The nervous system is a communication network
that controls and coordinates most body actions. People are not
generally conscious of some of these activities as they occur, such as
regulating body temperature, breathing, or heart rate. Others, we
consciously control, such as movement, talking, eating, and thinking.
The nervous system has two
main parts: the central and the peripheral nervous system. The brain
and spinal cord make up the central nervous system, or CNS. The nerves
branching off the central nervous system make up the peripheral nervous
system, or PNS. The PNS consists of nerves bundles made up of sensory
and motor neurons.
Neurons
The nervous system is made up
of cells called neurons. Neurons are long, thin cells that transmit
electrical impulses. Neurons have many branched endings, called
dendrites, which receive impulses from other neurons. An axon, or nerve
fiber, carries nerve impulses to other neurons or to muscle. Neurons do
not touch, but are separated by a tiny gap called a synapse. When an
impulse arrives at the end of an axon, it releases chemicals that
generate an impulse in the dendrites of the neighboring neuron.
There are three types of neurons: sensory, motor, and association.
Sensory neurons transmit nerve impulses from sense organs (eyes, ears,
nose, tongue, and touch) to the brain. They also carry nerve impulses
to the brain and spinal cord. Motor neurons transmit nerve impulses
from the brain and spinal cord to a specific area of the body. A nerve
impulse to a muscle, for example, may cause it to contract. Association
neurons make up 90 percent of all neurons and are found only in the
brain and spinal cord.
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