Anatomy of the Brain
Anatomy of the Brain
The brain serves many important functions. It gives meaning to
things that happen in the world surrounding us. Through the five senses
of sight, smell, hearing, touch and taste, the brain receives messages,
often many at the same time.
The brain controls thoughts, memory and speech, arm and leg
movements, and the function of many organs within the body. It also
determines how people respond to stressful situations (i.e. writing of
an exam, loss of a job, birth of a child, illness, etc.) by regulating
heart and breathing rates. The brain is an organized structure, divided
into many components that serve specific and important functions.
The weight of the brain changes from birth through adulthood. At
birth, the average brain weighs about one pound, and grows to about two
pounds during childhood. The average weight of an adult female brain is
about 2.7 pounds, while the brain of an adult male weighs about three
The Nervous System
The nervous system is commonly divided into the central nervous
system and the peripheral nervous system. The central nervous system is
made up of the brain, its cranial nerves and the spinal cord. The
peripheral nervous system is composed of the spinal nerves that branch
from the spinal cord and the autonomous nervous system (divided into the
sympathetic and parasympathetic nervous system).
The Cell Structure of the Brain
The brain is made up of two types of cells: neurons and glial
cells, also known as neuroglia or glia. The neuron is responsible for
sending and receiving nerve impulses or signals. Glial cells are
non-neuronal cells that provide support and nutrition, maintain
homeostasis, form myelin, and facilitate signal transmission in the
nervous system. In the human brain, glial cells outnumber neurons by
about 50 to one. Glial cells are the most common cells found in primary
When a person is diagnosed with a brain tumor, a biopsy may be
done, in which tissue is removed from the tumor for identification
purposes by a pathologist. Pathologists identify the type of cells that
are present in this brain tissue, and brain tumors are named based on
this association. The type of brain tumor and cells involved impact
patient prognosis and treatment.
The brain is housed inside the bony covering called the cranium.
The cranium protects the brain from injury. Together, the cranium and
bones that protect the face are called the skull. Between the skull and
brain is the meninges, which consist of three layers of tissue that
cover and protect the brain and spinal cord. From the outermost layer
inward they are: the dura mater, arachnoid and pia mater.
In the brain, the dura mater is made up of two layers of whitish,
nonelastic film or membrane. The outer layer is called the periosteum.
An inner layer, the dura, lines the inside of the entire skull and
creates little folds or compartments in which parts of the brain are
protected and secured. The two special folds of the dura in the brain
are called the falx and the tentorium. The falx separates the right and
left half of the brain and the tentorium separates the upper and lower
parts of the brain.
The second layer of the meninges is the arachnoid. This membrane
is thin and delicate and covers the entire brain. There is a space
between the dura and the arachnoid membranes that is called the subdural
space. The arachnoid is made up of delicate, elastic tissue and blood
vessels of varying sizes.
The layer of meninges closest to the surface of the brain is
called the pia mater. The pia mater has many blood vessels that reach
deep into the surface of the brain. The pia, which covers the entire
surface of the brain, follows the folds of the brain. The major arteries
supplying the brain provide the pia with its blood vessels. The space
that separates the arachnoid and the pia is called the subarachnoid
space. It is within this area that cerebrospinal fluid flows.
Cerebrospinal fluid (CSF) is found within the brain and surrounds
the brain and the spinal cord. It is a clear, watery substance that
helps to cushion the brain and spinal cord from injury. This fluid
circulates through channels around the spinal cord and brain, constantly
being absorbed and replenished. It is within hollow channels in the
brain, called ventricles, that the fluid is produced. A specialized
structure within each ventricle, called the choroid plexus, is
responsible for the majority of CSF production. The brain normally
maintains a balance between the amount of CSF that is absorbed and the
amount that is produced. However, disruptions in this system may occur.
The Ventricular System
The ventricular system is divided into four cavities called
ventricles, which are connected by a series of holes called foramen, and
Two ventricles enclosed in the cerebral hemispheres are called
the lateral ventricles (first and second). They each communicate with
the third ventricle through a separate opening called the Foramen of
Munro. The third ventricle is in the center of the brain, and its walls
are made up of the thalamus and hypothalamus.
The third ventricle connects with the fourth ventricle through a
long tube called the Aqueduct of Sylvius.
CSF flowing through the fourth ventricle flows around the brain
and spinal cord by passing through another series of openings.
Brain Components and Functions
Brainstem – The brainstem is the lower extension of the
brain, located in front of the cerebellum and connected to the spinal
cord. It consists of three structures: the midbrain, pons and medulla
oblongata. It serves as a relay station, passing messages back and forth
between various parts of the body and the cerebral cortex. Many simple
or primitive functions that are essential for survival are located here.
