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Group A nerve fibers are one of the three classes ofnerve fiber asgenerally classified byErlanger andGasser. The other two classes are thegroup B nerve fibers, and thegroup C nerve fibers. Group A are heavilymyelinated, group B are moderatelymyelinated, and group C are unmyelinated.[1][2]
The other classification is asensory grouping that uses the termstype Ia and type Ib,type II,type III, andtype IV, sensory fibers.[1]
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 andtherefore transmit signals atdifferent 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]
| Type | Erlanger-Gasser Classification | Diameter | Myelin | Conduction velocity | Associatedmuscle fibers |
|---|---|---|---|---|---|
| α | Aα | 13–20μm | Yes | 80–120 m/s | Extrafusal muscle fibers |
| γ | Aγ | 5–8μm | Yes | 4–24 m/s[3][4] | Intrafusal muscle fibers |
Differentsensory 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, andnociceptors andthermoreceptors by type III and IV sensory fibers.
| Type | Erlanger-Gasser Classification | Diameter | Myelin | Conduction velocity | Associatedsensory receptors |
|---|---|---|---|---|---|
| Ia | Aα | 13–20μm | Yes | 80–120 m/s[5] | Muscle spindle fibres |
| Ib | Aα | 13–20μm | Yes | 80–120 m/s | Golgi tendon organ |
| II | Aβ | 6–12μm | Yes | 33–75 m/s | Allcutaneous mechanoreceptors includingpacinian corpuscles |
| III | Aδ | 1–5μm | Thin | 3–30 m/s | Free nerve endings of touch and pressure Nociceptors ofneospinothalamic tract Coldthermoreceptors |
| IV | C | 0.2–1.5μm | No | 0.5–2.0 m/s | Nociceptors ofpaleospinothalamic tract Warmth receptors |
Type Aα fibers include thetype Ia andtype Ib sensory fibers of the alternative classification system, and are the fibers frommuscle spindle endings and theGolgi tendon, respectively.[1]
Type Aβ fibres, and type Aγ, are thetype IIafferent fibers fromstretch 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 theafferent fibers ofnociceptors. 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 toacute 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 awithdrawal reflex—initiated by the Aδ fibers in thereflex arc of activating withdrawal responses.[7][8] These are thetype III group. Aδ fibers carry cold, pressure, and acute pain signals; because they are thin (2–5 μm in diameter) andmyelinated, they send impulses faster than unmyelinatedC fibers, but more slowly than other, more thickly myelinated group A nerve fibers. Theirconduction velocities are moderate.[9]
Theircell bodies are located in thedorsal 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 theposterior grey column and terminate in Rexed laminae I to V.[10]
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