Activity-dependent variations in threshold and conduction velocity of human sensory fibers.

Activity-dependent changes in threshold with a sequential set of events (action potential, refractoriness, supernormal and subnormal period) can regularly be observed in sensory fibers of the human median nerve, the whole recovery cycle lasting for 120-200 ms in different subjects. Supernormality is probably due to an after-depolarization of the membrane potential and has its peak 5-8 ms after the conditioning pulse. The subsequent period of subnormal excitability results from a long-lasting hyperpolarization of the membrane and reaches its maximum 40-60 ms after the conditioning pulse. In contrast to some findings in slow-conducting fibers of different experimental animals, the fast-conducting fibers of the human median nerve do not reveal the phenomenon of supernormal conduction velocity.