A distributed-parameter model of the myelinated human motor nerve fibre: temporal and spatial distributions of electrotonic potentials and ionic currents.

The double cable model is used to investigate the electrotonic responses of the myelinated human motor nerve fibre to 100 ms depolarizing and hyperpolarizing current pulses. The model calculations provide estimates of the spatial and temporal distributions of the transaxonal and transmyelin components of the electrotonic potentials, both in different segments of the fibre and at different moments during and after the pulses. The temporal distributions of the potentials exhibit fast (rise time < 1 ms) and slow (from 10 to 100 ms) components, while the discontinuous spatial distributions of the potentials reflect the heterogeneous structure of the fibre. The distributions of the transaxonal and transmyelin currents along the fibre, and their contributions from different ionic channels, are also explored. The different axolemmal channel types beneath the myelin sheath make an important contribution to the responses to the long-lasting current pulses.