The motor innervation of the tympanic muscles in the guinea pig.

The number and the location of the motor neurons innervating the stapedius or tensor tympani muscles in the guinea pig were identified by retrograde axonal transport of the tracer horseradish peroxidase. Tracer injections were made either into the stapedius or tensor tympani muscle and effected the retrograde labeling of neurons in the ipsilateral brain stem. These findings showed that the stapedius motor neurons lie outside the traditionally recognized facial nucleus and are present in two cell columns: ventromedial and dorsomedial to the facial nucleus. These labeled neurons are dissimilar to cells within the facial nucleus, i.e. they are smaller and more fusiform in shape. The tensor tympani motor neurons were found outside the trigeminal motor nucleus. At a rostral level they were located in a region ventral and ventrolateral to the latter nucleus. These labeled neurons were smaller than the trigeminal motor neurons and polygonal in shape. In the animals studied there were about six times more tensor tympani motor neurons than stapedius motor neurons.