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 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
Motor and
sensory regions of the brain
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.
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.