Editing Axon (section)

==Classification==
{{Further |Nerve conduction velocity}}
The axons of neurons in the human [[peripheral nervous system]] can be classified based on their physical features and signal conduction properties. Axons were known to have different thicknesses (from 0.1 to 20&nbsp;μm)<ref name="LS"/> and these differences were thought to relate to the speed at which an action potential could travel along the axon{{Snd}}its ''conductance velocity''. [[Joseph Erlanger|Erlanger]] and [[Herbert Spencer Gasser|Gasser]] proved this hypothesis, and identified several types of nerve fiber, establishing a relationship between the diameter of an axon and its nerve conduction velocity. They published their findings in 1941 giving the first classification of axons.

Axons are classified in two systems. The first one introduced by Erlanger and Gasser, grouped the fibers into three main groups using the letters A, B, and C. These groups, [[group A nerve fiber|group A]], [[group B nerve fibres|group B]], and [[group C nerve fiber|group C]] include both the sensory fibers ([[afferent nerve fiber|afferents]]) and the motor fibers ([[efferent nerve fiber|efferents]]). The first group A, was subdivided into alpha, beta, gamma, and delta fibers{{Snd}}Aα, Aβ, Aγ, and Aδ. The [[motor neuron]]s of the different motor fibers, were the [[lower motor neuron]]s{{Snd}}[[alpha motor neuron]], [[beta motor neuron]], and [[gamma motor neuron]] having the Aα, Aβ, and Aγ nerve fibers, respectively.

Later findings by other researchers identified two groups of Aa fibers that were sensory fibers. These were then introduced into a system (Lloyd classification) that only included sensory fibers (though some of these were mixed nerves and were also motor fibers). This system refers to the sensory groups as Types and uses Roman numerals: Type Ia, Type Ib, Type II, Type III, and Type IV.

=== Motor === <!-- Motor fiber types redirects here -->
Lower motor neurons have two kind of fibers:

{| class="wikitable"
|+Motor fiber types
|-
! Type !! Erlanger-Gasser<br />Classification || Diameter<br />(μm) || Myelin || Conduction velocity<br />(meters/second) !! Associated [[muscle fiber]]s
|-
! [[Alpha motor neuron|Alpha (α) motor neuron]]
| Aα || 13–20 || Yes || 80–120 ||[[Extrafusal muscle fibers]]
|-
! [[Beta motor neuron|Beta (β) motor neuron]]
| Aβ || || || ||
|-
! [[Gamma motor neuron|Gamma (γ) motor neuron]]
| Aγ || 5-8 || Yes || 4–24<ref>{{cite journal | vauthors = Andrew BL, Part NJ | title = Properties of fast and slow motor units in hind limb and tail muscles of the rat | journal = Quarterly Journal of Experimental Physiology and Cognate Medical Sciences | volume = 57 | issue = 2 | pages = 213–25 | date = April 1972 | pmid = 4482075 | doi = 10.1113/expphysiol.1972.sp002151 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Russell NJ | title = Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat | journal = The Journal of Physiology | volume = 298 | pages = 347–60 | date = January 1980 | pmid = 7359413 | pmc = 1279120 | doi = 10.1113/jphysiol.1980.sp013085 }}</ref> || [[Intrafusal muscle fibers]]
|}

=== {{Visible anchor|Sensory}} === <!-- Sensory fiber types redirects here -->
Different [[sensory receptors]] are innervated by different types of nerve fibers. [[Proprioceptor]]s are innervated by type Ia, Ib and II sensory fibers, [[mechanoreceptor]]s by type II and III sensory fibers and [[nociceptor]]s and [[thermoreceptors]] by type III and IV sensory fibers.

{| class="wikitable"
|+Sensory fiber types
|-
! Type !! Erlanger-Gasser<br />Classification || Diameter<br />(μm) || Myelin || Conduction<br />velocity (m/s) !! Associated [[sensory receptor]]s !! Proprioceptors !! Mechanoceptors !! Nociceptors and<br />thermoreceptors
|-
! [[Type Ia sensory fiber|Ia]]
| Aα || 13–20 || Yes || 80–120 || Primary receptors of [[muscle spindle]] (annulospiral ending) || rowspan="3" align="center" | ✔ || rowspan=2 | || rowspan=3 |
|-
! Ib
| Aα || 13–20 || Yes || 80–120 || [[Golgi tendon organ]]
|-
! [[Type II sensory fiber|II]]
| Aβ || 6–12 || Yes || 33–75 || Secondary receptors of [[muscle spindle]] (flower-spray ending).<br />All [[cutaneous mechanoreceptor]]s|| rowspan="2" align="center" | ✔
|-
! III
| [[A delta fiber|Aδ]] || 1–5 || Thin || 3–30 || [[Free nerve ending]]s of touch and pressure<br />[[Nociceptor]]s of [[lateral spinothalamic tract]]<br />Cold [[thermoreceptors]] ||rowspan=2| || rowspan=2 align=center | ✔
|-
! IV
| [[Group C nerve fiber|C]] || 0.2–1.5 || No || 0.5–2.0 || [[Nociceptor]]s of [[anterior spinothalamic tract]]<br />[[Warmth receptors]] ||
|}

===Autonomic===
The [[autonomic nervous system]] has two kinds of peripheral fibers:

{| class="wikitable"
|+Fiber types
|-
! Type !! Erlanger-Gasser<br />Classification || Diameter<br />(μm) || Myelin<ref>{{cite book |first1=Gillian |last1=Pocock | first2 = Christopher D | last2 = Richards | name-list-style = vanc |title=Human Physiology |location=New York |publisher=Oxford University Press |edition=2nd |year=2004 |pages=187–189 |isbn=978-0-19-858527-5 |display-authors=etal}}</ref> || Conduction<br />velocity (m/s)
|-
! [[preganglionic fibers]]
| B || 1–5 || Yes || 3–15
|-
! [[postganglionic fibers]]
| C || 0.2–1.5 || No || 0.5–2.0
|}