THE SPINAL CORD IN MAMMALS In rats the bulbo avernosus BC muscles that surround

The spinal cord in mammals in rats the bulbo

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THE SPINAL CORD IN MAMMALS In rats, the bulbo- avernosus (BC) muscles that surround the base of the •enis are innervated by motor neurons in the spinal nucleus of the bulbocavernosus (SNB). Male rats liave about 200 SNB cells, but females have far fewer notor neurons in this region of the spinal cord. On the day before birth, female rats have BC mus- ies attached to the base of the clitoris that are nearly s large as those of males and that are innervated by notor neurons in the SNB region (Rand and Breedlove, 987). In the days just before and after birth, however, many SNB cells die, especially in females (Nordeen et aL, 1985), and the BC muscles of females die. A single injection of androgens given to a newborn Jemale rat permanently spares some SNB motor neurons and their muscles: a pow- 1 organizational effect. Castration of newborn males, accompanied by prenatal Dkxrkade of androgen receptors, causes the BC muscles and SNB motor neurons to ie as in females. The system also dies in newborn androgen- insensitive rats, so aro-matization seems to be unimportant for masculine development of the SNB. Androgens act on the BC muscles to prevent their demise, and this sparing of the muscles likewise causes the innervating SNB motor neurons to survive (Fish- n et al., 1990; C. L. Jordan et al., 1991). The exact site within the BC muscle where androgen has its effects is still unknown (Niel et al., 2009). But whatever the site of action is, the likely result is that the muscles are induced by androgen to provide a signal, such as a neurotrophic factor, that prevents SNB motor neurons from dying FIGURE 12.22) (Forger et al, 1997). The developmental rescue of SNB motor neurons is accomplished indirectly as a consequence of actions on muscle, but androgens can also directly affect the neurons themselves. SNB neurons contain androgen receptors and retain androgen sensitivity throughout life. In adulthood, androgen acts directly on the neurons to cause them ...and her SDN- POA is small. A normal female rat lacks both the perinatal and pubertal rises in testosterone levels... A female injected with testosterone at the time of the male perinatal surge... ...develops a large SDN-POA. Testosterone Injections at later times, such as puberty... .. .have no effect on the size of a female's SDN-POA. Adult 12.21 SEXUALLY DIMORPHIC NUCLEUS OF THE PREOPTIC AREA (SDN-POA) Testosterone can permanently enlarge the SDN-POA in rats, but only if given during a "sensitive period" early in life. spinal nucleus of the bulbocavernosus (SNB) A group of motor neurons in the spinal cord of rats that innervate striated muscles controlling the penis. SEX 393 fl (A) Male (B) Female (C) Female (D) Female Conception Birth Sensitive period
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to fro w (Watson et al., 2001) and start producing substances to aid in the formation of Male Female Male rat tt SNB 1 \ { ] ' | In males, circulating testosterone binds to the androgen receptors. In females, the receptors remain unoccupied.
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