Components of cochlear electric responses in the alligator lizard.

Electric responses to clicks and tones were recorded at the round windows of anesthetized alligator lizards before and after tetrodotoxin (TTX) was added to scala tympani. By combining click responses obtained in the presence and absence of TTX and at high and low click repetition rates, we trisected click responses into three components: (1) a rate-insensitive, TTX-insensitive component (that we identify as the cochlear microphonic potential or CM); (2) a rate-sensitive, TTX-sensitive component (that we identify as the neural component); (3) a rate-sensitive, TTX-resistant component (which has not been identified previously and which we call component X). Component X is generated in the inner ear and has a latency between that of the CM and the neural component. Several possible origins for component X are discussed of which the most likely is that component X represents the compound post-synaptic potential of the nerve terminals. Measurements of responses to tones in the presence and absence of TTX demonstrate that the contribution of the neural component to the response is appreciable below 1.5 kHz and negligible above this frequency.