Classification of Nerve Fibres
Axons are classified into 3 major groups based upon the amount of myelination, their diameters, and their propagation speeds:
A Fibres
Largest diameter 5-20 µm
Myelinated
AP speed 12 to 130 m/sec (27-280 mi/hr) Brief absolute refractory period Axons of sensory associated with touch, pressure, joint position, some thermal and pain sensation and motor nerves to skeletal muscles
B Fibres
Diameter 2-3 µm
Myelinated
AP speed up to 15 m/sec (32 mi/hr)
Longer absolute refractory period than A fibres Axons of sensory nerves from viscera to the CNS and all axons of the ANS from the CNS to autonomic ganglia (relay stations).
C Fibres
Smallest diameter 0.5 – 1.5 µm
Unmyelinated
AP speed 0.5 to 2 m/sec (1-4 mi/hr) Longest absolute refractory period
Axons of sensory impulses from pain, touch, pressure, heat, and cold from the skin, and pain impulses from viscera. ANS motor fibres that extend from autonomic ganglia to stimulate the heart, smooth muscle and glands.
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