Human Physiology, 14th edition (2016)

712 Chapter 20

(pigmented areas surrounding the nipples). Pronounced engorge-

ment of the outer third of the vagina produces what Masters and

Johnson, two scientists who performed pioneering studies of the

human sexual response, called the “orgasmic platform.”

In orgasm, which lasts only a few seconds, the uterus and

orgasmic platform of the vagina contract several times. This is

analogous to the contractions that accompany ejaculation in a

male. Orgasm is followed by the resolution phase, in which the

body returns to preexcitation conditions. Men, but not women,

immediately enter a refractory period following orgasm, dur-

ing which time they may produce an erection but are not able to

ejaculate. Women, by contrast, lack a refractory period and are

thus capable of multiple orgasms.

The age at which puberty begins is related to the amount
of body fat and level of physical activity of the child. The aver-
age age of menarche is later (age 15) in girls who are very active
physically than in the general population (age 12.6). This appears
to be due to a requirement for a minimum percentage of body
fat for menstruation to begin; this may represent a mechanism
favored by natural selection to ensure that a woman can success-
fully complete a pregnancy and nurse the baby. Recent evidence
suggests that the secretion of leptin from adipocytes (chapter 19)
is required for puberty. Later in life, women who are very lean
and physically active may have irregular cycles and amenorrhea
(cessation of menstruation). This may also be related to the per-
centage of body fat. However, there is also evidence that physical
exercise may act to inhibit GnRH and gonadotropin secretion.

Pineal Gland

The role of the pineal gland in human physiology is poorly
understood. It is known that the pineal, a gland located deep
within the brain, secretes the hormone melatonin as a derivative
of the amino acid tryptophan (chapter 11; see fig. 11.32) and that
production of this hormone is influenced by light-dark cycles.
The pineal glands of some vertebrates have photoreceptors
that are directly sensitive to environmental light. Although no
such photoreceptors are present in the pineal glands of mam-
mals, the secretion of melatonin has been shown to increase at
night and decrease during daylight. The inhibitory effect of light
on melatonin secretion in mammals is indirect. Pineal secretion
is stimulated by postganglionic sympathetic neurons that origi-
nate in the superior cervical ganglion; activity of these neurons,
in turn, is inhibited by nerve tracts that are activated by light
striking the retina. The physiology of the pineal gland was dis-
cussed in chapter 11 (see fig. 11.33).
There is abundant experimental evidence that melatonin
can influence the pituitary-gonad axis in seasonally breeding
mammals. However, the role of melatonin in the regulation of
human reproduction has not yet been clearly established.

Human Sexual Response

The sexual response, similar in both sexes, is often divided into
four phases: excitation, plateau, orgasm, and resolution. The
excitation phase, also known as arousal, is characterized by myo-
tonia (increased muscle tone) and vasocongestion (the engorge-
ment of a sexual organ with blood). This results in erection of the
nipples in both sexes, although the effect is more intense and evi-
dent in females than in males. The clitoris swells (analogous to
erection of the penis), and the labia minora swell to more than
twice their previous size. Vasocongestion of the vagina leads to
secretion of fluid, producing vaginal lubrication. Vasocongestion
also causes considerable enlargement of the uterus, and in women
who have not breast-fed a baby the breasts may enlarge as well.
During the plateau phase, the clitoris becomes partially hid-
den behind the labia minora because of the continued engorge-
ment of the labia with blood. Similarly, the erected nipples
become partially hidden by continued swelling of the areolae

| CHECKPOINT

3a. Using a flow diagram, show the negative feedback

control that the gonads exert on GnRH and

gonadotropin secretion. Explain the effects of

castration on FSH and LH secretion and the effects

of removal of the pituitary on the structure of the

gonads and accessory sex organs.

3b. Explain the significance of the pulsatile secretion of

GnRH and the gonadotropic hormones.

4a. Describe the two mechanisms that have been

proposed to explain the rise in sex steroid secretion

that occurs at puberty. Explain the possible effects

of body fat and intense exercise on the timing of

puberty.

4b. Describe the effect of light on the pineal secretion of

melatonin and discuss the possible role of melatonin

in reproduction.

4c. Compare the phases of the sexual response in males

and females.

20.3 MALE REPRODUCTIVE SYSTEM

The Leydig cells of the testes secrete testosterone, which stim-

ulates the male accessory sex organs, promotes the develop-

ment of male secondary sex characteristics, and is needed

for spermatogenesis. LH stimulates the Leydig cells, whereas

FSH stimulates the Sertoli cells of the seminiferous tubules.

LEARNING OUTCOMES

After studying this section, you should be able to:

5. Explain the functions of the two compartments of
   the testes, and how they are regulated.


6. Describe the stages of spermatogenesis, and the
   functions of Sertoli cells.


7. Explain how spermatogenesis is regulated.