You're in the middle of a meeting at work, but your mind
keeps drifting to the parent-teacher conference you have tonight
... and the car you have to pick up at the shop on the way home ...
and how you wish you hadn't skipped lunch because the rumbling
in your stomach is driving you nuts. Then, suddenly, you're
back in the moment, hoping nobody noticed your brief
"departure."
It may seem as if your brain is always on the go. And it is. The
brain not only controls what you think and feel, how you learn and
remember, and the way you move and talk, but also many things
you're less aware of - such as the beating of your heart, the
digestion of your food, and yes, even the amount of stress you
feel. Like you, your brain is quite the juggler.
Anatomy of the Nervous System
If you think of the brain as a central computer that controls
all bodily functions, then the nervous system is like a network
that relays messages back and forth from the brain to
different parts of the body. It does this via the spinal cord,
which runs from the brain down through the back and contains
threadlike nerves that branch out to every organ and body part.
When a message comes into the brain from anywhere in the body,
the brain tells the body how to react. For example, if you
accidentally touch a hot stove, the nerves in your skin shoot a
message of pain to your brain. The brain then sends a message back
telling the muscles in your hand to pull away. Luckily, this
neurological relay race takes a lot less time than it just took to
read about it.
Considering everything it does, the human brain is incredibly
compact, weighing just 3 pounds. Its many folds and grooves,
though, provide it with the additional surface area necessary for
storing all of the body's important information.
The spinal cord, on the other hand, is a long bundle of nerve
tissue about 18 inches long and 3/4 inch thick. It extends from the
lower part of the brain down through spine. Along the way, various
nerves branch out to the entire body. These are called the
peripheral nervous system.
Both the brain and the spinal cord are protected by bone: the
brain by the bones of the skull, and the spinal cord by a set of
ring-shaped bones called vertebrae. They're both cushioned by
layers of membranes called meninges as well as a special fluid
called cerebrospinal fluid. This fluid helps protect the nerve
tissue, keep it healthy, and remove waste products.
The brain is made up of three main sections: the forebrain, the
midbrain, and the hindbrain.
1. The Forebrain
The forebrain is the largest and most complex part of the brain.
It consists of the cerebrum - the area with all the folds and
grooves typically seen in pictures of the brain - as well as some
other structures beneath it.
The cerebrum contains the information that essentially makes us
who we are: our intelligence, memory, personality, emotion, speech,
and ability to feel and move. Specific areas of the cerebrum are in
charge of processing these different types of information. These
are called lobes, and there are four of them: the frontal,
parietal, temporal, and occipital.
The cerebrum has right and left halves, called hemispheres,
which are connected in the middle by a band of nerve fibers (the
corpus collosum) that enables the two sides to communicate. Though
these halves may look like mirror images of each other, many
scientists believe they have different functions. The left side is
considered the logical, analytical, objective side. The right side
is thought to be more intuitive, creative, and subjective. So when
you're balancing the checkbook, you're using the left side;
when you're listening to music, you're using the right
side. It's believed that some people are more
"right-brained" or "left-brained" while others
are more "whole-brained," meaning they use both halves of
their brain to the same degree.
The outer layer of the cerebrum is called the cortex (also known
as "gray matter"). Information collected by the five
senses comes into the brain from the spinal cord to the cortex.
This information is then directed to other parts of the nervous
system for further processing. For example, when you touch the hot
stove, not only does a message go out to move your hand but one
also goes to another part of the brain to help you remember not to
do that again.
In the inner part of the forebrain sits the thalamus,
hypothalamus, and pituitary gland. The thalamus carries messages
from the sensory organs like the eyes, ears, nose, and fingers to
the cortex. The hypothalamus controls the pulse, thirst, appetite,
sleep patterns, and other processes in our bodies that happen
automatically. It also controls the pituitary gland, which makes
the hormones that control our growth, metabolism, digestion, sexual
maturity, and response to stress.
2. The Midbrain
The midbrain, located underneath the middle of the forebrain,
acts as a master coordinator for all the messages going in and out
of the brain to the spinal cord.
3. The Hindbrain
The hindbrain sits underneath the back end of the cerebrum, and
it consists of the cerebellum, pons, and medulla. The cerebellum -
also called the "little brain" because it looks like a
small version of the cerebrum - is responsible for balance,
movement, and coordination.
