Onuf's nucleus X: a morphological study of a human spinal nucleus.

The first, second and third sacral segments of 59 human spinal cords were examined in order to localize and describe Onuf's nucleus X. The nucleus was found to be situated in the ventral horn of the segments S2 and S3; only in very few spinal cords did it extend into S1. A significant variation in the length of the nucleus was observed. Based on the cytoarchitecture the nucleus could be divided in three parts, a cranial, a dorsomedial and a ventrolateral. All parts of the nucleus consisted of chromatin-rich medium-sized neurons, and apparent direct appositions between different cells bodies as well as between cell bodies and large dendrites were observed. Characteristic findings in the neuropil surrounding the nucleus were the sparsity of myelinated fibers and the presence of dendritic bundles. The present observations are compared to the descriptions of a morphologically similar nucleus in experimental animals and a high degree of resemblance is found. As this nucleus in the experimental animals has been demonstrated to innervate muscles of the pelvic floor, including the striated sphincters of the urethra and the anus, it is suggested that the human Onuf's nucleus X has the same function.