General pattern and morphological specializations of the avian cochlea.

In different bird species, there is a common pattern in the hair-cell morphology and innervation of the basilar papilla; the absolute values, however, are species-specific. In the barn-owl papilla, an extreme being case, the basal high-frequency part of the papilla is greatly expanded. In this behaviorally most important frequency range of the barn owl, the number of afferent nerve terminals to neural hair cells is extensively increased. Instead of about 2 afferent terminals as in other species, up to 20 afferents are present. In the bird species studied (chicken, starling, emu, barn owl), the area of the afferent nerve terminals correlates well with the best hearing range. There is a continuous transition from neural to abneural, and from apical to basal in the morphological hair-cell parameters. Thus, the only precise and functionally relevant classification of avian hair-cell types (tall hair cells versus short hair cells) must be based on whether the hair cells have an afferent innervation or not. The differentiation of the evolutionarily-new short-hair-cell type is apparently essential in the high-frequency area of the papilla. This probably functionally supportive type has lost its afferent innervation; its function must therefore be within the papilla itself.