Anatomy of a Nerve
More than ten thousand million nerve cells and
their fibers, or axons, make up the nervous system. The axons are
grouped together in nerve trunks containing sensory fibers, which
conduct information from the sensory organs to the central nervous
system, and motor fibers, which carry information to the central
nervous system of the body. The nerve fibers which carry information
quickly are enclosed in a thick sheath made of the fatty substance
myelin. They are called myelinated nerve fibers. The myelinated
sheath has regular indentations along its length called the nodes of
Ranvier. The nonmyelinated nerve fibers, which carry information
slowly, are grouped together and enclosed in a single sheath.
The anatomy of a nerve includes: the cell body, which is composed of
cytoplasm and contains the cell nucleus; the nucleus, which contains
the information needed to control the activity of the neuron; the
dendrites, outgrowths of the cell body to which and from which they
conduct impulses; the epineurium, a fibrous sheath that surrounds
the whole nerve; the perineurium, the connective tissue sheath that
surrounds bundles of nerve fibers; the endoneurium, the fine sheath
of connective tissue around each nerve bundle; the axon, the
extended fiber of the nerve cell which carries impulses to and from
the cell body; the fatty myelin sheath, the insulating coat that
separates the axon in a nerve bundle; the Schwann cell nucleus, the
mechanism responsible for the production and maintenance of the
myelin sheath and the Nodes of Ranvier, constrictions in the myelin
sheath.