Onuf's nucleus of the sacral cord in a South American monkey (Saimiri): its location and bilateral cortical input from area 4.

A histological description of Onuf's nucleus, first described by Onuf as a sacral center for some striated muscles associated with the acts of erection and ejaculation in man, is reported in a South American monkey (Saimiri). Onuf's nucleus in Saimiri was organized into a well-defined cell group which consisted of medium-sized neurons with large and abundant Nissl bodies and was located in the ventrolateral aspect of the ventral horn of the first sacral segment of the spinal cord. Following horseradish peroxidase (HRP) injection into the external anal sphincter muscle in Saimiri, labeled neurons were found at the ipsilateral Onuf's nucleus. Additionally, a single injection of tritiated proline was made in the tail portion of area 4 of two Saimiri in order to study the cortical afferent connections of Onuf's nucleus. Autoradiographs revealed that corticospinal fibers might project directly to Onuf's nucleus. Onuf's nucleus, however, may be different in its cortical afferent connection compared with other somatic motor nuclei in the ventral horn of the cord. The termination is bilateral, i.e. some contralateral lateral corticospinal tract fibers apparently recross to end in the ipsilateral Onuf's nucleus.