The midbrain is an important center for ocular motion while the
pons is involved with coordinating eye and facial movements, facial
sensation, hearing and balance.
The medulla oblongata controls breathing, blood pressure, heart
rhythms and swallowing. Messages from the cortex to the spinal cord and
nerves that branch from the spinal cord are sent through the pons and
the brainstem. Destruction of these regions of the brain will cause
"brain death." Without these key functions, humans cannot survive.
The reticular activating system is found in the midbrain, pons,
medulla and part of the thalamus. It controls levels of wakefulness,
enables people to pay attention to their environments, and is involved
in sleep patterns.
Originating in the brainstem are 10 of the 12 cranial nerves that
control hearing, eye movement, facial sensations, taste, swallowing and
movements of the face, neck, shoulder and tongue muscles. The cranial
nerves for smell and vision originate in the cerebrum. Four pairs of
cranial nerves originate from the pons: nerves 5 through 8.
Cerebellum – The cerebellum is located at the back of the
brain beneath the occipital lobes. It is separated from the cerebrum by
the tentorium (fold of dura). The cerebellum fine tunes motor activity
or movement, e.g. the fine movements of fingers as they perform surgery
or paint a picture. It helps one maintain posture, sense of balance or
equilibrium, by controlling the tone of muscles and the position of
limbs. The cerebellum is important in one's ability to perform rapid and
repetitive actions such as playing a video game. In the cerebellum,
right-sided abnormalities produce symptoms on the same side of the body.
Cerebrum – The cerebrum, which forms the major portion of
the brain, is divided into two major parts: the right and left cerebral
hemispheres. The cerebrum is a term often used to describe the entire
brain. A fissure or groove that separates the two hemispheres is called
the great longitudinal fissure. The two sides of the brain are joined at
the bottom by the corpus callosum. The corpus callosum connects the two
halves of the brain and delivers messages from one half of the brain to
the other. The surface of the cerebrum contains billions of neurons and
glia that together form the cerebral cortex.
The cerebral cortex appears grayish brown in color and is called
the "gray matter." The surface of the brain appears wrinkled. The
cerebral cortex has sulci (small grooves), fissures (larger grooves) and
bulges between the grooves called gyri. Scientists have specific names
for the bulges and grooves on the surface of the brain. Decades of
scientific research have revealed the specific functions of the various
regions of the brain. Beneath the cerebral cortex or surface of the
brain, connecting fibers between neurons form a white-colored area
called the "white matter."
The cerebral hemispheres have several distinct fissures. By
locating these landmarks on the surface of the brain, it can effectively
be divided into pairs of "lobes." Lobes are simply broad regions of the
brain. The cerebrum or brain can be divided into pairs of frontal,
temporal, parietal and occipital lobes. Each hemisphere has a frontal,
temporal, parietal and occipital lobe. Each lobe may be divided, once
again, into areas that serve very specific functions. The lobes of the
brain do not function alone – they function through very complex
relationships with one another.
Messages within the brain are delivered in many ways. The signals
are transported along routes called pathways. Any destruction of brain
tissue by a tumor can disrupt the communication between different parts
of the brain. The result will be a loss of function such as speech, the
ability to read, or the ability to follow simple spoken commands.
Messages can travel from one bulge on the brain to another (gyri to
gyri), from one lobe to another, from one side of the brain to the
other, from one lobe of the brain to structures that are found deep in
the brain, e.g. thalamus, or from the deep structures of the brain to
another region in the central nervous system.
Research has determined that touching one side of the brain sends
electrical signals to the other side of the body. Touching the motor
region on the right side of the brain, would cause the opposite side or
the left side of the body to move. Stimulating the left primary motor
cortex would cause the right side of the body to move. The messages for
movement and sensation cross to the other side of the brain and cause
the opposite limb to move or feel a sensation. The right side of the
brain controls the left side of the body and vice versa. So if a brain
tumor occurs on the right side of the brain that controls the movement
of the arm, the left arm may be weak or paralyzed.
Cranial Nerves – There are 12 pairs of nerves that originate from
the brain itself. These nerves are responsible for very specific
activities and are named and numbered as follows:
- Olfactory: Smell
- Optic: Visual fields and ability to see
- Oculomotor: Eye movements; eyelid opening
- Trochlear: Eye movements
- Trigeminal: Facial sensation
- Abducens: Eye movements
- Facial: Eyelid closing; facial expression; taste sensation
- Auditory/vestibular: Hearing; sense of balance
- Glossopharyngeal: Taste sensation; swallowing
- Vagus: Swallowing; taste sensation
- Accessory: Control of neck and shoulder muscles
- Hypoglossal: Tongue movement
Hypothalamus – The hypothalamus is a small structure that
contains nerve connections that send messages to the pituitary gland.
