Neural tuning in the granite spiny lizard.

Scanning electron microscopy was used to examine the basilar papilla of the granite spiny lizard. The papilla contains three distinct hair cell populations: an apical and a basal population with free-standing cilia, and a central population with a tectorial membrane. In the free-standing populations, stereocilium length decreases towards the ends of the papilla. Ciliary tuft morphology differs in the free-standing and the tectorial membrane populations, except that several of the free-standing hair cells with the shortest stereocilia have a tuft morphology like the hair cells in the tectorial membrane population. On the basis of single-fiber physiology, auditory nerve fibers can be divided into a low characteristic frequency (CF) and a high CF population. Mappings of the tonotopic organization of the nerve demonstrated two groups of high CF fibers that correspond to the two free-standing hair cell populations. The low CF fibers are associated with the tectorial membrane hair cell population. Fiber CF correlated with hair cell cilium length, not position on basilar membrane, for hair cells with free-standing cilia. Tonotopic organization of high CF fibers could be predicted reasonably well from the histogram of fiber CFs.