[Innervation of the stapedius muscle. A study using horseradish peroxidase in the guinea pig]

The numbers and locations of motoneurons to the stapedius muscle were determined by retrograde axonal transport of horseradish peroxidase (HRP) in the guinea pig. With the aid of a drill the bulla was opened and the stapedius muscle exposed. HRP solution (0.2-0.3 microliter) was injected into the muscle. Following a survival time of 1-2 days the animals were perfused with a fixative and the brains were removed. Frozen sections were reacted fro the demonstration of HRP using tetramethylbenzidine as chromagen. All labelled neurons were observed not within the traditionally recognised facial nucleus but ventral and medial to the latter. All parent cells (up to 64) lay ipsilateral to the injection. These neurons were remarkably smaller in shape than the neurons in the facial nucleus.