Distribution of nerve fibers in the basilar papilla of normal and sound‐damaged chick cochleae

Epifluorescent light microscopy and confocal laser scanning microscopy were employed to visualize the distribution of nerve fibers in whole‐mount preparations of normal and sound‐damaged chick basilar papillae (BP). In normal cochleae, we identified a consistent pattern of nerve processes that ran transversely across the BP. The transverse processes increase in number from the proximal to the distal ends of the epithelium. However, when the processes are separated into populations of thin fibers and thick bundles, the thin fibers are more prevalent in distal regions whereas thick bundles are more extensive in proximal regions. Furthermore, the thick bundles form an elaborate longitudinal network in the border cell and hyaline cell region. Based on these data and on other previous studies, the thin fibers appear to be afferent nerves and the thick bundles represent efferent nerves. When birds are exposed to acoustic trauma, the normal pattern and number of nerve processes is not altered by levels of sound that produce moderate levels of damage, i.e., damage that leads to hair cell loss and regeneration. However, the nerve pattern is disrupted by severe levels of damage that destroy both hair cells and supporting cells. These findings indicate that the level of sound exposure that induces hair cell regeneration may damage the synaptic endings associated with the lost hair cells, but that the nerve processes that give rise to these endings remain intact within the sensory epithelium. In contrast, severe damage destroys both the hair cells and their associated nerve fibers. © 1996 Wiley‐Liss, Inc.