC): National Academies Press (US); 1992.

Discovering the Brain.

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2 Major Structures and Functions of the Brain

Outside the specialized world of neuroanatomy and for most of the uses of daily life, the brain is more or less an abstract entity. We do not experience our brain as an assembly of physical structures (nor would we wish to, perhaps); if we envision it at all, we are likely to see it as a large, rounded walnut, grayish in color.

This schematic image refers mainly to the cerebral cortex, the outermost layer that overlies most of the other brain structures like a fantastically wrinkled tissue wrapped around an orange. The preponderance of the cerebral cortex (which, with its supporting structures, makes up approximately 80 percent of the brain's total volume) is actually a recent development in the course of evolution. The cortex contains the physical structures responsible for most of what we call ''brainwork": cognition, mental imagery, the highly sophisticated processing of visual information, and the ability to produce and understand language. But underneath this layer reside many other specialized structures that are essential for movement, consciousness, sexuality, the action of our five senses, and more—all equally valuable to human existence. Indeed, in strictly biological terms, these structures can claim priority over the cerebral cortex. In the growth of the individual embryo, as well as in evolutionary history, the brain develops roughly from the base of the skull up and outward. The human brain actually has its beginnings, in the four-week-old embryo, as a simple series of bulges at one end of the neural tube.

FIGURE 2.1.

The brain owes its outer appearance of a walnut to the wrinkled and deeply folded cerebral cortex, which handles the innumerable signals responsible for perception and movement and also for mental processes. Below the surface of the cortex are packed (more...)

The bulges in the neural tube of the embryo develop into the hindbrain, midbrain, and forebrain—divisions common to all vertebrates, from sharks to squirrels to humans. The original hollow structure is commemorated in the form of the ventricles, which are cavities containing cerebrospinal fluid. During the course of development, the three bulges become four ventricles. In the hindbrain is the fourth ventricle, continuous with the central canal of the spinal cord. A cavity in the forebrain becomes the third ventricle, which leads further forward into the two lateral ventricles, one in each cerebral hemisphere.

The hindbrain contains several structures that regulate autonomic functions, which are essential to survival and not under our conscious control. The brainstem, at the top of the spinal cord, controls breathing, the beating of the heart, and the diameter of blood vessels. This region is also an important junction for the control of deliberate movement. Through the medulla, at the lower end of the brainstem, pass all the nerves running between the spinal cord and the brain; in the pyramids of the medulla, many of these nerve tracts for motor signals cross over from one side of the body to the other. Thus, the left brain controls movement of the right side of the body, and the right brain controls movement of the left side.

In addition to being the major site of crossover for nerve tracts running to and from the brain, the medulla is the seat of several pairs of nerves for organs of the chest and abdomen, for movements of the shoulder and head, for swallowing, salivation, and taste, and for hearing and equilibrium.

At the top of the brainstem is the pons—literally, a bridge—between the lower brainstem and the midbrain. Nerve impulses traversing the pons pass on to the cerebellum (or "little brain"), which is concerned primarily with the coordination of complex muscular movement. In addition, nerve fibers running through the pons relay sensations of touch from the spinal cord to the upper brain centers.

Many nerves for the face and head have their origin in the pons, and these nerves regulate some movements of the eyeball, facial expression, salivation, and taste. Together with nerves of the medulla, nerves from the pons also control breathing and the body's sense of equilibrium.

What had been the middle bulge in the neural tube develops into the midbrain, which functions mainly as a relay center for sensory and motor nerve impulses between the pons and spinal cord and the thalamus and cerebral cortex. Nerves in the midbrain also control some movements of the eyeball, pupil, and lens and reflexes of the eyes, head, and trunk.

• Thalamus And Hypothalamus

Deep in the core area of the brain, just above the top of the brainstem, are structures that have a great deal to do with perception, movement, and the body's vital functions. The thalamus consists of two oval masses, each embedded in a cerebral hemisphere, that are joined by a bridge. The masses contain nerve cell bodies that sort information from four of the senses—sight, hearing, taste, and touch—and relay it to the cerebral cortex. (Only the sense of smell sends signals directly to the cortex, bypassing the thalamus.) Sensations of pain, temperature, and pressure are also relayed through the thalamus, as are the nerve impulses from the cerebral hemispheres that initiate voluntary movement.

The hypothalamus, despite its relatively small size (roughly that of a thumbnail), controls a number of drives essential for the functioning of a wide-ranging omnivorous social mammal. At the autonomic level, the hypothalamus stimulates smooth muscle (which lines the blood vessels, stomach, and intestines) and receives sensory impulses from these areas. Thus it controls the rate of the heart, the passage of food through the alimentary canal, and contraction of the bladder.

The hypothalamus is the main point of interaction for the body's two physical control systems: the nervous system, which transmits information in the form of minute electrical impulses, and the endocrine system, which brings about changes of state through the release of chemical factors. It is the hypothalamus that first detects crucial changes in the body and responds by stimulating various glands and organs to release hormones.

