Studies on the whole nerve action potential and summating potential recorded from round window

Abstract

Electrical responses of the cochlea to acoustic stimuli recorded with gross electrode from the round window in guinea pigs. were recorded.Electrical potentials responding to click consist of cochlear microphonics (CM), summating potential (SP) and action potential (AP). Therefore, it is convenient to observe the relationship between SP and AP.In order to explain the relationship of each potentials, the experiment to be reported here was conducted.Owing to difference in latency, it is possible to separate AP from CM and SP. CM is an altenating electrical potential and SP appears as a displacement of the baseline on Which CM is superimposed. AP was divided into N, and N2, . On occasion N3 was seen. The duration of AP wave was measured about 4-5 miliseconds.When the stimulus intensity of the click is increased from low level to high level, the latency of N. and N2. was found to change. (1.3-2.0 miliseconds.) On the other hand, the intensity function of AP was not proportional to the increase of stimulus intensity but showed a non-linear increase.In the state of oxygen defieiency, the effect of asphyxiation on the responses was studied.The following results were obtained.1) SP was the most sensitive:Polarity changes of SP were initiated about 30sec. after asphyxiation, and negative SP could be reversed to positive.2) The polarity changes of SP was accompanied by the changes of latency of N1, 3) Under mild anoxia SP becomes negative.4) SP is observable while CM is evident.5) The course of alternation of SP seemed to be of the same type as that observed in the depression of CM.6) The relationship between SP and pure tone of background was investigated. It was found that CM for pure tone becomes nonsinusoidal at the same points where SP is present.7) AP was more labile than CM:AP was reduced in size immediatly after anoxia, and the course of their recovery depends upon the intensity of test tone.Recording of poststimulatory effect after exposure to white noise of moderate intensity (50-90db.), a depression of neural components were readily noticed by stimulation with white noise of 50db of 5 seconds duration.On the other hand, same the stimulation did not show any influence on the amplitude of CM and SP.With the same duration and intensity of overstimulation, the weaker the click, the greater reduction of the N1 component was seen. A prompt lowering of the N1 component compared to the relative than the stability of CM leads the author to the postulate that there is a part more susceptible to acoustic stimuli than the hair cells.Considering the mechanism of the N1 component generation, the area between the spiral ganglion cell and the hair cell bottom is suggested as such an affected part.By means of a pair of clicks, it was found that the amplitude of the second click response decreased with the changein interval between the two impulses. At an interval less than 30m sec., it was the reduction of the second click response was seen to be caused by the first click stimuli.