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][2]

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]

Types

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.^[2]

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 [3][4]	Intrafusal muscle fibers

Different

by type III and IV sensory fibers.

Sensory fiber types

Type	Erlanger-Gasser Classification	Diameter	Myelin	Conduction velocity	Associated sensory receptors
Ia	Aα	13–20 μm	Yes	80–120 m/s[5]	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	thermoreceptors
IV	C	0.2–1.5 μm	No	0.5–2.0 m/s	Warmth receptors

Type Aα fibers include the type Ia and type Ib sensory fibers of the alternative classification system, and are the fibers from

Type Aβ fibres, and type Aγ, are the type II afferent fibers from stretch receptors.^[1] 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.^[6]

Type Aδ fibers are the

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.^[10]

References