The nervous system is composed
of two divisions, the central nervous system
(CNS) and peripheral nervous system (PNS). The
CNS contains the brain and the spinal cord and
the PNS consists of thousands of nerves that
connect the spinal cord to muscles and sensory
receptors.
A peripheral nerve is composed of nerve bundles
(fascicles) that contain hundreds of individual
nerve fibers (neurons). Neurons consist of dendrites,
axon, and cell body. The dendrites are the tree-like
structures that receive signals from other neurons
and from special sensory cells that sense the
body's surrounding environment. The cell
body is the headquarters of the neuron and contains
its genetic information in the form of DNA. The
axon transmits signals away from the cell body
to other neurons.
Many neurons are insulated like pieces of electrical
wire. This insulation protects them and also
allows their signals to move faster along the
axon. Without this insulation, signals from the
brain might never reach the outlying muscle groups
in the limbs.
The operation of the nervous system depends
on the flow of communication between neurons.
For an electrical signal to travel between two
neurons, it must first be converted to a chemical
signal, which then crosses a space of about a
millionth of an inch wide. The space is called
a synapse, and the chemical signal is called
a neurotransmitter.
Neurotransmitters allow the billions of neurons
in the nervous system to communicate with one
another, making the nervous system the master
communication system of the body.
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