The hypothalamus is also the brain's intermediary for translating emotion into physical response. When strong feelings (rage, fear, pleasure, excitement) are generated in the mind, whether by external stimuli or by the action of thoughts, the cerebral cortex transmits impulses to the hypothalamus; the hypothalamus may then send signals for physiological changes through the autonomic nervous system and through the release of hormones from the pituitary. Physical signs of fear or excitement, such as a racing heartbeat, shallow breathing, and perhaps a clenched "gut feeling," all originate here.

Also in the hypothalamus are neurons that monitor body temperature at the surface through nerve endings in the skin, and other neurons that monitor the blood flowing through this part of the brain itself, as an indicator of core body temperature. The front part of the hypothalamus contains neurons that act to lower body temperature by relaxing smooth muscle in the blood vessels, which causes them to dilate and increases the rate of heat loss from the skin. Through its neurons associated with the sweat glands of the skin, the hypothalamus can also promote heat loss by increasing the rate of perspiration. In opposite conditions, when the body's temperature falls below the (rather narrow) ideal range, a portion of the hypothalamus directs the contraction of blood vessels, slows the rate of heat loss, and causes the onset of shivering (which produces a small amount of heat).

The hypothalamus is the control center for the stimuli that underlie eating and drinking. The sensations that we interpret as hunger arise partly from a degree of emptiness in the stomach and partly from a drop in the level of two substances: glucose circulating in the blood and a hormone that the intestine produces shortly after the intake of food. (Receptors for this hormone gauge how far digestion has proceeded since the last meal.) This system is not a simple "on" switch for hunger, however: another portion of the hypothalamus, when stimulated, actively inhibits eating by promoting a feeling of satiety. In experimental animals, damage to this portion of the brain is associated with continued excessive eating, eventually leading to obesity.

In addition to these numerous functions, there is evidence that the hypothalamus plays a role in the induction of sleep. For one thing, it forms part of the reticular activating system, the physical basis for that hard-to-define state known as consciousness (about which more later); for another, electrical stimulation of a portion of the hypothalamus has been shown to induce sleep in experimental animals, although the mechanism by which this works is not yet known. In all, the hypothalamus is a richly complex cubic centimeter of vital connections, which will continue to reward close study for some time to come. Because of its unique position as a midpost between thought and feeling and between conscious act and autonomic function, a thorough understanding of its workings should tell us much about the earliest history and development of the human animal.

• Pituitary And Pineal Glands

The pituitary and the pineal glands function in close association with the hypothalamus. The pituitary responds to signals from the hypothalamus by producing an array of hormones, many of which regulate the activities of other glands: thyroid-stimulating hormone, adrenocorticotropic hormone (which stimulates an outpouring of epinephrine in response to stress), prolactin (involved in the production of milk), and the sex hormones follicle-stimulating hormone and luteinizing hormone, which promote the development of eggs and sperm and regulate the timing of ovulation. The pituitary gland also produces several hormones with more general effects: human growth hormone, melanocyte-stimulating hormone (which plays a role in the pigmentation of skin), and dopamine, which inhibits the release of prolactin but is better known as a neurotransmitter (see Chapter 5 ).

The pineal gland produces melatonin, the hormone associated with skin pigmentation. The secretion of melatonin varies significantly over a 24-hour cycle, from low levels during the day to a peak at night, and the pineal gland has been called a "third eye" because it is controlled by neurons sensitive to light, which originate in the retina of each eye and end in the hypothalamus. In animals with a clear-cut breeding season, the pineal gland is a link between the shifting hours of daylight and the hormonal responses of the hypothalamus, which in turn guide reproductive functions. In humans, who can conceive and give birth throughout the year, the pineal gland plays no known role in reproduction, although there is evidence that melatonin has a share in regulating ovulation.

• The "Little Brain" At The Back Of The Head

While autonomic and endocrine functions are being maintained by structures deep inside the brain, another specialized area is sorting and processing the signals required to maintain balance and posture and to carry out coordinated movement. The cerebellum (the term in Latin means "little brain") is actually a derived form of the hindbrain—as suggested by its position at the back of the head, partly tucked under the cerebral hemispheres. In humans, with our almost unlimited repertoire of movement, the cerebellum is accordingly large; in fact, it is the second-largest portion of the brain, exceeded only by the cerebral cortex. Its great surface area is accommodated within the skull by elaborate folding, which gives it an irregular, pleated look. In relative terms, the cerebellum is actually largest in the brain of birds, where it is responsible for the constant streams of information between brain and body that are required for flight.

