Nodal and internodal strength-duration time constants and rheobase currents for human myelinated motor and sensory nerve axons
Stephanova, D.; Daskalova, M.
Dokladi na B"lgarskata Akademiya na Naukite 56(10): 113-118
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 human motor and sensory nerve axons. The time constants and rheobases based on action potential generation are defined as nodal when the case of point fibre polarization 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 is used. The spatial extracellular potentials presented show that the case of point fibre polarization closely approximates the effects of point application of current extra-axonally at the node, whereas the case of periodic kind of uniform polarization closely approximates the effects of external surface stimulation with a large electrode. 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 calculated from the curve-fitting equation (a polynomial function of degree 2) for the resulting charge-duration curves. The results are consistent with the interpretation that the considerably different nodal and internodal time constants and rheobase currents depend not only on the cable properties of the fibres, but on the methods of stimulation.