The Parasympathetic Innervation of the Inner Ear and the Problem of Cochlear Efferents: Enzyme and Autoradiographic Studies

Abstract

It has been assumed for many years that the matter of the existence of olivocochlear and vestibular efferents terminating on hair cells in the end organs of the inner ear is non-controversial. Opposing concepts that the centrally originating efferent fibers of the cochlear and vestibular nerves are parasympathetic (Ross 1969; Eyries and Chouard, 1970), and that the efferent-type terminals on hair cells might be collateral branches of spiral and vestibular ganglion nerve fibers (Ross, 1973) attracted little notice. However, two recent anatomical observations have served to refocus attention on these ideas. These are 1) the finding of multipolar neurons with synapses on their dendrites in human spiral ganglia (Kimura et al., 1979; Ota and Kimura, 1980), which supports the concept of a parasympathetic innervation of the inner ear; and 2) the observation that reciprocal synapses occur on outer hair cells in the human organ of Corti (Nadol, 1980), which substanties the notion that both afferent and efferent-type terminals can originate from the same parent nerve fiber. Additionally, a combined histochemical-physiological study (Goldberg and Fernandez, 1980) has shown that vestibular efferents originate bilaterally in the brain stem, lateral to the facial genu, in the monkey.