| Abstract #569, Date 2/16/99, Session R3, Poster (B69) |
| The contributions of ionic conductances in the dynamic response of an octopus cell model |
| *Y. Cai, J. McGee, E.J. Walsh (Boys Town National Research Hospital)
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| The responses of octopus cells in the ventral cochlear nucleus have been variably attributed to intrinsic membrane properties, low membrane impedance, and/or synaptic inputs. While the importance of a low membrane impedance is generally acknowledged, views differ on the role of ionic channels. On one extreme, ionic channels are modeled as a static leakage conductance (Levy & Kipke, 1997). Results from other studies emphasize the importance of a hyperpolarization-activated inward rectifier (Ih) in producing the low membrane impedance (Golding et al., 1995), and still others suggest that a low-threshold K channel (KLT) is a key element affecting responses in this cell group (Feng et al., 1994; Cai et al., 1997; Ferragamo & Oertel, 1998).
In this study, a computer model was used to simulate octopus cell responses to both current injection and acoustic stimulation. Experimentally recorded spike trains from auditory-nerve fibers served as the model's input to simulate acoustic stimulation. Our original model (Cai et al., 1997) was modified by increasing the contribution of KLT to overall resting membrane conductance and decreasing the contribution of Ih to accommodate recent experimental data. Two criteria were used to evaluate the relative contribution of the ionic channels: the model's sensitivity to rate of change in membrane voltage when initiating an action potential under current stimulation (Ferragamo & Oertel, 1998), and the PSTH pattern produced by acoustic stimulation.
Our results indicate that, during the ramp-up stage of current stimulation, the membrane is depolarized, activating KLT, increasing membrane conductance and dynamically increasing the current required to evoke an action potential. We conclude that, under these conditions, KLT is key to the model's sensitivity to rate of change in membrane voltage, and its dynamic role cannot be provided by a static membrane leakage conductance. Results also demonstrated that a model based mainly on KLT conductance produces realistic onset responses to acoustic stimulation. Systematically replacing the KLT with the Ih or leakage conductance reduces the model's sensitivity to the rate of change in membrane voltage and makes the model's response to acoustic stimulation more chopper-like. These findings support the idea that KLT, which operates during depolarization, is the primary membrane conductance shaping the dynamic response of octopus cells. Ih, on the other hand, is activated during hyperpolarization and probably does not play a major role in determining the responses of octopus cells under normal physiological conditions. |
| Supported by the NIDCD DC01007, and NIDCD P60 DC00982-06 (YC) |
Citation for this abstract:
Cai, Y., J. McGee, and E.J. Walsh (1999). The contributions of ionic conductances in the dynamic response of an octopus cell model Assoc. Res. Otolaryngol. Abstr.
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