In humans, the cerebellum relays impulses for movement from the motor area of the cerebral cortex to the spinal cord; from there, they pass to their designated muscle groups. At the same time, the cerebellum receives impulses from the muscles and joints that are being activated and in some sense compares them with the instructions issued from the motor cortex, so that adjustments can be made (this time by way of the thalamus). The cerebellum thus is neither the sole initiator of movement nor a simple link in the chain of nerve impulses, but a site for the rerouting and in some cases refining of instructions for movement. There is evidence, too, that the cerebellum can store a sequence of instructions for frequently performed movements and for skilled repetitive movements—those that we think of as learned "by rote."

The right and left hemispheres of the cerebellum each connect with the nerve tracts from the spinal cord on the same side of the body, and with the opposite cerebral hemisphere. For example, nerve impulses concerned with movement of the left arm originate in the right cerebral hemisphere, and information about the orientation, speed, and force of the movement is fed back to the right cerebral hemisphere, through the left half of the cerebellum. The nerves responsible for movement at the ends of the arms and legs tend to have their origin near the outer edges of the cerebellum. By contrast, nerves that have their origin near the center of the cerebellum serve to monitor the body's overall orientation in space and to maintain upright posture, in response to information about balance that is transmitted by nerve impulses from the inner ear, among other sources.

• Reticular Network

Some nerve fibers from the cerebellum also contribute to the reticular formation, a widespread network of neurons ("reticular" is derived from the Latin word for "net"). This formation and some neurons in the thalamus, together with others from various sensory systems of the brain, make up the reticular activating system—the means by which we maintain consciousness. The reticular activating system also comes into play when we deliberately focus our attention, "tuning out" distractions to some degree. At the midline of the brainstem are the raphe nuclei, whose axons extend down into the spinal cord and up to the cerebral cortex—a reach that makes it possible for many areas of the nervous system to be contacted simultaneously. The reticular formation plays a role in movement, particularly those forms of movement that do not call for conscious attention: it is also involved in transmitting or inhibiting sensations of pain, temperature, and touch. Less tangibly, the reticular activating system appears to work as a filter for the countless stimuli that can act on the nervous system both from within and from outside the body. It is this filtering of signals that allows a passenger on an airplane, for example, to doze off undisturbed by sounds of nearby conversation and steady jet engines, but to awake and become alert when the pitch of the engines changes and the plane tilts into its descent.

• The "Emotional Brain"

The limbic system (from the Latin limbus , for "hem" or "border") is another assembly of linked structures that form a loose circuit throughout the brain. This system is a fairly old part of the brain and one that humans share with many other vertebrates; in reptiles, it is known as the rhinencephalon, or "smell-brain," because it reacts primarily to signals of odor. In humans, of course, the stimuli that can affect the emotional brain are just about limitless in their variety.

The limbic system is responsible for most of the basic drives and emotions and the associated involuntary behavior that are important for an animal's survival: pain and pleasure, fear, anger, sexual feelings, and even docility and affection. As with the rhinencephalon, the sense of smell is a powerful factor. Nerves from the olfactory bulb, by which all odor is perceived, track directly into the limbic system at several points and are then connected through it to other parts of the brain; hence the ability of pheromones, and perhaps of other odors as well, to influence behavior in quite complex ways without necessarily reaching our conscious awareness.

Also feeding into the limbic system are the thalamus and hypothalamus, as well as the amygdala, a small, almond-shaped complex of nerve cells that receive input from both the olfactory system and the cerebral cortex. These connections are illustrated in an unusual way in the context of epilepsy. Perhaps because the amygdala is located near a common site of origin of epileptic seizures—that is, in the temporal lobe of the cerebral hemispheres—epileptics sometimes experience unidentifiable or unpleasant odors or changes of mood as part of the aura preceding a seizure. The limbic system is not thought to be involved in the causes of epilepsy, but it is indirectly stimulated by the electric discharge in the brain that sets off a seizure and gives evidence of the stimulation in its own characteristic ways.

• Hippocampus

The hippocampus is another major structure of the limbic system. Named for its fanciful resemblance to the shape of a sea horse, the hippocampus is located at the base of the temporal lobe near several sets of association fibers. These are bundles of nerve fibers that connect one region of the cerebral cortex with another, so that the hippocampus, as well as other parts of the limbic system, exchanges signals with the entire cerebral cortex. The hippocampus has been shown to be important for the consolidation of recently acquired information. (In contrast, long-term memory is thought to be stored throughout the cerebral cortex. The means by which short-term memory is converted into long-term memory has posed a particularly challenging riddle that only now is beginning to yield to investigation; see Chapter 8 .)

Recent work with a variety of animals has found dense clusters of receptor sites for tetrahydrocannabinol, the active ingredient of marijuana and related drugs, in the hippocampus and other nearby structures of the limbic system. This localization helps explain the effects of marijuana, which range from mild euphoria to wavering attention to temporarily weakened short-term memory. A loss of short-term memory is also seen in certain syndromes of alcoholism and in Alzheimer's disease, which involves some degeneration of the hippocampus and other limbic structures.

