Nerve Structure and Classification

What is a nerve you ask?

A nerve is simply a bundle of axons held together by connective tissue, outside of the brain and spinal cord. To the naked eye, they look an awful lot like a linguine noodle. These nerves are responsible for carrying all sensory information to the spinal cord, and for carrying all motor information from the spinal cord to the muscles of the body, and in some cases, glands.

This information travels along these nerves in the form of action potentials along individual axons. It is important to remember that only axons are found in nerves, and therefore nerves are white matter and responsible solely for signal transduction, not information processing.


Nerve Categorization

Nerves fall into one of three basic categories based on the direction of the action potentials they carry.

Sensory Nerves: These nerves carry action potentials from the bodies periphery to the spinal cord. From here the action potentials are carried along spinal tracts and sent to the appropriate cortical region.

Motor Nerves: These nerves carry action potentials from the spinal cord to the bodies periphery. These nerves can include both somatic and autonomic motor information.

Mixed Nerves: A combination of axons carrying information to the body’s periphery, and axons carrying sensory information to the CNS.

Nerve Structure:

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As mentioned earlier nerves are simply bundles of axons outside of the CNS packaged in connective tissue. Specifically the arrangement of connective tissue is as follows:

Endoneurium: connective tissue sheath that surrounds each individual axon (nerve fiber)

Perineurium: This connective tissue sheath bundles groups of axons into fascicles.

Epineurium: The outer most connective tissue sheath. This sheath surrounds the entire nerve. When you look at a nerve, you are actually looking at this layer of connective tissue.

The perineurium and epineurium are often confused. Remember that it is the epineurium that is the outermost connective tissue sheath. Peri- means around or surrounding. Epi means above, outside of, etc. If this doesn’t help I try to associate the epi in epineurium with the word epic, which helps me remember it is the outermost sheath for some reason!

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