Nerve fibers – Classification

Nerve fibers can be classified as A, B and C and A type fibers can be further classified into alpha, beta, gamma and delta. The size and myelination (thus conduction) progressively decreases in the descending order.

A alpha:

1. Efferent (Somatic motor) – To extrafusal fibers (muscle spindle)
2. Afferent (Proprioception):
   1. Ia – Muscle spindle (Annulospiral endings)
   2. Ib – Golgi tendon organ

spindLE = LEngth of muscle
TENdon = TENsion of muslcle

A beta:

• Mechanoafferents of skin (Fine touch, Pressure, Vibration)
• Type II afferent fibers (secondary fibers from muscle spindle)

A gamma:

Efferents – To intrafusal fibers (muscle spindel)

A delta:

Fast (sharp) pain and Temperature (cold)

B: Preganglionic sympathetic (Autonomic nervous system)

C:

1. Dorsal root: Slow pain, Temperature (hot)
2. Sympathetic: Postganglionic

Motor fibers:

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	Intrafusal muscle fibers

Afferent fibers:

Type	Erlanger-Gasser Classification	Diameter	Myelin	Conduction velocity	Associated sensory receptors
Ia	Aα	13–20 μm	Yes	80–120 m/s	Muscle spindle
Ib	Aα	13–20 μm	Yes	80–120 m/s	Golgi tendon organ
II	Aβ	6–12 μm	Yes	33–75 m/s	Secondary receptors of muscle spindle All cutaneous mechanoreceptors
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

Susceptibilities of the nerve fibers to LA, pressure and hypoxia:

• A fibers are most susceptible to pressure
• B fibers are most susceptible to hypoxia
• C fibers are most susceptible to local anaesthetics
• C fibers are least susceptible to hypoxia and pressure
• A fibers are least susceptible to local anaesthetics

If you have any mnemonics or ideas for remembering these, share with the world by writing in the comments.