Abstract
The perineal muscles and their spinal motoneurons are sexually dimorphic in adult mammals. The onset of this sexual dimorphism underlies the production of sex specific behaviors. In fact, in male rats the bulbocavernosus (BC), the ischiocavernosus (IC) and the levator ani (LA) muscles attach to the penis and participate in copulatory reflexes such as erection. In adult female rats these muscles are atrophied (Cihak et al., 1970) and made up of few muscle fibers (Tobin and Joubert, 1988). The spinal motor nuclei innervating perineal muscles in the rat are located in the lumbosacral spinal cord (Schroeder, 1980), being the homologous of Onuf’s nucleus in man (Onuf, 1900). In males two distinct neuronal columns are recognizable: a dorsolateral one, generally for the IC, the external vesical sphincter and the external anal sphincter, and a dorsomedial one, for the BC and the LA muscles overall (Schroeder, 1980). In females, these motor nuclei are made up of fewer and smaller motoneurons, mostly innervating the nondimorphic anal and vesical sphincters (McKenna and Nadelhaft, 1986).
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