Intracellular study of synaptic events related to phase‐locking responses of cat cochlear nucleus cells to low‐frequency tones

Abstract

Intracellular recording techniques were used to record from 55 cochlear nucleus cells in 46 pentobarbital anesthetized cats. Computer‐generated peristimulus‐time histograms, stimulus phase‐locked histograms, and interspike interval histograms were used to study each unit. For units which show phase locking of discharges an excitatory postsynaptic potential (EPSP) is generated in response to each period of the sine wave. With lower‐frequency stimulation, the probability of a unit discharge occurring with each EPSP was high but never exceeded 0.8. As the frequency of the stimulus increased, the probability of firing to a given period dropped, even though the period of the stimulus was much longer than the refractory period of the neuron. Units showed variable degrees of phase locking. Primary‐like units of the same low characteristic frequency might or might not phase lock to the characteristic frequency. Units which showed phase locking at lower frequencies but not at higher frequencies often also changed response configuration from primary to onset, built‐up or pause. A decrease in stimulus intensity caused a decrease in the amplitude of the EPSD and a lowered probability of firing. Both the EPSP and the phase‐locked unit discharge can contribute to the frequency‐following response recorded with macroelectrodes in the cochlear nucleus. [Work supported by Bank of America—Gianinni postdoctoral fellowship.]