C H A P T E R 3 9
Introduction to the reproductive system
603
occurs. This is similar to female menopause. The hypo-
thalamus and anterior pituitary put out larger amounts
of GnRH, FSH and LH in an attempt to stimulate the
gland. If no increase in testosterone or inhibin occurs,
the levels of GnRH, FSH and LH eventually return to
normal levels.
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The testes produce sperm in the seminiferous tubules
in response to FSH stimulation, and testosterone in
the interstitial cells in response to LH stimulation.
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Testosterone is responsible for the development of
male sex characteristics. These characteristics can be
maintained by the androgens from the adrenal gland
once the body has undergone the changes of puberty.
KEY POINTS
THE HUMAN SEXUAL RESPONSE
Many animals require particular endocrine stimuli,
called an oestrous cycle, for sexual response to occur.
Humans and ferrets are the only animals known to be
sexually stimulated and responsive at will. Humans can
be sexually stimulated by thoughts, sights, touch or a
variety of combined stimuli. The human sexual response
consists of four phases:
• A period of stimulation with mild increases in
sensitivity and beginning stimulation of the
sympathetic nervous system
• A plateau stage when stimulation levels off
• A climax, which results from massive sympathetic
stimulation of the body
• A period of recovery or resolution, when the effects of
the sympathetic stimulation are resolved (Figure 39.6)
Previously, it was believed that male and female
responses were very different. However, it is now thought
that the physiology of the responses is quite similar.
Sexual stimulation and activity are a normal response
and, in healthy individuals, are probably necessary for
complete health of the body’s systems. The sympathetic
stimulation causes increased heart rate, increased blood
pressure, sweating, pupil dilation, glycogenolysis (break-
down of stored glycogen to glucose for energy) and other
sympathetic responses. This stimulation could be dan-
gerous in some cardiovascular conditions that could be
exacerbated by the sympathetic effects. In the male, the
increased blood flow to the penis causes erection, which
is necessary for penetration of the female and deposition
of the sperm. Any drug therapy or disease process that
interferes with the sympathetic response or the innerva-
tion of the sexual organs will change the person’s ability
to experience the human sexual response. This is import
ant to keep in mind when teaching and when evaluating
the effects of a drug.
Growth of male and sexual accessory organs (penis,
prostate gland, seminal vesicles, vas deferens)
Growth of testes and scrotal sac
Thickening of vocal cords, producing the deep, male
voice
Hair growth on the face, body, arms, legs and trunk
Male-pattern baldness
Increased protein anabolism and decreased protein
catabolism (this causes larger and more powerful
muscle development)
Increased bone growth in length and width, which
ends when the testosterone stimulates closure of the
epiphyses
Thickening of the cartilage and skin, leading to the male
gait
Vascular thickening
Increased haematocrit
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BOX 39.3
Effects of testosterone
CNS
Hypothalamus
GnRH
Anterior pituitary
FSH
Seminiferous tubules
Sperm
Inhibin, oestrogens
LH
Interstitial or Leydig cells
Testosterone
FIGURE 39.5
Interaction of the hypothalamic, pituitary, and testicular
hormones that underlies the male sexual hormone system. CNS,
central nervous system; FSH, follicle-stimulating hormone; GnRH,
gonadotropin-releasing hormone; LH, luteinising hormone.
Climax (orgasm)
Plateau
Resolution
Excitement
FIGURE 39.6
Human sexual response.
