Synaptic morphology of inner and outer hair cells of the human organ of Corti.

Innervations of inner and outer hair cells of the organ of Corti of the human cochlea were studied by serial section electron microscopy. At the base of inner hair cells, presumed afferent fibers were of varying size and demonstrated synaptic specialization consisting of a presynaptic body, vesicles, and asymmetrical synaptic membrane specialization. Two types of neurons, vesiculated presumably efferent and nonvesiculated presumably afferent, synapsed at the base of outer hair cells. The synaptic specialization of afferent fibers included presynaptic body, vesicles, and asymmetrical membrane thickening, whereas efferent synapses demonstrated presynaptic vesicles and a subsynaptic cisterna. Some presumably afferent nerve terminals formed a reciprocal synapse with outer hair cells in both the human and the chimpanzee. Such a synaptic relationship demonstrated morphologic specialization consistent with both hair cell-to-neuron and neuron-to-hair cell transmission between the same outer hair cell and nerve terminal. The innervation density of inner and outer hair cells and the comparative anatomy of the afferent and efferent innervation are discussed.