A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and (in some cases) a myelin sheath. There are nerve fibers in the central nervous system and peripheral nervous system. 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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Central nerve fibers[edit]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).
Descending nerve fibers (Upper motor neurons)[edit]
The A group consists of four types of nerve fibers:
C group[edit]See also: Group C nerve fiberThe C group fibers are unmyelinated and as the B group fibers have a small diameter and low conduction velocity. These fibers include:
See also[edit]References[edit]
https://en.wikipedia.org/wiki/Nerve_fiber
Contents [hide]
- 1Central nerve fibers
- 2Peripheral nerve fiber types
- 3Components of peripheral nerve fiber
- 4Classification of peripheral nerve fibers
- 5Motor fibers of the A group
- 6Sensory fibers of the A group
- 7Regeneration of peripheral nerve fibers
- 8See also
- 9References
- 10Further reading
Central nerve fibers[edit]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).
Descending nerve fibers (Upper motor neurons)[edit]
- Pyramidal system
- Corticospinal tract (or Pyramidal tract)
- Corticobulbar tract
- Extrapyramidal tracts
- Descending autonomic fibers
- Spinothalamic tract
- Fasciculus cuneatus
- Fasciculus gracilis
- Spinoreticular tract
- Spinotectal tract
- Spinocerebellar tract
- Spinocervical tract
- Sensory nerve fibers (afferent fibers)
- Motor nerve fibers (efferent fibers)
- Autonomic nerve fibers (autonomic fibers)
- An axon (or a long dendrite of sensory fiber that also is known as an axon)
- Axolemma
- Myelin sheath (not always)
- Schwann's sheath (neurolemma)
- Endoneurium
- A group
- B group
- C group
The A group consists of four types of nerve fibers:
- A alpha fibers (afferent or efferent fibers)
- A beta fibers (afferent or efferent fibers)
- A gamma fibers (efferent fibers)
- A delta fibers (afferent fibers)
C group[edit]See also: Group C nerve fiberThe C group fibers are unmyelinated and as the B group fibers have a small diameter and low conduction velocity. These fibers include:
- Postganglionic fibers in the autonomic nervous system (ANS)
- Nerve fibers at the dorsal roots (IV fiber). These fibers carry the following sensory information:
- Nociception (pain)
- Temperature
- Touch
- Pressure
- Itch
- Characteristics:
- High conduction velocity
- By alpha motor neuron for muscle contraction.
- Alpha motor neurons innervate extrafusal muscle fibers (slow and fast fibers)
- Characteristics:
- By beta motor neuron for muscle spindle fibers contraction.
- Beta motor neurons innervate intrafusal muscle fibers of muscle spindles (nuclear bag and nuclear chain fibers), with collaterals to extrafusal muscle fibers.
- Characteristics:
- By gamma motor neuron for muscle spindle fibers contraction.
- Gamma motor neurons innervate intrafusal muscle fibers of muscle spindles (nuclear bag and nuclear chain fibers).
- Characteristics:
- High conduction velocity
- Ia fibers are related to muscle spindle primary endings (muscle sense)
- Ib fibers are related to golgi tendon organs (muscle sense)
- II fibers carry sensory information related to muscle spindle secondary endings, touch, and kinesthesia.
- III fibers carry sensory information related to pain and cold temperature.
See also[edit]References[edit]
- Jump up ^ Burnett & Zager, Mark & Eric. "Pathophysiology of Peripheral Nerve Injury: A Brief Review: Nerve Regeneration". Medscape Article. Medscape. Retrieved 2011-10-26.
- Peter L. Williams et. al, ed. (1989). Gray's anatomy (37th ed.). Edinburgh: Churchill Livingstone. ISBN 0-443-04177-6.
- Guyton, Arthur C. (1986). Textbook of medical physiology (7th ed.). Philadelphia: Saunders. ISBN 0-7216-1260-1.
- http://www.unmc.edu/physiology/Mann/mann12.html
- http://www.boddunan.com/education/20-medicine-a-surgery/12730-nerve-fiber-types-and-function.html
- "Pathophysiology of Peripheral Nerve Injury: A Brief Review: Nerve Regeneration." 2004.
- http://www.medscape.com/viewarticle/480071_5 (accessed 5 December 2010).
- Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. Fifth ed. New York: McGraw Hill, 2010.
https://en.wikipedia.org/wiki/Nerve_fiber