The pons and the medulla, along with the midbrain, are often
called the brainstem. The brainstem takes in, sends out, and
coordinates all of the brain's messages. It is also controls
many of the body's automatic functions, like breathing, heart
rate, blood pressure, swallowing, digestion, and blinking.
How the Nervous System Works
The basic functioning of the nervous system depends a lot on
tiny cells called neurons. The brain has billions of them, and they
have many specialized jobs. For example, sensory neurons take
information from the eyes, ears, nose, tongue, and skin to the
brain. Motor neurons carry messages away from the brain and back to
the rest of the body.
All neurons, however, relay information to each other through a
complex electrochemical process, making connections that affect the
way we think, learn, move, and behave.
Intelligence, learning, and memory.
At birth, the nervous system contains all the neurons you will ever
have, but many of them are not connected to each other. As you grow
and learn, messages travel from one neuron to another over and
over, creating connections, or pathways, in the brain. It's why
driving seemed to take so much concentration when you first learned
but now is second nature: The pathway became established.
In young children, the brain is highly adaptable; in fact, when
one part of a young child's brain is injured, another part can
often learn to take over some of the lost function. But as we age,
the brain has to work harder to make new neural pathways, making it
more difficult to master new tasks or change established behavior
patterns. That's why many scientists believe it's important
to keep challenging your brain to learn new things and make new
connections- it helps keeps the brain active over the course of a
lifetime.
Memory is another complex function of the brain. The things
we've done, learned, and seen are first processed in the
cortex, and then, if we sense that this information is important
enough to remember permanently, it's passed inward to other
regions of the brain (such as the hippocampus and amygdala) for
long-term storage and retrieval. As these messages travel through
the brain, they too create pathways that serve as the basis of our
memory.
Movement.
Different parts of the cerebrum are responsible for moving
different body parts. The left side of the brain controls the
movements of the right side of the body, and the right side of the
brain controls the movements of the left side of the body. When you
press the accelerator with your right foot, for example, it's
the left side of your brain that sends the message allowing you to
do it.
Basic body functions.
A part of the peripheral nervous system called the autonomic
nervous system is responsible for controlling many of the body
processes we almost never need to think about, like breathing,
digestion, sweating, and shivering. The autonomic nervous system
has two parts: the sympathetic and the parasympathetic nervous
systems.
The sympathetic nervous system prepares the body for sudden
stress, like if you see a robbery taking place. When something
frightening happens, the sympathetic nervous system makes the heart
beat faster so that it sends blood more quickly to the different
body parts that might need it. It also causes the adrenal glands at
the top of the kidneys to release adrenaline, a hormone that helps
give extra power to the muscles for a quick getaway. This process
is known as the body's "fight or flight"
response.
The parasympathetic nervous system does the exact opposite: It
prepares the body for rest. It also helps the digestive tract move
along so our bodies can efficiently take in nutrients from the food
we eat.
The senses.
Your spouse may be a sight for sore eyes at the end of a long day -
but without the brain, you wouldn't even recognize him or her.
Pepperoni pizza sure is delicious - but without the brain, your
taste buds wouldn't be able to tell if you were eating pizza or
the box it came in. None of your senses would be useful without the
processing that occurs in the brain.
- Sight.
Sight probably tells us more about the world than any other
sense. Light entering the eye forms an upside-down image on the
retina. The retina transforms the light into nerve signals for
the brain. The brain then turns the image right-side up and tells
us what we are seeing.
- Hearing.
Every sound we hear is the result of sound waves entering our
ears and causing our eardrums to vibrate. These vibrations are
then transferred along the tiny bones of the middle ear and
converted into nerve signals. The cortex then processes these
signals, telling us what we are hearing.
- Taste.
The tongue contains small groups of sensory cells called taste
buds that react to chemicals in foods. Taste buds react to sweet,
sour, salty, and bitter. Messages are sent from the taste buds to
the areas in the cortex responsible for processing taste.
- Smell.
Olfactory cells in the mucous membranes lining each nostril react
to chemicals we breathe in and send messages along specific
nerves to the brain- which, according to experts, can distinguish
between more than 10,000 different smells. With that kind of
sensitivity, it's no wonder research suggests that smells are
very closely linked to our memories.
