Intracellularly labeled stapedius-motoneuron cell bodies in the cat are spatially organized according to their physiologic responses

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Abstract
This study examines whether the locations of stapedius-motoneuron cell bodies are correlated with their responses to sound. Single-unit recordings and injections of horseradish peroxidase were made in axons of stapedius motoneurons in the fascicles which run from the facial nerve to the stapedius muscle in the cat. Single units were characterized physiologically by their responses to ipsilateral, contralateral, and binaural sounds. Labeled cell bodies (N = 28) were found in all of the brainstem regions previously identified as containing stapedius motoneurons. Motoneurons characterized as having similar response properties had cell bodies in relatively circumscribed locations. Most (eight of 12) motoneurons excited by sound in either ear had cell bodies in a narrow band around the facial nucleus. Most (seven of eight) motoneurons excited by ipsilateral but not contralateral sound had cell bodies in the cleft between the superior olivary complex and the facial nucleus. All four motoneurons excited by contralateral but not ipsilateral sound had cell bodies located ventromedial to the facial nucleus. The three motoneurons excited only by binaural sound had cell bodies located dorsal to the superior olivary complex. (Two of these were also in the cleft between the superior olivary complex and the facial nucleus.) The cell body of the one motoneuron showing activity in the absence of sound stimulation was located dorsolateral to the facial nucleus. These results show that the cell bodies of stapedius motoneurons with similar electrophysiologic properties tend to have similar locations in the brainstem. The results are consistent with the idea that the stapediusmotoneuron pool is divided into subgroups that are spatially segregated in terms of their patterns of input from the two ears.