group A nerve fiber

There are four subdivisions of group A nerve fibers: alpha (α) Aα; beta (β) Aβ; , gamma (γ) Aγ, and delta (δ) Aδ. These subdivisions have different amounts of myelination and axon thickness and therefore transmit signals at different speeds. Larger diameter axons and more myelin insulation lead to faster signal propagation.

Group A nerves are found in both motor and sensory pathways.

class="wikitable" |+Motor fiber types
Type	Erlanger-Gasser Classification || Diameter || Myelin || Conduction velocity	Associated muscle fibers
α | Aα || 13–20 μm || Yes || 80–120 m/s ||Extrafusal muscle fibers
γ | Aγ || 5–8 μm || Yes || 4–24 m/s {{cite journal | last1 = Andrew | first1 = BL | last2 = Part | first2 = NJ | year = 1972 | 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 = 213–225 | doi = 10.1113/expphysiol.1972.sp002151 | pmid = 4482075 | doi-access = free }}{{cite journal | author = Russell NJ | year = 1980 | title = Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat | journal = J Physiol | volume = 298 | pages = 347–360 | doi = 10.1113/jphysiol.1980.sp013085 | pmid = 7359413 | pmc = 1279120 }} || Intrafusal muscle fibers

Different sensory receptors are innervated by different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers, and nociceptors and thermoreceptors by type III and IV sensory fibers.

class="wikitable" |+Sensory fiber types
Type	Erlanger-Gasser Classification || Diameter || Myelin || Conduction velocity	Associated sensory receptors
Ia | Aα || 13–20 μm || Yes || 80–120 m/s{{cite book | last1=Siegel | first1=Allan | last2=Sapru | first2=Hreday | title=Essential Neuroscience | isbn=978-0781750776 | year=2005 | page=[https://archive.org/details/essentialneurosc0000sieg/page/257 257] | publisher=Lippincott Williams & Wilkins | url-access=registration | url=https://archive.org/details/essentialneurosc0000sieg/page/257 }} || Muscle spindle fibres
Ib | Aα || 13–20 μm || Yes || 80–120 m/s || Golgi tendon organ
II | Aβ || 6–12 μm || Yes || 33–75 m/s || All cutaneous mechanoreceptors including pacinian corpuscles
III | Aδ || 1–5 μm || Thin || 3–30 m/s || Free nerve endings of touch and pressure Nociceptors of neospinothalamic tract Cold thermoreceptors
IV | C || 0.2–1.5 μm || No || 0.5–2.0 m/s || Nociceptors of paleospinothalamic tract Warmth receptors

Type Aα fibers include the type Ia and type Ib sensory fibers of the alternative classification system, and are the fibers from muscle spindle endings and the Golgi tendon, respectively.

Type Aβ fibres, and type Aγ, are the type II afferent fibers from stretch receptors. Type Aβ fibres from the skin are mostly dedicated to touch. However a small fraction of these fast fibres, termed "ultrafast nociceptors", also transmit pain.{{cite journal|last1=Nagi|first1=Saad S.|last2=Marshall|first2=Andrew G.|last3=Makdani|first3=Adarsh|last4=Jarocka|first4=Ewa|last5=Liljencrantz|first5=Jaquette|last6=Ridderström|first6=Mikael|last7=Shaikh|first7=Sumaiya|last8=O’Neill|first8=Francis|last9=Saade|first9=Dimah|last10=Donkervoort|first10=Sandra|last11=Foley|first11=A. Reghan|last12=Minde|first12=Jan|last13=Trulsson|first13=Mats|last14=Cole|first14=Jonathan|last15=Bönnemann|first15=Carsten G.|last16=Chesler|first16=Alexander T.|last17=Bushnell|first17=M. Catherine|last18=McGlone|first18=Francis|last19=Olausson|first19=Håkan|title=An ultrafast system for signaling mechanical pain in human skin|journal=Science Advances|volume=5|issue=7|year=2019|pages=eaaw1297|issn=2375-2548|doi=10.1126/sciadv.aaw1297|pmid=31281886|pmc=6609212|bibcode=2019SciA....5.1297N}}

Type Aδ fibers are the afferent fibers of nociceptors. Aδ fibers carry information from peripheral mechanoreceptors and thermoreceptors to the dorsal horn of the spinal cord. This pathway describes the first-order neuron. Aδ fibers serve to receive and transmit information primarily relating to acute pain (sharp, immediate, and relatively short-lasting). This type of pain can result from several classifications of stimulants: temperature-induced, mechanical, and chemical. This can be part of a withdrawal reflex—initiated by the Aδ fibers in the reflex arc of activating withdrawal responses.{{Cite journal|last1=Skljarevski|first1=V.|last2=Ramadan|first2=N. M.|title=The nociceptive flexion reflex in humans – review article|journal=Pain|volume=96|issue=1|pages=3–8|doi=10.1016/s0304-3959(02)00018-0|pmid=11932055|year=2002|s2cid=23881420}}{{Cite book|title=Neurobiology : a functional approach|author=Striedter, Georg F.|isbn=9780195396157|edition=Instructor's|location=New York|oclc=919041751|year = 2016}} These are the type III group. Aδ fibers carry cold, pressure, and acute pain signals; because they are thin (2–5 μm in diameter) and myelinated, they send impulses faster than unmyelinated C fibers, but more slowly than other, more thickly myelinated group A nerve fibers. Their conduction velocities are moderate.{{Cite book|title=Neuroscience|date=2012|publisher=Sinauer Associates|others=Purves, Dale.|isbn=9780878936953|edition=5th|location=Sunderland, Mass.|oclc=754389847}}

Their cell bodies are located in the dorsal root ganglia and axons are sent to the periphery to innervate target organs and are also sent through the dorsal roots to the spinal cord. Within the spinal cord the axons reach the posterior grey column and terminate in Rexed laminae I to V.{{cite journal|last1=Basbaum|first1=Allan I.|last2=Bautista|first2=Diana M.|last3=Scherrer|first3=Grégory|last4=Julius|first4=David|title=Cellular and Molecular Mechanisms of Pain|journal=Cell|date=October 2009|volume=139|issue=2|pages=267–284|doi=10.1016/j.cell.2009.09.028|pmc=2852643|pmid=19837031}}

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{{Nervous tissue}}

Category:Neurohistology

Category:Neurophysiology

Category:Sensory systems