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The excitability of various regions of the spinal motorneuron can be specified by solving the partial differential equation of a nerve fiber whose diameter and membrane properties vary with distance. For our model geometrical factors for the myelinated axon, initial segment and cell body were derived from anatomical measurements, the dendritic tree was represented by its equivalent cylinder, and the current-voltage relations of the membrane were described by a modification of the Hodgkin-Huxley model that fits voltage-clamp data from the motorneuron. In order to compute spike waveforms that match experimental observations, 1) the dendritic membrane must be inexcitable, 2) the voltage threshold of the initial segment of the axon must be ten millivolts lower than that of the cell body, and 3) the density of sodium conductance in the initial segment must be ten times greater than in a typical unmyelinated axon.
Dodge et al. (Tue,) studied this question.