The temporal relationship between basilar membrane motion and nerve impulse initiation in auditory nerve fibers of guinea pigs.

When the helicotrema was obstructed in guinea pigs, trapezoidal displacement of the round window membrane produced a trapezoidal microphonic which indicated a unidirectional displacement of the entire length of the basilar membrane. Responses of single auditory nerve fibers to the trapezoidal displacement of the round window membrane were recorded after obstruction of the helicotrema. About 39% of the 424 fibers showed tonic responses which demonstrated directional sensitivity. More than 90% of these fibers increased their discharge rate during displacement of the basilar membrane toward the scala tympani and decreased their discharge rate during oppositely directed displacement. Less than 5% of the tonic fibers responded in the reverse manner. About 23% of the auditory nerve fibers responded to onset and/or cessation of trapezoidal motion of the basilar membrane. About 28% showed a combination of the tonic and phasic responses. However it is probably that all phasic responses we observed do not arise from velocity-sensitive fibers but some may represent artifacts. Both possibilities remain open until further studies are performed. The transduction mechanism of the cochlea is discussed on the basis of our data obtained by unidirectional displacement of the basilar membrane.