Physiology and anatomy of single olivocochlear neurons in the cat.

A surgical approach to the cat's VIIIth nerve has been developed which allows recordings to be made from efferent fibers of the olivocochlear bundle (OCB) as well as primary afferent fibers, without compromising the acoustic responsiveness of either ear. The designation of OCB fibers as those with regular interspike intervals was confirmed in five cases by intracellular labeling with horseradish peroxidase. Labeled fibers could be traced centrally to somata in the superior olivary complex and peripherally to large endings on outer hair cells. The locations of the labeled neurons are consistent with a classification as cells of the medial olivocochlear system [Warr and Guinan (1979): Brain Res. 173, 152-155]. Within the cochlea, efferent neurons branched profusely to innervate as many as 84 outer hair cells over as much as 2.8 mm of the organ of Corti. Efferent units had tuning curves which were similar to those of primary afferents, although most were somewhat more broadly-tuned than afferents from the same animal. The cochlear region innervated by an efferent neuron was always close to the place where afferent fibers of the same characteristic frequency (CF) would be found. Most efferents (89%) were excited by only one ear and showed no spontaneous activity. Neurons with an ipsilateral response (n = 3) had cell bodies in the contralateral brainstem and vice versa (n = 2). Binaural units (none of which were labeled) often had spontaneous discharge and were generally restricted to low-CF regions. Differences between low- and high-CF units, which cut across the monaural/binaural distinction, were seen in the dynamic range and minimum latency. Interanimal differences seen in the responses of the efferent neurons may be related to differences in the level of anesthesia.