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Central Nervous System
The central nervous system works closely with the peripheral nervous
system. It receives information from the sensory neurons and
responds by sending messages through motor neurons to various parts
of the body.
Mission Control
The brain is the largest organ in the nervous system. It has hundreds of
different jobs. Many of the processes that the brain controls happen
automatically and are referred to as involuntary. For example, you
couldn’t stop digesting the food you have eaten even if you tried.
Other types of activities controlled by your brain are voluntary. When
you want to move your arm, your brain sends signals along motor
neurons to muscles in your arm telling it to move. The brain has three
major parts—the cerebrum, the cerebellum, and the medulla. Each
part has its own functions it is responsible for.
Your Thinking Cap
The largest part of your brain is called the cerebrum. This domeshaped area is where you think and where most memories are stored.
It controls voluntary movements and allows you to detect touch, light,
sound, odors, taste, pain, heat, and cold.
The cerebrum is divided into two halves called hemispheres. The left
hemisphere controls the right side of the body, and the right
hemisphere controls the left side of the body. This is because the way
our bodies are wired, with axons crossing over to the opposite side of
the body in the spinal cord. Most brain activity requires the use of both
hemispheres.
The Balancing Act
The second largest part of your brain is the cerebellum. The
cerebellum controls your balance. It lies underneath the back of your
cerebrum and receives sensory impulses from skeletal muscles and
joints. This allows the brain to keep track of your body’s position. For
example, if you are beginning to lose your balance, the cerebellum
sends impulses to different skeletal muscles to make them contract,
keeping you upright.
The Mighty Medulla
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The part of your brain that connects to your spinal cord is called the
medulla. The medulla is only about 3 centimeters long, but you
couldn’t live without it. The medulla controls your blood pressure,
heart rate, involuntary breathing, and other involuntary activities.
Your medulla constantly receives sensory impulses from your blood
vessels. It uses this information to regulate your blood pressure. If your
blood pressure gets too low, the medulla sends out impulses that tell
blood vessels to tighten up to increase the blood pressure. The
medulla also sends impulses to the heart to make it beat faster or
slower as necessary.
The Spinal Cord
Your spinal cord is part of the central nervous system and is about as
big around as your thumb. It contains neurons and bundles of axons
that pass impulses to and from the brain. The spinal cord is surrounded
by protective bones called vertebrae.
The nerve fibers in your spinal cord enable you brain to communicate
with your peripheral nervous system. Sensory neurons in your skin and
muscles send impulses along their axons to your spinal cord. The spinal
cord then conducts impulses to your brain, where they can be
interpreted as pain, heat, cold, or other sensations. Impulses moving
from the brain down the cord are relaying to motor neurons, which
carry the impulses along their axons to muscles and glands all over
your body.
Spinal Cord Injury
If the spinal cord is injured, any sensory information coming into it
below where the damaged occurred may be unable to travel to the
brain. Likewise, any motor commands the brain send to an area below
the injury may not get through to the peripheral nerves. Thousands of
people each year are paralyzed by spinal cord injuries. Many of these
injuries occur in automobile accidents. Among young people, spinal
cord injuries are often sports related.
Ouch, That Hurt!
Have you ever stepped on something sharp? You probably pulled your
foot up without thinking. This quick, involuntary action that helps
protect your body from damage is called a reflex.
When you step on a sharp object, the message “pain” travels to your
spinal cord. Your spinal cord reacts and tells your foot to move before
sending the message to your brain. The muscles in your leg respond
before the information ever reaches the brain. By the time your brain
finds out what happened, your foot has already moved. If you had to
wait for your brain to get the message, your foot might have been
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seriously injured!
Answer the following questions in complete sentences.
1.
Describe the function of each major part of the brain.
Cerebrum
Cerebellum
Medulla
2.
How do reflexes help to protect our bodies?
3.
Write a paragraph to explain how spinal cord injury can cause paralysis. Use the following words
in your explanation: impulse, spinal cord, sensory neuron, and motor neuron.
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