THE NERVOUS SYSTEM
The brain is likened to a control center, then the
nervous system can be thought of as a two-way communications network through
which informational messages flow to the control center and command messages
are transmitted from the center. The informational, or sensory, messages come
from the outside world through the sense organs (eyes, ears, etc.); they also
come from within the body itself-there are billions of receptors all over the
body concerned with various functions.
The nervous system is organized to give
you essential voluntary control over many actions. It is also set up to relieve
you of concern with routine matters. Thus, for example, you eat dinner and
decide whether you like or dislike a certain dish and wish to finish it.
On the other hand, you walk along, stumble on an
object; without thought on your part, the muscles of the legs are automatically
commanded to react, and one leg is extended and the other flexed so you
maintain your balance. How Nerves Work Messages travel along nerves, at speeds
of as much as 250 miles an hour, as the result of both electrical and chemical
action.
A nerve cell, or neuron, when viewed under a microscope, looks like a
tiny blob, rounded or irregular in shape, with one or more threads extending
from it. The blob is the actual nerve cell body; the threads are nerve fibers.
Shorter fibers, called dendrites, bring messages to the cell body; they may
range from a very small fraction of an inch to several feet in length.
One fiber, longer than the others and called the
axon, transmits messages away from the cell body. A nerve impulse, going
through the nerve network, travels over the fibers of many cells. As it reaches
the end of one fiber, it jumps a gap, called a synapse, to the next fiber.
Chemicals produced and stored around synapses can help the impulse to jump the
gaps or can block the impulse. Some drugs that act in the nervous system-some of
those for high blood pressure, for example-accomplish their tasks by affecting
the chemicals at the synapses.