• Cerebral Cortex

The cerebral cortex occupies by far the greatest surface area of the human brain and presents its most striking aspect. Also known as the neocortex, this is the most recently evolved area of the brain. In fact, the enormous expansion in the area of the cerebral cortex is hypothesized to have begun only about 2 million years ago, in the earliest members of the genus Homo ; the result today is a brain weighing approximately three times more than would be expected for a mammal our size. The cortex is named for its resemblance to the bark of a tree, because it covers the surface of the cerebral hemispheres in a similar way. Its wrinkled convoluted appearance is due to a growth spurt during the fourth and fifth months of embryonic development, when the gray matter of the cortex is expanding greatly as its cells grow in size. The supporting white matter, meanwhile, grows less rapidly; as a result, the brain takes on the dense folds and fissures characteristic of an object with great surface area crowded into a small space.

FIGURE 2.2.

The brain is divided into a left and a right hemisphere by a deep groove that runs from the front of the head (at left) to the back (at right). In each hemisphere, the cerebral cortex falls into four main divisions, or lobes, set off from one another (more...)

FIGURE 2.3.

Two miniature ''maps" represent the body on the cerebral cortex. One of these, in the motor area, assigns a specific portion of the cortex to each part of the body that calls for muscular control; the portions assigned to the fingers, lips, and tongue (more...)

Although the folds in the cerebral cortex appear at first to be random, they include several prominent bulges, or gyri, and grooves, or sulci, that act as landmarks in what is in fact a highly ordered structure (the finer details of which are still not completely known). The deepest groove extends from the front to the back of the head, dividing the brain into the left and right hemispheres. The central sulcus, which runs from the middle of the brain outward to both left and right, and the lateral sulcus, another left-to-right groove somewhat lower on the hemispheres and toward the back of the head, further divide each hemisphere into four lobes: frontal, parietal, temporal, and occipital. A fifth lobe, known as the insula, is located deep within the parietal and temporal lobes and is not apparent as a separate structure on the outer surface of the cerebral hemispheres.

Two noticeable bulges, the precentral gyrus and the postcentral gyrus, are named for their positions just in front of and just behind the central sulcus, respectively. The precentral gyrus is the site of the primary motor area, responsible for conscious movement. From eyebrows to toes, the movable parts of the body are "mapped" on this area of the cortex, with each muscle group or limb represented here by a population of neurons. In complementary fashion, the job of receiving sensations from all parts of the body is managed by the primary somatosensory area, which is located in the postcentral gyrus. Here, too, the human form is mapped, and, as with the precentral gyrus, the areas devoted to the hand and the mouth are disproportionately large. Their size reflects the elaborate brain circuitry that makes possible the precision grip of the human hand, the fine motor and sensory signals required for striking up a violin arpeggio or sharpening a tool, and the coordination of the lips, tongue, and vocal apparatus to produce the highly arbitrary and significant sounds of human language.

Close observations of animals and humans after injury to particular sites of the brain indicate that many areas of the cortex control quite specific functions. Additional findings have come from stimulating sites on the cortex with a small electrical charge in experimental procedures or during surgery; the result might be an action in some part of the body (if the motor cortex is involved) or (for a sensory function) a pattern of electrical discharges in other parts of the cortex. Careful exploration has established, for example, that the auditory area in the temporal lobe is made up of smaller regions, each attuned to different sound frequencies.

But for much of the cortex, no such direct functions have been found, and for a time these areas were known as "silent" cortex. It is now clear that "association" cortex is a better name for them because they fill the crucial role of making sense of received stimuli, piecing together the signals from various sensory pathways and making the synthesis available as felt experience. For instance, if there is to be not merely perception but conscious understanding of sounds, the auditory association area (just behind the auditory area proper) must be active. In the hemisphere that houses speech and other verbal abilities—the left hemisphere, for most people—the auditory association area blends into the receptive language area (which also receives signals from the visual association area, thereby providing a neural basis for reading as well as for the comprehension of speech in most languages).

A large portion of the association cortex is found in the frontal lobes, which have expanded most rapidly over the past 20,000 or so generations (about 500,000 years) of human evolution. Medical imaging shows increased activity in the association cortex after other areas of the brain have received electrical stimulation and also before the initiation of movement. On present evidence, it is in the association cortex that we locate long-term planning, interpretation, and the organization of ideas—perhaps the most recently developed elements of the modern human brain.

Visual functions occupy the occipital lobe, the bulge at the back end of the brain. The primary area for visual perception is almost surrounded by the much larger visual association area. Nearby, extending into the lower part of the temporal lobe, is the association area for visual memory —a specialized area in the cortex. Clearly, this function has been important for an omnivorous foraging primate that probably spent a long evolutionary period ranging among scattered food sources. (For an account of the intricate mechanisms that underlie depth perception and color vision, see Chapter 7 .)

