THE ANTIDROMIC COMPOUND ACTION-POTENTIAL OF THE AUDITORY-NERVE

1. The antidromic compound action potential (ACAP) of the auditory nerve was evoked by shocks to the auditory nerve root and recorded at the round window of the cochlea in anesthetized guinea pigs. The goal of this study was to determine the characteristics of the ACAP and compare these characteristics with those of the orthodromic, sound-evoked compound action potential (CAP). 2. The ACAP consists of an initial complex of a positive peak (p(1)) followed by a negative peak (n(1)). In contrast, the CAP consists of a negative peak (N-1) followed by a positive peak (P-1). These differences in waveform are likely due to the differences in conduction direction, antidromic for the ACAP vs. orthodromic for the CAP. 3. After the initial complex, the ACAP has a second complex of peaks (p(2), n(2)) at a latency of similar to 1 ms; this complex is much smaller in amplitude than the initial complex (p(1), n(1)). It is likely that the initial ACAP complex reflects firing of auditory-nerve fibers whereas the second complex reflects firing of neurons further centrally, perhaps in the cochlear nucleus, that are activated by orthodromic firing of auditory-nerve fibers. 4. Experiments with shock pairs are consistent with the idea that for auditory nerve fibers, the absolute refractory period is <0.5 ms, and the relative refractory period is between 0.5 and at least 5 ms. 5. Experiments with click-shock pairs indicate that a shock interferes with the response to a click when the click and shock are given at about the same time. A click interferes with the response to a shock when it precedes the shock by similar to 2 ms. These time differences are present because ACAP and CAP have different latencies, the latter having a synaptic delay via the hair cells. 6. The ACAP is much more resistant than the CAP to the effects of noise masking, probably because the generation of the ACAP does not involve the hair cell-nerve fiber synapse, where adaptation arises. The ACAP is also more resistant than the CAP to the effects of asphyxia, probably because the generation of the ACAP does not involve inner hair cells, which have responses that are quickly reduced during asphyxia.