Anatomy of cochlear innervation.

In this review of cochlear innervation, the differences in the innervation of outer and inner hair cells are emphasized. Of the afferent neurons, 90 to 95 per cent are large, myelinated type I neurons, exclusively connected in an essentially radial unbranched manner to the inner hair cells; 5 to 10 per cent are small, mostly unmyelinated type II neurons connected to the outer hair cells with considerable spiral extension and branching. The few small type II neurons, with their thin unmyelinated axons, probably have a minor functional importance for centripetal information transfer. The functional emphasis of the outer hair cell system is likely at the level of the receptor cells where the outer hair cells monitor receptor function. The efferent innervation also consists of at least two types of neurons. Small neurons from the lateral superior olivary nucleus project to the inner hair cell area in a predominantly homolateral fashion, making almost exclusively synaptic contacts with the afferent dendrites associated with the inner hair cells. Larger neurons from the medial nucleus of the trapezoid body and periolivary nucleus provide the abundant efferent nerve supply of the outer hair cells, predominantly contralateral. They have mostly large synaptic contacts, and, in some species exclusively, with the receptor cells, indicating again the functional emphasis of the outer hair cell system at the receptor cell level.