Basilar membrane motion in the alligator lizard: Its relation to tonotopic organization and frequency selectivity

Abstract

The configuration of the inner ear in the alligator lizard allows (from a single approach) electrical and mechanical probing of the entire sensory organ. Studies of nerve fiber responses [Weiss, Mulroy, Turner, and Pike, Brain Res. 115, 71–90 (1977)] and intracellular responses in the receptor organ [Weiss, Mulroy, and Altmann, J. Acoust. Soc. Am. 55, 606–619 (1974)] have demonstrated that responses can be placed in two categories based on their frequency selectivity [viz., a high characteristic frequency (CF) group associated with the basal end of the organ and a low CF group associated with the apical end]. These two regions of the cochlea differ in morphology of the cilia and tectorial membrane [Mulroy, Brain Behav. Evol. 10, 69–87 (1974)]. Measurements of velocity using the Mossbauer method indicate that the motion of the basilar membrane is virtually the same in both regions. Thus, tonotopic organization and frequency selectivity are not determined by the basilar membrane motion. This finding suggests that other mechanisms, perhaps involving the cilia and tectorial membrane, provide spatially varying frequency selectivity in this species. Similar mechanisms may play a role in mammalian cochleas, where the basilar membrane apparently provides tonotopic organization and at least some of the frequency selectivity. [Supported by NIH.]