Impact of auditory nerve parameters on threshold and spike initiation site for cochlear implant users : a modeling study

Abstract

Cochlear implants restore hearing by bypassing damaged or lost hair cells in the cochlea and directly stimulating the auditory nerve fiber via electrode-arrays. While the basic principle is straightforward, the complex geometry of the auditory nerve fiber poses sig-nificant challenges in selecting the optimal electrode positions and current magnitudes.The aim of this thesis is to identify key parameters that influence action potential propagation and initiation.For this thesis, two multi-compartment models of the auditory nerve fiber have benn developed in Matlab R2023b. The first is a simulation of a healthy auditory nerve fiber to analyze which parameters have an impact on conduction velocity, presomatic delayand action potential propagation. Insights have been applied to the second model which simulates a pathological case with hair cell loss. Stimulation is provided by an extra cellularpoint electrode and its effects are investigated across different positions.Results have shown that factors like myelination state, fiber diameter, node of Ranviergeometry and ion channel density influence conduction velocity and presomatic delay. Themost critical parameters for reliable action potential initiation and propagation have been found to be the soma diameter and the length of the presomatic region. Extra cellularstimulation introduces an additional factor influencing propagation and initiation of action potentials - the electrode position and electrode current magnitude.This thesis highlights the importance of accounting for variability in auditory nerve fiber geometry between patients. While some parameters have only a marginal influence on action potential initiation, differences in soma diameter and presomatic length in the μm range in extreme cases lead to the failure of action potential propagation and have an impact on current magnitude depending on the electrode position. It is crucial to consider these factors when selecting stimulation parameters and electrode position in cochlear implant design.