Serial section reconstruction of the neural poles of hair cells in the human organ of Corti. II. outer hair cells.

Study of the anatomy of the cochlea, and in particular the morphology of synaptic relationships between hair cells and cochlear neurons, is essential for elucidation of the mechanisms of transduction of mechanical acoustic signals into electrical neural events. Because considerable gaps remain in our understanding of the microscopic anatomy of these synapses, particularly in the human, a reconstruction of the neural pole of outer hair cells of the human organ of Corti was performed. The data are based on 577 serial sections from the basal turn and 368 sections from the middle turn. This provided complete data on 11 and partial data on 9 outer hair cells. Terminal size of afferent fibers on outer hair cells was much more uniform than that found at the base of inner hair cells. Only small bouton-like terminals were found. Branching of afferent fibers was also seen at the base of outer hair cells. Each outer hair cell received approximately two to eight afferent nerve terminals. Multiple synaptic contacts between a single afferent terminal and an outer hair cell were common. Junctional membrane specialization consisted of synapses, desmosomes, coated vesicles and arrays of microtubules and membrane cisternae. Specialization at synapses consisted of asymmetrical membrane thickening. At outer hair cells the presynaptic membrane was thicker than the postsynaptic membrane. At inner hair cells the converse was true. At outer hair cells 35% of synapses had presynaptic bodies, compared to 83% of synapses at inner hair cells. Reciprocal synapses, with both hair cell to neuron and neuron to hair cell polarities, were found only on outer hair cells. Vesiculated efferent terminals were common at the base of outer hair cells. Both axosomatic and axodendritic efferent synapses were found. In addition, the same efferent fibers were found to synapse both on an outer hair cell and on an afferent dendrite. One example of a probable dendro-dendritic synapse in the outer spiral bundle is presented.