A less specific kind of function has been attributed to the prefrontal cortex, located on the forward-facing part of the frontal lobes. This area is connected by association fibers with all other regions of the cortex and also with the amygdala and the thalamus, which means that it, too, makes up part of the "emotional brain," the limbic system. Injury to the prefrontal cortex or its underlying white matter results in a curious disability: the patient suffers from a reduced intensity of emotion and can no longer foretell the consequences of things that are said or done. (The injury must be bilateral to produce such an effect; if only one hemisphere is injured, the other can compensate and avert this strange, potentially crippling social deficit.) Among its other functions, the prefrontal cortex is responsible for inhibiting inappropriate behavior, for keeping the mind focused on goals, and for providing continuity in the thought process.

Long-term memory has not yet been found to reside in any exclusive part of the brain, but experimental findings indicate that the temporal lobes contribute to this function. Electrical stimulation of the cerebral cortex in this area gives rise to sensations of déjà vu ("already seen") and its opposite, jamais vu ("never seen"); it also conjures up images of scenes witnessed or speech heard in the past. That the association areas for vision and hearing and the language areas are all nearby may suggest pathways for the storage and retrieval of memories that include several types of stimuli.

The function of language itself is housed in the left hemisphere (in most cases), in several discrete sites on the cortex.

The expressive language area, responsible for the production of speech, is found toward the center of the frontal lobe; this is also called Broca's area, after the French anatomist and anthropologist of the mid-1800s who was among the first to observe differences in function between the left and right hemispheres. The receptive language area, which is located near the junction of the parietal and temporal lobes, allows us to comprehend both spoken and written language, as described above. This is often called Wernicke's area, after the German neurologist Karl Wernicke, who in the late 1800s laid the basis for much of our current understanding of how the brain encodes and decodes language. A bundle of nerve fibers connects Wernicke's area directly to Broca's area. This tight linkage is important, since before any speech at all can be uttered, its form and appropriate words must first be assembled in Wernicke's area and then relayed to Broca's area to be mentally translated into the requisite sounds; only then can it pass to the supplementary motor cortex for vocal production.

For nine of ten right-handed people and almost two-thirds of all left-handers, language abilities are sited in the left hemisphere. No one knows why there should be this asymmetrical distribution rather than an even balance or, for that matter, a consistent location of language in the left brain. What is clear is that in all cases, the hemisphere that does not contain language abilities holds the key to other functions of a less distinct, more holistic nature. The appreciation of forms and textures, the recognition of the timbre of a voice, and the ability to orient oneself in space all appear to lodge here, as do musical talent and appreciation—a host of perceptions that do not lend themselves well to analysis in words.

The limited specialization of the two hemispheres is efficient in terms of the use of space: it increases the functional abilities of the brain without adding to its volume. (The skull of the human infant, it is calculated, is already as large as can be accommodated through the birth canal, which in turn is constrained by the skeletal requirements for upright walking.) Moreover, the bilateral arrangement allows for some flexibility if one hemisphere is injured; often the other hemisphere can compensate to some degree, depending on the age at which injury occurs (a young, still-developing brain readjusts more readily).

The two hemispheres are connected mainly by a thick bundle of nerve fibers called the corpus callosum, or "hard body," because of its tough consistency. A smaller bundle, the anterior commissure, connects just the two temporal lobes. Although the corpus callosum is a good landmark for students of brain anatomy, its contribution to behavior has been difficult to pin down. Patients in whom the corpus callosum has been severed (a way of ameliorating epilepsy by restricting seizures to one side of the brain) go about their everyday business without impairment. Careful testing does turn up a gap between sensations processed by the right brain and the language centers of the left brain—for instance, a person with a severed corpus callosum is unable to name an object placed unseen in the left hand (because stimuli perceived by the left half of the body are processed in the right hemisphere). On the whole, though, it appears that the massive crossing-over of nerve fibers that takes place in the brainstem is quite adequate for most purposes, at least those related to survival.

Although the cerebral cortex is quite thin, ranging from 1.5 to 4 millimeters deep (less than 3/8 inch), it contains no fewer than six layers. From the outer surface inward, these are the molecular layer, made up for the most part of junctures between neurons for the exchange of signals; the external granular layer, mainly interneurons, which serve as communicating nerve bodies within a region; an external pyramidal layer, with large-bodied "principal" cells whose axons extend into other regions; an internal granular layer, the main termination point for fibers from the thalamus; a second, internal pyramidal layer, whose cells project their axons mostly to structures below the cortex; and a multiform layer, again containing principal cells, which in this case project to the thalamus. The layers vary in thickness at different sites on the cortex; for example, the granular layers (layers 2 and 4) are more prominent in the primary sensory area and less so in the primary motor area.

• Building Blocks Of The Brain

Extensive and intricate as the human brain is, and with the almost limitless variation of which it is capable, it is built from relatively few basic units. The fundamental building block of the human brain, like that of nervous systems throughout the animal kingdom, is the neuron, or nerve cell. The neuron conducts signals by means of an axon, which extends outward from the soma, or body of the cell, like a single long arm. Numerous shorter arms, the dendrites ("little branches"), conduct signals back to the soma.

