Incidence of reciprocal synapses on outer hair cells of the human organ of Corti.

The incidence of reciprocal synapses at the base of outer hair cells of the human organ of Corti was studied by electron-microscopic serial section reconstruction. Reciprocal synapses were found in each of six ears from five individuals. A total of 1,386 serial sections and the neural poles of 25 outer hair cells in two individuals were examined. Fourteen of the 25 outer hair cells (56%) possessed reciprocal synapses. The incidence of reciprocal synapses increased from the first to the third rows of outer hair cells, as determined by the percentage of outer hair cells and afferent nerve terminals of each row demonstrating reciprocal synapses and the average number of reciprocal synapses per hair cell in each row. Hair cells with reciprocal synapses had more afferent and fewer efferent terminals per hair cell than hair cells without reciprocal synapses.