Excitation and inhibition in cochlear nucleus. I. Tone-burst stimulation.

Abstract

Firing patterns and membrane potentials were studied in neurons of the cochlear nucleus by means of intracellular electrodes. Stimuli consisted of sequences of short tone bursts of different frequencies. Stimuli were controlled and data was analyzed by an on-line computer (LINC). Previous workers have shown that, for most of these neurons, spike discharges are sustained for the duration of the tonal stimulus. Sustained discharges were associated with a sustained membrane depolarization. In many cases, the sustained discharge contained interruptions which were functions of both tonal frequency and time after stimulus onset. For some neurons the duration of a suppression of firing either during or after a stimulus tone was linearly related to the logarithm of the stimulus frequency; for other neurons, the suppression of firing could not readily be interpreted in terms of a frequency coding process. Hyperpolarization of the neuron membrane was rarely observed during suppression of firing, even where depolarization was observed during spike firing. This suggests that the inhibitory synaptic inflow, as well as a trigger zone, were placed remotely from the somatic location of the electrode. The lack of correlation between the firing patterns and the observed depolarizations suggest that the time courses of the inhibitory and excitatory synaptic inflow are different. Modulation of firing patterns by inhibition plays an important role in stimulus coding at the cochlear nucleus.