Acoustic responses recorded from the saccular bundle on the eighth nerve of the guinea pig

Previous studies have demonstrated the existence of acoustic responses from the saccule in guinea pigs for stimulus intensities above 75 dB SPL. It is well known that the saccule receives a double afferent innervation: a bundle of the superior branch of the eighth nerve reaches its anterior part and another bundle of the inferior branch innervates its posterior part. These two bundles join just at the exit of the internal auditory meatus. An electrophysiological experiment was performed to investigate whether saccular acoustic responses could be similarly recorded from both bundles. Recordings from an electrode tip of 0.2 mm diameter were obtained in normal animals and also in animals with a complete and selective destruction of the cochlear sensory epithelium induced by treatment with an ototoxic antibiotic combined with a loop diuretic. In both normal and treated animals acoustic responses with the peculiar very short latency were specifically recorded in the area of the junction of the two bundles, they were of maximal amplitude on the bundle of the inferior branch. These data confirm in a more direct way the saccular origin of the short latency acoustic responses and indicate that they are probably conveyed in both saccular nerve bundles but mostly in that of the inferior branch of the eighth nerve. Finally these data show unequivocally that saccular acoustic responses exist in normal guinea pig ears.