Spiral ganglion cell site of excitation II: numerical model analysis.

An anatomically based model of cochlear neuron electrophysiology has been developed and used to interpret the physiological responses of the auditory neuron to electrical summation and refractory pulse-pair stimuli. For summation pulses, the summation time constant, tau(sum), indicates the ability of the membrane to hold charge after cessation of a pulse. When a spiral ganglion cell with a cell body was simulated, the value of tau(sum) was elevated at the peripheral node adjacent to the cell body. For refraction pulses, the refraction time constant, tau(ref), indicates the duration of the relative refractory period of the membrane. In spiral ganglion cell simulations, tau(ref) was decreased at the peripheral node adjacent to the cell body and slightly elevated at other peripheral nodes. The extent of the cell body influence on tau(sum) and tau(ref) was high localized. Excitation times for the nodes adjacent to the cell body were either simultaneous or near simultaneous resulting in similar response latencies. Results indicate that values of tau(sum) and tau(ref) may be useful for distinguishing central and peripheral excitation sites while latency measures alone are not a good indication of site of excitation.

Spiral ganglion cell site of excitation I: comparison of scala tympani and intrameatal electrode responses.

To determine the site of excitation on the spiral ganglion cell in response to electrical stimulation similar to that from a cochlear implant, single-fiber responses to electrical stimuli delivered by an electrode positioned in the scala tympani were compared to responses from stimuli delivered by an electrode placed in the internal auditory meatus. The response to intrameatal stimulation provided a control set of data with a known excitation site, the central axon of the spiral ganglion cell. For both intrameatal and scala tympani stimuli, the responses to single-pulse, summation, and refractory stimulus protocols were recorded. The data demonstrated that summation pulses, as opposed to single pulses, are likely to give the most insightful measures for determination of the site of excitation. Single-fiber summation data for both scala tympani and intrameatally stimulated fibers were analyzed with a clustering algorithm. Combining cluster analysis and additional numerical modeling data, it was hypothesized that the scala tympani responses corresponded to central excitation, peripheral excitation adjacent to the cell body, and peripheral excitation at a site distant from the cell body. Fibers stimulated by an intrameatal electrode demonstrated the greatest range of jitter measurements indicating that greater fiber independence may be achieved with intrameatal stimulation.