Strength-duration properties of human myelinated motor and sensory axons in normal case and in amyotrophic lateral sclerosis.

The time constants and rheobase currents are properties of axonal membrane and their calculation may shed light on axonal properties when taken in conjunction with examinations of axonal excitability. Using double cable models of human myelinated motor and sensory axons and of their three simulated types amyotrophic lateral sclerosis (termed as ALS1, ALS2 and ALS3, respectively), the time constants and rheobase currents (nodal/internodal) are calculated in the case of action potential propagation and in the case of a uniformly polarized fibre. A polynomial function of degree 2 (parabola), which relates threshold charge to stimulus duration, provides an accurate fit for the axon data. The results are consistent with the interpretation that the considerably different nodal/internodal time constants and nodal/internodal rheobase currents depend not only on the cable properties of the axons, as well as on the nodes, but on the methods of stimulation.