Efferent vestibular system in the squirrel monkey: anatomical location and influence on afferent activity

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Abstract
The retrograde transport of horse-radish peroxidase from the labyrinth was used to label the cells of origin of the efferent vestibular system (EVS) in S. sciuresus. The EVS projection to one labyrinth has a bilaterally symmetric origin from about 200 neurons on each side of the brain stem. The main efferent group, consisting of a circumscribed column of cells interposed between the abducens and the superior vestibular nuclei, could also be identified in the adult. The intracranial course of the efferent pathways was traced by acetylcholinesterase histochemistry. Afferent activity was monitored in barbiturate-anesthetized animals while the EVS was electrically stimulated. The response to brain stem stimulation could not be changed from excitation to inhibition by variations in shock intensity, shock frequency, or stimulating locus. Nor could the response be reversed in polarity by large, calorically induced changes in background discharge. The excitation must be due to an activation of the EVS, rather than to a blocking of a tonic inhibitory influence. The response of many units can be desribed as a summation of fast and slow components, having time constants of 10-100 ms and 5-20 s, respectively. Rarely was a unit''s response composed almost entirely of a fast component. The fast and slow components were compared in irregularly discharging afferents. The 2 components differ not only in their time constants and latent periods, but also in their dependency on shock frequency, shock intensity and shock-train duration. Except for the fact that it is excitatory, the fast component resembles the efferent responses observed in other hair cell systems and may be synaptically mediated. The properties of the slow component suggest that it is the result of a peculiar synaptic process or else that it is not synaptically mediated. Efferent activation can alter the magnitude or gain of the response to natural stimulation. In afferents with little or no background discharge, response magnitude is increased. Tonically active, irrregularly discharging neurons can show a modest and transient decrease in afferent gain, provided that the natural stimulus, when presented alone, does not come close to silencing discharge. Consistent effects on afferent gain were not observed in regularly discharging units. The EVS probably functions to extend the dynamic range of the afferents during the large accelerations accompanying intended head movements.