Fine structure of cochlear innervation in the cat.

Examination of adult and juvenile cat cochleas by electron microscopy and semi-serial sections permitted identification of the cytological features characteristic of the afferent and efferent nerve fiber populations identified in Golgi impregnations of the cochlea. This study demonstrated the distribution of synaptic contacts made by these fiber populations. As in the Golgi findings, radial and outer spiral afferent fibers were identified in well separated zones of the inner spiral bundle. The trunks of the outer spiral fibers, containing many microtubules and few neurofilaments, at first coursed spirally below the inner hair cells on the proximal face of the inner pillar, turned abruptly between adjacent pillar cells and entered the tunnel without branching. Radial afferents, containing many neurofilaments and a few microtubules, coursed through the inner spiral bundle, maintaining a radial or oblique orientation and proceeded directly toward the inner hair cells. Efferent fibers in the region of the inner spiral bundle were distinguishable by size, by orientation, and, to a lesser extent, by cytology. Small (1 micron) efferent fibers, containing few neurofilaments, an occasional microtubule, and mitochondria, occurred in the inner and tunnel spiral bundles and formed large varicosities, which contacted radial afferents. A separate population of much thicker efferents, containing many neurofilaments, mitochondria and dense-cored vesicles, but no microtubules, did not enter the inner spiral bundle but coursed directly to the level of the tunnel spiral bundle on the proximal face of the inner pillar cells. These fibers crossed the tunnel at the level of the tunnel spiral bundle and, upon reaching the outer hair cells, formed large synaptic contacts on outer hair cells and on outer spiral fibers as well. Some of these efferent fibers also synapse on afferent fibers while crossing the tunnel. The findings agree with previous observations with the Golgi method showing that entirely separate populations of spiral ganglion cells innervate the inner and outer hair cells. Likewise, there are efferent fibers innervating only inner or outer hair cells, but the probability of efferent fibers to both inner and outer hair cells cannot be ruled out.