The characteristics of the cochlear after-potential studied in the guinea-pig by perfusion and stimulation.

1. An after-potential which comprises three components can be recorded from scala tympani or scala vestibuli following the termination of a tone burst of more than 20-30 msec duration. The major component is of opposite polarity to the negative summating potential (SP) when recorded from scala tympani (ST) and of the same polarity as the SP when recorded from scala vestibuli.2. The size of this after-potential is dependent on the duration and strength of the tonal stimulus but not on the stimulus frequency or its rate of turn-off.3. A single short tone-pip (10 msec) produces no visible after-potential, but an after-potential is seen following the end of a train of such pips separated by intervals of similar duration.4. Replacement of the perilymph in ST by Ringer solution has a negligible effect on the cochlear responses. However, replacing the sodium ion in Ringer solution by lithium or adding DNP to the perfusate abolishes the after-potential just as in the case of a neuronal after-potential. The latter manoeuvre affects also the N(1), CM and SP, while the former does not.5. It is suggested that there exists an active membrane mechanism at the hair cell/nerve junction, which is responsible for the after-potential and which can be influenced by perfusion of ST. It is further suggested that this neural mechanism makes a contribution to the recorded SP over and above the SP component due to non-linear factors.