Perineal musculature and its innervation by spinal motoneurons in the male rabbit: effects of testosterone.

Abstract

Striated muscles in the perineum, which include the ischiocavernosus (IC), bulbocavernosus (BC), and levator ani (LA), were identified in an attempt to understand motor regulation of penile erection in the male rabbit. The IC surrounded the corpus spongiosum of the penis whereas the BC attached to the dorsum of the penis at the midline. The LA encircled the rectum and attached to the base of the penis. This anatomy suggested that the IC plays the primary role in the erection in the rabbit, whereas the BC may cause flips of the erected penis. Spinal motoneurons that innervate the IC were identified by retrograde labeling by horseradish peroxidase (HRP). As in other mammals, spinal labels from the IC appeared ipsilaterally in the ventral horn that encompassed the sixth lumbar (L6) and the first sacral segments. HRP injections into the BC labeled a small number of cells bilaterally at the same spinal levels. Rabbits are peculiar in having the IC motoneurons scattered among other motoneurons, unlike the rat and other rodents that have their IC motoneurons aggregated to form a spinal nucleus. Castration caused significant decreases in both the wet weight of IC muscles and the size of IC motoneurons within 2 weeks. Testosterone supplement following castration maintained the IC muscle weight and the neuronal size. Neither castration nor testosterone supplement induced changes in the number of IC motoneurons.