Excitability changes during the recovery cycle of human myelinated motor and sensory axons in normal case and in amyotrophic lateral sclerosis.

The excitability changes of human axons during 100-ms recovery cycles are investigated by paired stimulations. Using double cable models of normal axons and of three simulated types amyotrophic lateral sclerosis (termed as ALS1, ALS2 and ALS3, respectively), the recovery cycles are presented in the case of action potential propagation and in the case of a uniformly polarized fibre. Following the testing pulse, human axons go through an oscillating sequence of excitability changes, the recovery cycle: they are initially inexcitable (absolute refractory period), than excitable with a raised threshold (early subnormal period) and after about 2-ms they become more excitable than normal (superexcitable period). For uniformly polarized axons in the normal and ALS1 types, the superexcitable period usually followed by a late subnormal period. With increasing the degree of ALS, the length-dependent potentials propagate with complex, desynchronized amplitudes and velocities, and abnormalities in the recovery cycles are obtained for the ALS3 axonal types.