Modelled response of the electrically stimulated human auditory nerve fibre

Abstract

This study determined whether the Hodgkin-Huxley model for unmyelinated nerve fibres could be more comprehensively modified to predict excitation behaviour at Ranvier nodes of a human sensory nerve fibre, as specifically applied to the prediction of temporal characteristics of the human auditory system. The model was developed in three phases. Firstly, the Hodgkin-Huxley model was modified to describe action potential dynamics at Ranvier nodes using recorded ionic membrane current data from single human myelinated peripheral nerve fibres. A nerve fibre cable model, based on a combination of two existing models, was subsequently developed using human sensory nerve fibre morphometric data. Lastly the morphological parameters of the nerve fibre model were changed to resemble a Type I peripheral auditory nerve fibre and coupled to a volume-conduction model of the cochlea. This study is the first to show that the Hodgkin-Huxley model equations can be modified successfully to predict excitation behaviour of a generalised human peripheral sensory nerve fibre without using the Goldman-Hodgkin-Katz equations. The model includes a more comprehensive establishment of temperature dependence of the physiological and electrical parameters compared to existing models. Two versions of the human Type I auditory nerve fibre model were developed, one simulating an undamaged (non-degenerate) fibre and another a damaged (degenerate) fibre. Comparison between predicted and measured results indicated similar transient and persistent sodium, as well as slow potassium ionic membrane currents to those found in generalised sensory nerve fibres. Results confirm that chronaxie, rheobase current, mean latency, threshold and relative refractive periods depend on the amount of degeneracy of fibres. The model could account for threshold differences observed between different asymmetric waveforms. The combination of persistent sodium and slow potassium ionic membrane currents could in part predict non-monotonic excitation behaviour observed experimentally. A simplified method was developed to calculate electrically evoked compound action potential responses following neural excitation. It provided a computationally effective way to obtain an estimate of profile widths from the output of models that calculate neural excitation profiles, and an indirect way to estimate stimulus attenuation by calculating the value of the parameter that produces the best fit to experimental data. Results also confirmed that electrode arrays located closer to the modiolus produce more focussed neural excitation spread than more laterally located arrays.