Cochlear innervation in the greater horseshoe bat: demonstration of an acoustic fovea.

The innervation of the cochlea of the greater horseshoe bat was investigated by different methods. The regional densities of the spiral ganglion neurons and of the inner and outer receptors were determined from surface specimens and histological sections. The pattern of the unmyelinated fibers was reconstructed in EM serial sections and the efferent pattern separately by localization of cholinesterase activity. The study reveals three regions each adapted to different auditory functions: (1) The region 1.3--5.4 mm from the basal end where the constant frequency segment of the orientation signal (around 83 kHz) is analysed. The neuronal structures of this region are similar to other mammals studied. Since, however, this region has widely expanded frequency mapping, the innervation density per octave is very high. In the region of this 'acoustic fovea' 25% of the receptors and 21% of the spiral ganglion neurons of the cochlea represent 10% of a single octave. (2) The region from 5.4 to 8 mm with frequencies from 40--80 kHz encompasses the frequency modulated segment of the orientation signal. This region is characterized by a high density of spiral ganglion neurons together with a short spiral course of the afferent fibers to the outer receptors. (3) The region from 8 mm to the apex (16 mm) represents frequencies lower than 40 kHz. Here neuronal elements, except for the efferents, are comparable to those of other mammalian cochleae. As important and surprising finding was that there is no efferent fiber to the outer hair cells in any part of the cochlea.