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.

muscle spindle model
Neuromechanics / Public domain

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)


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

Motor fibers:

DiameterMyelinConduction velocityAssociated muscle fibers
α13–20 μmYes80–120 m/sExtrafusal muscle fibers
γ5–8 μmYes4–24 m/sIntrafusal muscle fibers

Afferent fibers:

DiameterMyelinConduction velocityAssociated sensory receptors
Ia13–20 μmYes80–120 m/sMuscle spindle
Ib13–20 μmYes80–120 m/sGolgi tendon organ
II6–12 μmYes33–75 m/sSecondary receptors of muscle spindle
All cutaneous mechanoreceptors
III1–5 μmThin3–30 m/sFree nerve endings of touch and pressure
Nociceptors of neospinothalamic tract
Cold thermoreceptors
IVC0.2–1.5 μmNo0.5–2.0 m/sNociceptors 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

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