Permanent noise-induced damage to stereocilia: a scanning electron microscopic study of the lizard's cochlea.

Alligator lizards were exposed to broadband noise ranging in intensity from 106 to 132 dB SPL for two hours and permitted to recover from 19 to 62 days. Hearing loss was assessed by comparing the auditory nerve component of the cochlear potential recorded at the end of the recovery period with that recorded before the noise exposure. The stereocilia in these ears were examined with a scanning electron microscope. These sensory hairs showed pathological changes similar to those described in mammalian cochleas with noise-induced damage. In decreasing order of severity the damage included completely missing auditory papillas, missing hair cells, missing hairs, hairs fallen over, and hairs that were only moderately splayed apart compared with their normal appearance. Long lasting hearing loss seems to be associated with all of these sensory hair pathologies.