The ability of the axon to conduct nerve impulses is greatly enhanced by the myelin sheath that surrounds it, interrupted at intervals by nodes. Myelin is a fatty substance, a natural electrical insulator, that protects the axon from interference by other nearby nerve impulses. The arrangement of nodes increases the speed of conductivity, so that an electrical impulse sent along the axon can literally jump from node to node, reaching velocities as high as 120 meters per second.

The site of communication between any two neurons—actually not a physical contact but an infinitesimal cleft across which signals are transmitted—is called a synapse, from the Greek word for "conjunction." An axon may extend over a variable distance to make contact with other neurons at a synapse. The end of an axon near a synapse widens out into a bouton, or button; the bouton contains mitochondria, which supply energy, and a number of synaptic vesicles. It is these vesicles, each less than 200 billionths of a meter in diameter, that contain the chemical neurotransmitters to be released into the synaptic cleft. On the other side of the synapse is usually a dendrite, sometimes with a dendritic spine—a small protuberance that expands the surface area of the dendrite and provides a receptive site for incoming signals.

A completely different arrangement for transmitting signals is the electrical synapse, at which the cell membranes of two neurons are extremely close together and are linked by a bridge of tubular protein molecules. This bridge allows passage of water and electrically charged small molecules; any change in electrical charge in one neuron is instantaneously transmitted to the other. Hence this mechanism for relaying signals relies entirely on direct electrical coupling; an electrical synapse is about 3 nanometers (nm), or billionths of a meter, wide, as compared with the 25-nm gap of a chemical synapse. Outside of nervous tissue, electrical synapses (and other, similar gap junctions) are the messengers of choice.

The brain is sometimes said to be full of "gray matter," which is supposed to be the stuff of intelligence. The material referred to is actually grayish pink in living brain, and only gray in specimens that have been chemically preserved; it consists of nerve cell bodies and dendrites and the origins and boutons of axons. It is gray matter that forms sheets of cortex on the surface of the cerebral hemispheres. White matter receives its name from the appearance of the myelin enclosing the elongated region of axons. The third main form of matter in the brain is the neuroglia, or "glue" cells. These cells do not connect the neurons, as their name implies; connections are already far from scarce, with the vast system of neural soma, axons, and dendrites packed so densely into the brain. Rather, the neuroglia provide structural support and a source of metabolic energy for the roughly 100 billion nerve cells of the human brain.

• Chemical And Electrical Signals

The actual signals transmitted throughout the brain come in two forms, electrical and chemical. The two forms are interdependent and meet at the synapse, where chemical substances can alter the electrical conditions within and outside the cell membrane.

A nerve cell at rest holds a slight negative charge (about -70 millivolts, or thousandths of a volt, mV) with respect to the exterior; the cell membrane is said to be polarized. The negative charge, the resting potential of the membrane, arises from a very slight excess of negatively charged molecules inside the cell.

A membrane at rest is more or less impermeable to positively charged sodium ions (Na + ), but when stimulated it is transiently open to their passage. The Na + ions thus flow in, attracted by the negative charge inside, and the membrane temporarily reverses its polarity, with a higher positive charge inside than out. This stage lasts less than a millisecond, and then the sodium channels close again. Potassium channels (K + ) open, and K + ions move out through the membrane, reversing the flow of positively charged ions. (Both these channels are known as voltage-gated, meaning that they open or close in response to changes in electrical charge occurring across the membrane.) Over the next 3 milliseconds, the membrane becomes slightly hyperpolarized, with a charge of about -80 mV, and then returns to its resting potential. During this time the sodium channels remain closed; the membrane is in a refractory phase.

An action potential—the very brief pulse of positive membrane voltage—is transmitted forward along the axon; it is prevented from propagating backward as long as the sodium channels remain closed. After the membrane has returned to its resting potential, however, a new impulse may arrive to evoke an action potential, and the cycle can begin again.

Gated channels, and the concomitant movement of ions in and out of the cell membrane, are widespread throughout the nervous system, with sodium, potassium, and chlorine being the most common ions involved. Calcium channels are also important, particularly at the presynaptic boutons of axons. When the membrane is at its resting potential, positively charged calcium ions (Ca 2+ ) outside the cell far outnumber those inside. With the advent of an action potential, however, calcium ions rush into the cell. The influx of calcium ions leads to the release of neurotransmitter into the synaptic cleft; this passes the signal to a neighboring nerve cell.

