Denervation study of synapses of organ of corti of old world monkeys

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Abstract
Fine structural characteristics of synapses in the spiral organ of Corti were examined, with reference to differences between inner and outer haircell systems, and to location of neurons of origin of efferent axons. Surgical interruption of crossed olivocochlear bundle, of vestibular nerve, of facial nerve, and excision of superior cervical ganglia were used to determine the pathways of efferent axons. Interruption of the vestibular nerve near the brainstem results in degeneration of all efferent terminals on outer hair cells. Mid‐line lesions at, and caudal to, the facial colliculus result in degeneration of about half of these efferent terminals. Efferent synaptic bulbs to the inner hair‐cell system are small, of the order of one micron, and form type 2 junctions with afferent dendrites. They tend to have more large dense‐core vesicles (about 80 nm) than the large efferent terminals of the outer hair‐cell system, and appear to be the terminals of axons in the habenula perforata, which exhibit varicosities laden with large dense core vesicles. The varicosities are unaffected by excision of the superior cervical ganglia. So far as our material can reveal, it appears that the varicosities in the habenula perforata do not survive vestibular root interruption, nor do the efferent processes in the internal spiral bundle or at the base of inner hair cells. Most interestingly, the afferent processes of the inner hair‐cell system, as identified for example by their relation to pre‐synaptic bodies in the inner hair cells, are subject to a trans‐synaptic reaction after severance of the vestibular root. They undergo a dramatic cytological transformation, characterized by increase of volume, engorgement with microtubules, microfilaments, microvesicles of various sizes, and clusters of lysosomes. Thus, both the efferent and afferent terminals of the inner hair‐cell system show marked cytological differences from the corresponding terminals of the outer hair cell system.