The spinal cord in mammals in rats the bulbo

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

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.