Having taken a close look at the electrical side of the picture, we are in a better position to see where the chemistry comes in. Molecules of neurotransmitter are released into a synaptic cleft and bind to specific receptor sites on the postsynaptic side (the dendrite or dendrite spine), thereby altering the ion channels in the postsynaptic membrane. Some neurotransmitters cause sodium channels to open, allowing the influx of Na + ions and thus a lessening of negative charge inside the cell membrane. If a considerable number of these potentials are received within a short interval, they can depolarize the membrane enough to trigger an action potential; the result is the transmission of a nerve impulse. The substances that can cause this to occur are the excitatory neurotransmitters. By contrast, other chemical compounds cause potassium channels to open, increasing the outflow of K + ions from the cell and making excitation less likely; the neurotransmitters that bring about this state are considered inhibitory.

A given neuron has a great quantity of sites available on its dendrites and cell body and receives signals from many synapses simultaneously, both excitatory and inhibitory. These signals often amount to a rough balance; it is only when the net potential of the membrane in one region shifts significantly up or down from the resting level that a particular neurotransmitter can be said to be exerting an effect. Interestingly, in the membrane's overall balance sheet, the importance of a particular synapse varies with its proximity to where the axon leaves the nerve cell body, so that numerous excitatory potentials out at the ends of the dendrites may be overruled by several inhibitory potentials closer to the soma. Other kinds of synapse regulate the release of neurotransmitters into the synaptic cleft, where they go on to affect the postsynaptic channels as described above.

The list of known neurotransmitters, once thought to be quite short, continues to grow as more substances are found to be synthesized by neurons, contained in presynaptic boutons, and bound on the postsynaptic membrane by specific receptors. Despite stringent requirements for identifying a substance as a neurotransmitter (see Chapter 5 ), well over two dozen have been so named, and another several dozen strong candidates are under review.

The most cursory look at the human brain can excite awe at its complex functions, the intricacy of its structure, and the innumerable connections all maintained on microscopic fibers a few millionths of a meter in diameter. But a slightly more intimate acquaintance with this 3-pound organ inside our heads, an acquaintance that builds on observation of the brain in action and discovery of the principles by which it works, can yield something more satisfying than awe: the sense of mastery and of rewarded curiosity that comes with understanding. With the rewarding of curiosity as our goal, let us take a closer look at a few aspects of the functioning brain.

• Cite this Page Ackerman S. Discovering the Brain. Washington (DC): National Academies Press (US); 1992. 2, Major Structures and Functions of the Brain.
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Psychology. Brain Structure and Behavior Essay

Introduction.

The brain stem is one of the most important parts of the brain that plays vital roles in basic attention, arousal and consciousness. “It forms the path through which all information from all parts of the body on the way to and from the brain and is located within the bony protrusions that make it vulnerable to damage during trauma” (Fiona & Spar, 2006). According to Fiona & Spar (2006), it consists of “medulla- which contains various nuclei (most of which are vital for life, including those controlling cardiac function and respiration) as well as a profusion of ascending and descending nerve pathways.”

From the discussion on the vital roles played by brain stem, it can be discerned that a small damage on some parts may have fatal consequences. If the brain is cut above the position of the medulla, normal body functioning such as heart beat and normal breathing may be maintained. Damage that directly affects the medulla is inevitably fatal because of the critical roles it plays in balancing osmotic body functions.

A mild brain stem injury, commonly refereed to as a “concussion may lead to temporary malfunctioning of the brain system such as brief loss of consciousness, sudden memory loss, and focal neurological deficits” (Fiona & Spar, 2006). In cases where the symptoms of brain stem damage are not immediately recognized, the victim may be look normal on the surface but may develop serious complications later. Moderate brain damage is characterized by “confusion that persists for weeks, state of consciousness that may run for hours and cognitive and behavioral impairments that may run for months” (Fiona & Spar, 2006).The most devastating form of brain stem damage is the severe brain stem injury that may lead to fatal consequences. According to Fiona & Spar (2006), “s evere brain stem injuries are classified by a major limitation in mental functioning, prolonged state of unconsciousness or coma, persistent vegetative state, or minimally responsive state.”

In addition to the above, brain stem damage may lead to a number of devastating signs and symptoms that may have long time consequences on the entire life time of the victim. These are demonstrated by Strauer, Schannwell and Brehmm (2009) to include “abnormalities in the function of cranial nerves which may lead to visual disturbances, pupil abnormalities, changes in sensation, muscle weakness, hearing problems, vertigo, swallowing and speech difficulty, voice change, and co-ordination problems.”

The receipt of timely treatment for brain stem damage is vital to a patient’s recovery. Approaches that have been used within the medical circles to rectify brain stem damage depend primarily on the extent of the damage. “The necessary treatment for a brain stem injury may include medication, surgery, diagnostic testing and rehabilitation while in more serious cases; a comprehensive team of care providers is usually required” (Fiona & Spar, 2006). This calls for an immediate response to provide treatment for a victim because there is a possibility that victims may fully recover and resume normal lives. However, because of the fact that brain stem injuries may be as result of intentional and unintentional human actions, it is best to seek a brain stem injury attorney for further instructions.

