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.

Cerebrum
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.

Brainstem
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.

Cerebellum
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.

Amygdala
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.

• 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 vestibulocochlearnerve 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.