Mathematical models of cochlear nucleus onset neurons: II. Model with dynamic spike-blocking state

Onset ( On) neurons in the cochlear nucleus ( CN), characterized by their prominent response to the onset followed by little or no response to the steady- state of sustained stimuli, have a remarkable ability to entrain ( firing 1 spike per cycle of a periodic stimulus) to low- frequency tones up to 1000 Hz. In this article, we present a point- neuron model with independent, excitatory auditory- nerve ( AN) inputs that accounts for the ability of On neurons to both produce onset responses for high- frequency tone bursts and entrain to a wide range of low- frequency tones. With a fixed- duration spike- blocking state after a spike ( an absolute refractory period), the model produces entrainment to a broad range of low- frequency tones and an On response with short interspike intervals ( chopping) for high- frequency tone bursts. To produce On response patterns with no chopping, we introduce a novel, more complex, active membrane model in which the spike- blocking state is maintained until the instantaneous membrane voltage falls below a transition voltage. During the sustained depolarization for a high- frequency tone burst, the new model does not chop because it enters a spike- blocking state after the first spike and fails to leave this state until the membrane voltage returns toward rest at the end of the stimulus. The model entrains to low- frequency tones because the membrane voltage falls below the transition voltage on every cycle when the AN inputs are phase- locked. With the complex membrane model, On response patterns having moderate steady- state activity for high- frequency tone bursts ( On- L) are distinguished from those having no steady- state activity ( On- I) by requiring fewer AN inputs. Voltage- gated ion channels found in On- responding neurons of the CN may underlie the hypothesized dynamic spike- blocking state. These results provide a mechanistic rationale for distinguishing between the different physiological classes of CN On neurons.