The nerve fibre is essentially a highly irritable conductor. Along it the dynamic nervous excitation propagates in waves, faster in large than in small axons.
During the conduction of excitation the activity of one portion of the axon, serves as a stimulate activating the next portion, and so on. As the nerve fibres become active it changes in its electric potential, the outside of each active portion becoming negative relative to resting portion.
Muscle action currents the flow of impulse between active and resting regions. Conduction of nervous impulse is influenced by the permeability of axon membrane, permitting g sodium ions to enter the axon and potassium ions to migrate from it.
Nerve fibres can be classified into three types A B and C according to the speed of conduction.
The ‘A’ fibres, with high velocity of conduction have a large diameter, a thick myelin sheath and long internodes.
The B fibres are small myelinated fibres with relatively short internodes. The ‘C’ fibres with the lowest conduction rate, have little or on myelin.
