Contributions of ion conductances to the onset responses of octopus cells in the ventral cochlear nucleus: Simulation results

The onset response pattern displayed by octopus cells has been attributed t o intrinsic membrane properties, low membrane impedance, and/or synaptic in puts. Although the importance of a low membrane impedance generally is ackn owledged as an essential component, views differ on the role that ion chann els play in producing the onset response. In this study, we use a computer model to investigate the contributions of ion channels to the responses of octopus cells. Simulations using current ramps indicate that, during the "r amp-up" stage, the membrane depolarizes, activating a low-threshold K+ chan nel, K-LT, which increases membrane conductance and dynamically increases t he current required to evoke an action potential. As a result, the model is sensitive to the rate that membrane potential changes when initiating an a ction potential. Results obtained when experimentally recorded spike trains of auditory-nerve fibers served as model inputs (simulating acoustic stimu lation) demonstrate that a model with K-LT conductance as the dominant cond uctance produces realistic onset response patterns. Systematically replacin g the K-LT conductance by a h-type conductance (which corresponds to a hype rpolarization-activated inward rectifier current, I-n) or by a leakage cond uctance reduces the model's sensitivity to rate of change in membrane poten tial, and the model's response to "acoustic stimulation" becomes more chopp er-like. Increasing the h-type conductance while maintaining a large K-LT c onductance causes an increase in threshold to both current steps and acoust ic stimulation but does not significantly affect the model's sensitivity to rate of change in membrane potential and the onset response pattern under acoustic stimulation. These findings support the idea that K-LT, which is a ctivated during depolarization, is the primary membrane conductance determi ning the response properties of octopus cells, and its dynamic role cannot be provided by a static membrane conductance. On the other hand, I-n, which is activated during hyperpolarization, does not play a large role in the b asic onset response pattern but may regulate response threshold through its contribution to the membrane conductance.