Progress report on a hormonally sensitive neuromuscular system

Abstract

The bulbocavernosus (BC) muscles are attached to the base of the penis and are active during copulation. In several rodent species, BC muscles are innervated by motoneurons that form the spinal nucleus of the bulbocavernosus (SNB). The morphology of SNB motoneurons and of the BC muscles that they innervate, as well as the behavior mediated by those muscles, is sensitive to androgenic hormones both during development and upon maturity. The BC muscles and SNB cells of adult male rats shrink after castration, but the administration of androgens can prevent these changes. Androgens also influence developing SNB cells: at birth, female rats have BC muscles innervated by SNB motoneurons, both of which die soon thereafter. Testicular androgen secretion in male rats prevents the death of these cells, resulting in sexual dimorphism. Perinatal androgens can masculinize females by preventing the death of SNB cells and their targets, but only during that critical period in which they normally die. This androgen-induced sparing of SNB motoneurons is an indirect result of hormone action upon the peripheral target muscle. There is also a sexual dimorphism in the spinal nucleus innervating the BC in humans.