| Nerve fiber | |
|---|---|
| Micrograph showing nerve fibers in a nerve root. HPS stain. | |
| Latin | neurofibra |
| MeSH | Nerve+Fibers |
| Code | TA A14.0.00.001 |
A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath (if present) in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system (CNS), myelin is produced by oligodendroglia cells. Schwann cells form myelin in the peripheral nervous system (PNS). Schwann cells can also make a thin covering for an axon which does not consist of myelin (in the PNS). A peripheral nerve fiber consists of an axon, myelin sheath, Schwann cells and its endoneurium. There are no endoneurium and Schwann cells in the central nervous system.
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In the central nervous system, nerve fibers differ in terms of size, conduction velocity, and presence or lack of myelin. For example, the olfactory nerve fibers are short and without myelin, but the optic nerve fibers are myelinated (the olfactory and optic nerves are considered as a parts of the CNS, while other cranial nerves are a component of the PNS). A bundle of nerve fibers constitutes a tract in the central nervous system. The pyramidal tract and extrapyramidal tracts have long nerve fibers that descend from the brain to the spinal cord. These fibers have an important role in motor control, and are known as descending tracts. There are other bundles of nerve fibers in the CNS that are called ascending tracts.These carry sensory information from the periphery to the different areas of the brain (such as the cerebral cortex, cerebellum, and brain stem).
A nerve may be sensory, motor or sensory-motor (mixed). There are three types of nerve fibers in a mixed nerve that include:
Each peripheral nerve fiber contains:
There are three types of peripheral nerve fibers based on their diameter:
Fibers of the A group have a large diameter and high conduction velocity, and are myelinated fibers.
The A group consists of four types of nerve fibers:
Nerve fibers in these group, are myelinated with a small diameter. they are the preganglionic fibers of the autonomic nervous system. Preganglionic fibers have a low conduction velocity.
The C group fibers are unmyelinated and as the B group fibers have a small diameter and low conduction velocity. These fibers include:
A alpha fibers
A beta fibers
A gamma fibers
A alpha fibers (Ia fiber or Ib fibers)
A beta fibers (II fibers)
A delta fibers (III fibers)
Nerves can be easily damaged in a traumatic event due to their sometimes vulnerable positions in the body. However, if a nerve is damaged, it does have the ability to regenerate if its soma and a small portion of the neurilemma remain. The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud. When one of the growth processes finds the regeneration tube, it begins to grow rapidly towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is very slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect.[1]
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