Efferent acoustic neurons within the lateral superior olivary nucleus of the guinea pig.

The cells of origin of the olivocochlear bundles in the guinea pig were labeled by retrograde axonal transport with either the fluorescent dye 'Fast Blue' or horseradish peroxidase (HRP). Labeled cells were found bilaterally in the superior olivary complex, as described previously. Moreover, an extremely large population of labeled cells occurred within the cell mass of the lateral superior olivary nucleus (LSO) ipsilaterally to the injection site. Preliminary electron microscopical observations confirmed that the labeled LSO cells were distinctly smaller than the unlabeled LSO cells. The labeled cells contained a large nucleus that exhibited extensive invaginations.