The hypothalamus handles information that comes from the autonomic
nervous system. It plays a role in controlling functions such as eating,
sexual behavior and sleeping; and regulates body temperature, emotions,
secretion of hormones and movement. The pituitary gland develops from
an extension of the hypothalamus downwards and from a second component
extending upward from the roof of the mouth.
Frontal Lobes – The frontal lobes are the largest of the
four lobes responsible for many different functions. These include motor
skills such as voluntary movement, speech, intellectual and behavioral
functions. The areas that produce movement in parts of the body are
found in the primary motor cortex or precentral gyrus. The prefrontal
cortex plays an important part in memory, intelligence, concentration,
temper and personality.
The premotor cortex is a region found beside the primary motor
cortex. It guides eye and head movements and a person’s sense of
orientation. Broca's area, important in language production, is found in
the frontal lobe, usually on the left side.
Occipital Lobes – These lobes are located at the back of
the brain and enable humans to receive and process visual information.
They influence how humans process colors and shapes. The occipital lobe
on the right interprets visual signals from the left visual space, while
the left occipital lobe performs the same function for the right visual
Parietal Lobes – These lobes interpret simultaneously,
signals received from other areas of the brain such as vision, hearing,
motor, sensory and memory. A person’s memory and the new sensory
information received, give meaning to objects.
Temporal Lobes – These lobes are located on each side of
the brain at about ear level, and can be divided into two parts. One
part is on the bottom (ventral) of each hemisphere, and the other part
is on the side (lateral) of each hemisphere. An area on the right side
is involved in visual memory and helps humans recognize objects and
peoples' faces. An area on the left side is involved in verbal memory
and helps humans remember and understand language. The rear of the
temporal lobe enables humans to interpret other people’s emotions and
Limbic System – This system is involved in emotions.
Included in this system are the hypothalamus, part of the thalamus,
amygdala (active in producing aggressive behavior) and hippocampus
(plays a role in the ability to remember new information).
Pineal Gland – This gland is an outgrowth from the
posterior or back portion of the third ventricle. In some mammals, it
controls the response to darkness and light. In humans, it has some role
in sexual maturation, although the exact function of the pineal gland
in humans is unclear.
Pituitary Gland – The pituitary is a small gland attached
to the base of the brain (behind the nose) in an area called the
pituitary fossa or sella turcica. The pituitary is often called the
"master gland" because it controls the secretion of hormones. The
pituitary is responsible for controlling and coordinating the following:
- Growth and development
- The function of various body organs (i.e. kidneys, breasts and
- The function of other glands (i.e. thyroid, gonads, and
Posterior Fossa – This is a cavity in the back part of the
skull which contains the cerebellum, brainstem, and cranial nerves 5-12.
Thalamus – The thalamus serves as a relay station for
almost all information that comes and goes to the cortex. It plays a
role in pain sensation, attention and alertness. It consists of four
parts: the hypothalamus, the epythalamus, the ventral thalamus, and the
dorsal thalamus. The basal ganglia are clusters of nerve cells
surrounding the thalamus.
Language and Speech Functions
In general, the left hemisphere or side of the brain is
responsible for language and speech. Because of this, it has been called
the "dominant" hemisphere. The right hemisphere plays a large part in
interpreting visual information and spatial processing. In about one
third of individuals who are left-handed, speech function may be located
on the right side of the brain. Left-handed individuals may need
specialized testing to determine if their speech center is on the left
or right side prior to any surgery in that area.
Many neuroscientists believe that the left hemisphere and perhaps
other portions of the brain are important in language. Aphasia is
simply a disturbance of language. Certain parts of the brain are
responsible for specific functions in language production. There are
many types of aphasias, each depending upon the brain area that is
affected, and the role that area plays in language production.
There is an area in the frontal lobe of the left hemisphere
called Broca’s area. It is next to the region that controls the movement
of facial muscles, tongue, jaw and throat. If this area is destroyed, a
person will have difficulty producing the sounds of speech, because of
the inability to move the tongue or facial muscles to form words. A
person with Broca's aphasia can still read and understand spoken
language, but has difficulty speaking and writing.
There is a region in the left temporal lobe called Wernicke's
area. Damage to this area causes Wernicke's aphasia. An individual can
make speech sounds, but they are meaningless (receptive aphasia) because
they do not make any sense.