Group A nerve fiber

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Group A nerve fibers are one of the three classes of nerve fiber as generally classified by Erlanger and Gasser. The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B are moderately myelinated, and group C are unmyelinated.[1]

The other classification is a sensory grouping that uses the terms type Ia and type Ib, type II, type III, and type IV, sensory fibers.[1]


There are four subdivisions of group A nerve fibers: alpha (ɑ), beta (β), gamma (ɣ), and delta (δ).

Type Aɑ 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.[1]

Type Aβ fibres, and type Aɣ, are the type II afferent fibers from stretch receptors.[1]

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.[2][3] These are the type III group. Aδ fibers carry cold, pressure, and acute pain signals, and because they are thin (2 to 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.[4]

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.[5]


  1. ^ a b c d Hall, John (2011). Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, Pa.: Saunders/Elsevier. pp. 563–564. ISBN 978-1-4160-4574-8.
  2. ^ Skljarevski, V.; Ramadan, N. M. "The nociceptive flexion reflex in humans – review article". Pain. 96 (1): 3–8. doi:10.1016/s0304-3959(02)00018-0.
  3. ^ 1962-, Striedter, Georg F. Neurobiology : a functional approach (Instructor's ed.). New York. ISBN 9780195396157. OCLC 919041751.
  4. ^ Neuroscience. Purves, Dale. (5th ed.). Sunderland, Mass.: Sinauer Associates. 2012. ISBN 9780878936953. OCLC 754389847.CS1 maint: others (link)
  5. ^ Basbaum, Allan I.; Bautista, Diana M.; Scherrer, Grégory; Julius, David (October 2009). "Cellular and Molecular Mechanisms of Pain". Cell. 139 (2): 267–284. doi:10.1016/j.cell.2009.09.028.