View source for Efferent nerve fiber

{{Short description|Axonal projections that exit a particular region}}
{{Use American English|date=March 2021}}
{{Use mdy dates|date=March 2021}}
{{Infobox nerve
| Name        = Efferent nerve fiber
| Latin       = neurofibrae efferentes
| Image       = Afferent and efferent neurons en.svg
| Caption     = In the peripheral nervous system, afferent and efferent projections are always from the perspective of the [[spinal cord]].
| Image2      =
| Caption2    =
| System = [[Peripheral nervous system]]
}}

'''Efferent nerve fibers''' are [[axon]]s (nerve fibers) of [[efferent neuron]]s that exit a particular region. These terms have a slightly different meaning in the context of the [[peripheral nervous system]] (PNS) and [[central nervous system]] (CNS). The efferent fiber is  a long process projecting far from the [[soma (biology)|neuron's body]] that carries [[action potential|nerve impulses]] away from the [[central nervous system]] toward the peripheral effector organs ([[muscle]]s and [[gland]]s). A bundle of these fibers constitute an [[efferent nerve]].<ref>{{Cite web |title=Efferent nerve fibre {{!}} anatomy {{!}} Britannica |url=https://www.britannica.com/science/efferent-nerve-fiber |access-date=2024-05-13 |website=www.britannica.com |language=en}}</ref> The opposite direction of neural activity is afferent conduction,<ref>Mader S. S. (2000): Human biology. McGraw-Hill, New York, {{ISBN|0-07-290584-0}}; {{ISBN|0-07-117940-2}}.</ref><ref>Hall J.  E., Guyton A. C. (2006): Textbook of medical physiology, 11th edition. Elsevier Saunders, St. Louis, Mo, {{ISBN|0-7216-0240-1}}.</ref><ref>Warrell D. A., Cox T. M., Firth J. D. (2010): [http://otm.oxfordmedicine.com/ The Oxford Textbook of Medicine] {{webarchive|url=https://web.archive.org/web/20120321002102/http://otm.oxfordmedicine.com/ |date=2012-03-21 }} (5th ed.). Oxford University Press</ref> which carries impulses by way of the [[afferent nerve fiber]]s of [[sensory neuron]]s.

In the nervous system, there is a "closed loop" system of sensation, decision, and reactions. This process is carried out through the activity of sensory neurons, [[interneuron]]s, and motor neurons.

In the CNS, afferent and efferent projections can be from the perspective of any given brain region. That is, each brain region has its own unique set of afferent and efferent projections. In the context of a given brain region, '''a'''fferents are '''a'''rriving fibers while '''e'''fferents are '''e'''xiting fibers.

==Structure==
{{See also|Afferent nerve fiber}}

===Motor nerve===
{{Further|Motor neuron}}
[[File:Motoneuron.svg|thumb|left|260px|[[Myelinated]] [[general somatic efferent fibers|GS efferent fiber]] leaving cell body of motor neuron to form a neuromuscular junction]]
The efferent nerve fibers of motor neurons are involved in [[muscle]] control, both [[skeletal muscle|skeletal]] and [[smooth muscle]]. The [[cell (biology)|cell]] [[soma (biology)|body]] of the motor neuron is connected to a single, long [[axon]] and several shorter [[dendrite]]s projecting out of the cell body itself. This axon then forms a [[neuromuscular junction]] with the effectors. The cell body of the motor neuron is [[satellite]]-shaped. The motor neuron is present in the [[grey matter]] of the [[spinal cord]] and [[medulla oblongata]], and forms an electrochemical pathway to the [[wikt:effector|effector organ]] or muscle. Besides motor nerves, there are efferent sensory nerves that often serve to adjust the sensitivity of the signal relayed by the afferent sensory nerve.{{citation needed|date=March 2017}}

===Types===
There are three types of efferent fibers: [[general somatic efferent fiber]]s (GSE), [[general visceral efferent fiber]]s (GVE) and [[special visceral efferent fiber]]s (SVE).

Subtypes of general somatic efferent fibers include:  [[alpha motor neuron]]s (α) – these target [[extrafusal muscle fiber]]s, and [[gamma motor neuron]]s (γ) that target [[intrafusal muscle fiber]]s. [[Beta motor neuron]]s target both types of muscle fiber and there are two types known as [[Beta motor neuron#Types|''static'' and ''dynamic'']].

'''Alpha motor neurons''' are large lower motor neurons located in the anterior (ventral) horn of the spinal cord, and they serve as the final common pathway for producing skeletal muscle movement. Their axons exit through the ventral roots and innervate extrafusal muscle fibers, the main contractile fibers responsible for generating force. When activated, alpha motor neurons release acetylcholine at the neuromuscular junction, triggering depolarization and contraction of the muscle. They receive input from multiple sources, including upper motor neurons from the corticospinal tract, sensory afferents from muscle spindles (especially Ia fibers involved in the stretch reflex), spinal interneurons, and motor control centers such as the cerebellum and basal ganglia. Alpha motor neurons play a central role in voluntary movement, reflexes, muscle tone, and posture. Damage to these neurons results in classic lower motor neuron signs, such as muscle weakness, flaccid paralysis, fasciculations, decreased tone, and atrophy, seen in conditions like poliomyelitis, ALS, and spinal muscular atrophy. They work alongside gamma motor neurons, which adjust spindle sensitivity, but alpha motor neurons are primarily responsible for producing actual muscle force and movement.<ref>@mcgraw hill</ref>

==Etymology and mnemonics==
Both '''afferent''' and '''efferent''' come from [[French language|French]], evolved from [[Latin]] (the basis of many terms in medicine and biology) for the terms, respectively, ''ad ferens'' (Latin verb ''ferre'': carry), meaning ''carrying into'', and ''ex ferens'', meaning ''carrying away'' (''ad'' literally means ''to'', and ''e'' = ''ex'' means ''from''). ''Ad'' and ''ex'' give an easy [[mnemonic device]] for remembering the relationship between ''afferent'' and ''efferent'': '''''a'''fferent connection '''a'''rrives'' and an '''''e'''fferent connection '''e'''xits''.<ref>{{MedicalMnemonics|3502|3463|367|115}}</ref>

Afferent and efferent are connected to affect and effect through their common Latin roots: Afferent nerves affect the subject, whereas efferent nerves allow the subject to effect change.

== See also ==
* [[Sensory nerve]]
* [[Motor nerve]]
* [[Afferent nerve fiber]]
* [[Sensory neuron]]
* [[Motor neuron]] (efferent neuron)

==References==
{{Reflist}}

{{Nervous tissue}}
{{Authority control}}

[[Category:Peripheral nervous system]]
[[Category:Medical mnemonics]]