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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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