Nerve fibers are axons that conduct action potentials; they are classified chiefly by diameter and conduction velocity in the Erlanger-Gasser classification.
Type A fibers are fastest, conducting at 70 to 120 m/sec with diameter 12 to 20 µm.
Properties of Nerve Fibers
Nerve fibers show seven properties:excitability, conductivity, unfatigability, refractory period, all-or-none law, summation and accommodation.
Excitability: cells respond to changes in the internal or external environment.
A wave of depolarization demonstrates excitation; stimulus strength, duration and extracellular Ca2+ influence it.
Conductivity: an action potential generated on stimulation propagates along the entire fiber to the axon terminal; if initiated mid-axon, it travels in both directions.
Summation: a subthreshold stimulus alone evokes no action potential, but subthreshold stimuli applied in rapid succession summate to produce one.
Accommodation: continuous stimulation decreases nerve fiber excitability, chiefly through adaptation of nerve endings, decreasing impulse transmission across neurons.
Unfatigability: nerve fibers do not fatigue even with continuous stimulation, because conduction (propagation of the action potential) does not expend energy.