Excitability properties of normal and demyelinated human motor nerve axons

The strength-duration time constants and rheobase currents, which provide an indirect indication of the axonal properties are calculated in two cases of stimulation, using our previous double cable models of normal and demyelinated human motor fibres. The time constants and rheobases are defined as nodal when the case of point fibre polarization (intracellular current application at the first node) is used for their calculations, whereas the time constants and rheobases are defined as internodal when the case of periodic kind of uniform fibre polarization (simultaneous intracellular current application at each axon segment) is used. Four fibre demyelinations (termed as paranodal focal 1 systematic and internodalfocal 1 systematic demyelinations) are studied. For both investigating cases of current application, the stimulus duration is increased in 0.025-ms steps from 0.025-ms to 1-ms and the strength-duration and charge-duration curves are plotted for the axons. The strength-duration time constants are calculatedfrom the curve-fitting equation for the resulting charge-duration curves. The results are consistent with the interpretation that the time constants depend not only on the types of the demyelinated axon, but on the methods of fibre stimulation. The strength-duration time constants (nodal 1 internodal) are almost the same for the normal axons and focally demyelinated axons, however, they are shorter for the paranodally systematically demyelinated axons, and longer for the internodally systematically demyelinated axons. For all investigated cases, the internodal time constants are greater than the nodal time constants and there is an inverse relationship between the time constants and rheobase currents.