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Nerves

Nerves are the wires or cables connecting the processing centers of the CNS to structures throughout the body.  Some are called sensory nerves because they only carry sensory information into the CNS.  Most nerves are considered mixed nerves because they carry sensory input and motor output.  Keep this in mind as you view the structure of nerves.  You are looking at sensory and motor axons passing through these nerves!

Most tissues are held together via connective tissues and this is true for nerves also.  Dense connective tissues surround entire nerves as epineurium.  Inside the epineurium covering, groups of axons are isolated into cylindrical groups called fascicles.  These are surrounded by a connective tissue component called perineurium.  Extending inward from the perineurium are more delicate collagen and elastic fibers that form the endoneurium, connective tissue wrappings around and between axons.

Nerves of the PNS

Axons are either myelinated or un-myelinated.  In the PNS the myelin sheath is a product of a neuroglial cell called the Schwann Cell.  At high magnification, these are visible as spiral wrappings around axons.

From a functional standpoint, myelin wrappings increase the speed and efficiency of impulse conduction along neurons.  Impulses jump from one exposed site to another along the axon membrane in a saltatory or "jumping" conduction.  These exposed membrane sites along axons are called Nodes of Ranvier and are visible in longitudinal views of nerves at higher magnification.

Here are some important points to remember about nerves!

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