Origin of N1 wave of the cochlear nerve action potential recorded at the bony wall of the cochlea

: As an objective audiometry, the cochlear nerve action potential (AP) evoked by tone stimulation has been clinically utilized. But the origin of AP wave is still obscure. In order to know the origin of the N1 wave of AP, the author intended to compare the AP waves (AP(W)) recorded at the bony wall of the cochlea with action potentials of the individual spiral ganglion cell (spikes) and group of the spiral ganglion cells (AP(R)) in the Rosenthal canal of the guinea pig. Spikes and AP(R) were recorded by glass pipettes filled with 2M NaCl. The results showed that waves of AP(R) demonstrated a wide monophasic negative wave in contrast to diphasic AP(W) wave. The Input-Output Curves (I.O.C.) of the AP(W) and AP(R) were compared in order to ascertain whether both APs were derived from the cochlear nerve or not. The I.O.C. of latency, onset, amplitude, offset and width of both AP(R) and AP(W) N1 waves showed a same pattern with high correlation coefficient. So it was confirmed that both AP(R) and AP(W) N1 waves were the direct and indirect cochlear nerve action potential respectively. The thresholds of both AP(W) and AP(R) at the characteristic frequency of a single ganglion cell showed a high correlation. The time of onset and latency of AP(R) were longer than those of AP(W). Based on anatomy and electrophysiology, it is concluded that AP(W) N1 wave originates from the action potential generated at the habenula perforata, where spikes are first produced. The curves of the AP(R) were flat in respect of frequency-threshold relationship. The threshold was the lowest in the spiral ganglion cell followed by AP(R) and AP(W).