A model of the electrically excited human cochlear neuron I. Contribution of neural substructures to the generation and propagation of spikes

Differences in neural geometry and the fact that the soma of the human coch lear neuron typically is not myelinated are reasons for disagreements betwe en single fiber recordings in animals and the neural code evoked in cochlea r implant patients. We introduce a compartment model of the human cochlear neuron to study the excitation and propagation process of action potentials . The model can be used to predict (i) the points of spike generation, (ii) the time difference between stimulation and the arrival of a spike at the proximal end of the central axon, (iii) the vanishing of peripherally evoke d spikes at the soma region under specific conditions, (iv) the influence o f electrode positions on spiking behavior, and (v) consequences of the loss of the peripheral axon. Every subunit of the cochlear neuron is separately modeled. Ion channel dynamics are described by a modified Hodgkin-Huxley m odel. Influence of membrane noise is taken into account. Additionally, the generalized activating function is introduced as a tool to give an envision of the origin of spikes in the peripheral and in the central axon without any knowledge of the gating processes in the active membranes. Comparing th e reactions of a human and cat cochlear neuron, we find differences in spik ing behavior, e.g, peripherally and centrally evoked spikes arrive with a t ime difference of about 400 mus in man and 200 mus in cat. (C) 2001 Elsevie r Science B.V. All rights reserved.