The ability to arrive at a perfect solution to most medical problems has been a challenge in medical fields and brain stem injury total treatment has not been fully achieved. This has attracted concerted research efforts on both stem cell behavior and transplantation combined with a deeper understanding of the complications may open up the doors to alleviating the suffering on the victims.

Fiona, M. and Spar, D. (2006). Bit Player or Powerhouse? China and Stem-Cell Research, New England Journal of Medicine . Vol.13. No. 4. 145-69.

Strauer, B. E. Schannwell, C. M. and Brehm, M. (2009). Therapeutic potentials of stem cells in cardiac diseases. Journal of Minerva Cardioangio. Vol. 57. No. 2: 249–67.

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How does the brain think?

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

How does the brain think? – Tom, age 16, San Diego, California

Have you ever wondered how your brain creates thoughts or why something randomly popped into your head? It may seem like magic – but actually the brain is like a supercomputer inside your head that helps you think, learn and make decisions .

Imagine your brain as a busy city with lots of streets and buildings. Each part of the brain has a specific job to do, just like certain areas of a city or certain buildings serve different purposes. When you have a thought, it’s like a message traveling through the city, passing from one area to another.

As a professor of psychology and neuroscience , I have studied the brain for almost 20 years. Neurologists , neuroscientists and neurosurgeons work every day to understand the brain better. And there’s still a lot to learn.

Practice and repetition create skills

The neuron is a key player in the brain – these are tiny cells that send and receive signals and messages so they can communicate with each other.

Your brain has somewhere between 80 billion and 100 billion neurons . Neurons tend to group together to form neural tracts , which would be like the streets and highways in the city analogy. When you have a thought, neurons in your brain fire up and create electrical impulses. These impulses tend to travel along similar pathways and release tiny chemicals called neurotransmitters along the way.

These neurotransmitters are like the construction crew that builds the roads, making it easier for the messages to be delivered. You can imagine it as a dirt road, but as more traffic – that is, neuron signals – travel the dirt road, the road gets upgraded to a paved street. If the traffic continues, it gets upgraded to a highway.

As you learn new things and experience the world around you, these connections grow stronger . For example, when you are learning to ride a bike, you may be unsteady and find it hard to coordinate all of the different muscles along with your ability to balance. But the more you practice, the more the neurons controlling your muscles and your ability to balance fire together, which makes it much easier as you practice. Neurons are wiring together and forming neural networks.

That’s why practice and repetition are important for improving your skills, whether playing the piano or learning a language. Neural networks are created and then strengthened the more times they communicate together. Scientists have a saying in this field: “Neurons that fire together wire together.” Certain thinking or behavior patterns can be chalked up to this kind of repeated synchronized activity.

Developing creativity

You are conscious of only a very small portion of the information your brain takes in. It is constantly receiving input from your senses – sights, sounds, tastes, smells and touch. When you see a cute puppy or hear your favorite song, your senses send signals to the brain, triggering a chain reaction of thoughts and emotions.

The brain also stores memories , which are like files in a computer that you can access whenever you need them. Memories help shape your thoughts and influence how you see the world.

If you remember a fun day at the beach, it might make you feel happy and relaxed. If you smell an apple pie, it may remind you of your grandma’s baking. These thoughts are triggered because these pleasant associations have been formed in your brain, and through repetition, strengthened over time.

Creativity is another superpower of the brain. When you let your imagination run wild, your brain can come up with new ideas, stories and inventions . Artists, writers and scientists all use their creative brains to explore new possibilities and solve problems.

Have you ever experienced a “eureka” moment when a brilliant idea pops into your head out of nowhere? That’s your brain’s way of connecting the dots and coming up with a solution.

Keeping your brain healthy

Most scientists agree that sleep is really important for your brain to process information from the day and to allow it to rest and form new connections. A lot of people find that they have new ideas or thoughts after a good night’s sleep. The opposite is true, too – without enough sleep, you may feel like you can’t think straight.

Along with enough sleep, eat healthy foods and exercise . Just like a car needs fuel to run smoothly, your brain needs nutrients and oxygen to function at its best and to boost your thinking power.

Activities that challenge you are also great: reading , doing puzzles, playing music, making art, doing math , writing essays and book reports and journaling . Positive thinking also helps. Keep in mind that whatever you are consuming – what you’re eating or what you’re watching, listening to or reading – has the power to influence your brain.

Conversely, smoking cigarettes , vaping , drinking alcohol and using drugs kills brain cells. So might head injuries that can occur when playing sports such as football, soccer and bicycling – but wearing a helmet can make a big difference .

The brain is a fascinating organ that works tirelessly to create thoughts, memories and ideas. As technology continues to improve, scientists will learn more and more about how biological processes give rise to our conscious experiences. The challenges of learning about the brain are like a neuroscientific moonshot – we have a long way to go before we completely understand how it works.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

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