- Touch.
The skin contains more than 4 million sensory receptors - mostly
concentrated in the fingers, tongue, and lips - that gather
information related to touch, pressure, temperature, and pain and
send it to the brain for processing and reaction.
Things That Can Go Wrong With the Brain
Because the brain controls just about everything, when something
goes wrong with it, it's often serious and can affect many
different parts of the body. Inherited diseases, brain disorders
associated with mental illness, and head injuries can all affect
the way the brain works and upset the daily activities of the rest
of the body.
Problems that can affect the brain include:
Brain tumors.
A tumor is a swelling caused by overgrown tissue. A tumor in the
brain may grow slowly and produce few symptoms until it becomes
large, or it can grow and spread rapidly, causing severe and
quickly worsening symptoms. Brain tumors in children can be benign
or malignant. Benign tumors usually grow in one place and may be
curable through surgery if they're located in a place where
they can be removed without damaging the normal tissue near the
tumor. A malignant tumor is cancerous and more likely to grow
rapidly and spread.
Cerebral palsy.
Cerebral palsy is the result of a developmental defect or damage to
the brain before or during birth. It affects the motor areas of the
brain. A person with cerebral palsy may have average intelligence
or can have severe developmental delays or mental retardation.
Cerebral palsy can affect body movement in many different ways. In
mild cases of cerebral palsy, there may be minor muscle weakness of
the arms and legs. In other cases, there may be more severe motor
impairment - a child may have trouble talking and performing basic
movements like walking.
Epilepsy.
This condition is made up of a wide variety of seizure disorders.
Partial seizures involve specific areas of the brain, and symptoms
vary depending on the location of the seizure activity. Other
seizures, called generalized seizures, involve a larger portion of
the brain and usually cause uncontrolled movements of the entire
body and loss of consciousness when they occur. Although the
specific cause is unknown in many cases, epilepsy can be related to
brain injury, tumors, or infections. The tendency to develop
epilepsy may be inherited in families.
Headaches.
Of the many different types of headaches, the most frequently
occurring include tension headache (the most common type), caused
by muscle tension in the head, neck, and shoulders; migraine, an
intense, recurring headache with an unclear cause; and cluster
headache, considered by some to be a form of migraine. Migraines
occur with or without warning and may last for several hours or
days. There seems to be an inherited predisposition to migraines as
well as certain triggers that can lead to them. People with
migraines may experience dizziness, numbness, sensitivity to light,
and nausea, and may see flashing zigzag lines before their
eyes.
Meningitis and encephalitis.
These are infections of the brain and spinal cord that are usually
caused by bacteria or viruses. Meningitis is an inflammation of the
coverings of the brain and spinal cord, and encephalitis is an
inflammation of the brain tissue. Both conditions may result in
permanent injury to the brain.
Mental illness.
Mental illnesses are psychological and behavioral in nature and
involve a wide range of problems in thought and function. Certain
mental illnesses are now known to be linked to structural
abnormalities or chemical dysfunction of the brain. Some mental
illnesses are inherited, but often the cause is unknown. Injuries
to the brain and chronic drug or alcohol abuse also can trigger
some mental illnesses. Signs of chronic mental illnesses such as
bipolar disorder or schizophrenia may first show up in childhood.
Mental illnesses that can be seen in younger people include
depression, eating disorders such as bulimia or anorexia nervosa,
obsessive-compulsive disorder (OCD), and phobias.
Head injuries.
Head injuries fit into two categories: external (usually scalp)
injuries and internal head injuries. Internal injuries may involve
the skull, the blood vessels within the skull, or the brain.
Fortunately, most childhood falls or blows to the head result in
injury to the scalp only, which is usually more frightening than
threatening. An internal head injury could have more serious
implications because the skull serves as the protective helmet for
the delicate brain.
Concussions are also a type of internal head injury. A
concussion is the temporary loss of normal brain function as a
result of an injury. Repeated concussions can result in permanent
injury to the brain. One of the most common reasons kids get
concussions is through sports, so it's important to make sure
they wear appropriate protective gear and don't continue to
play if they've had a head injury.
Reviewed by:
Steven Dowshen, MD
Date reviewed: April 2007