Chapter 9. THE HUMAN
MENSTRUAL CYCLE
Reid L. Norman, PhD
Professor, Pharmacology and
Neuroscience, TTUHSC
Learning Objectives
• Know the hormonal changes that occur
throughout a menstrual cycle
• List the reproductive hormones and where
they are produced
• Diagram how secretion of pituitary and
ovarian hormones is regulated
• Explain how birth control pills work
Hypothalamus
• Releases GnRH (gonadotropin releasing
hormone) in pulses to stimulate pituitary
FSH (follicle stimulating hormone) and LH
(luteinizing hormone) secretion.
• The hypothalamus is under the control of
higher brain centers and feedback from
ovarian steroids
Pituitary
• Secretes LH and FSH in response to GnRH
• Estrogen, inhibin and progesterone from the ovary
regulate the amount of LH and FSH released in a
feedback relationship
• The pituitary also secretes prolactin, particularly
during pregnancy and lactation
• TSH (thyroid stimulating hormone), ACTH
(adrenocorticotripic hormone) and GH (growth
hormone) are secreted by the pituitary and
important for normal fertility
Ovary
• Responds to FSH and LH
• Follicular growth and maturation
• Secretion of estrogen, progesterone and
inhibin
• Ovulation in response to LH
• Corpus luteum function
• Progesterone secretion
Hormonal changes in cycle
E
P
LH
FSH
-14
0
Days from LH peak
+14
Feedback relationships
HYPOTHALAMUS
(GnRH)
PITUITARY
negative
feedback
(LH, FSH)
OVARY
(Estradiol,
Progesterone)
(Inhibin)
negative
and
positive
feedback
Hormonal birth control
• Birth control pills contain estrogens and/or
progestins
• Act by suppressing LH and FSH through
negative feedback mechanisms
• Formulated to mimic the changes that
naturally occur during the menstrual cycle
• Stimulate the uterus so menstruation occurs
Fallopian tube
• Ovum is transported to the uterus through
the fallopian tubes
• Responds to estrogen and progesterone by
altering motility (muscular contractions)
• Fertilization occurs in fallopian tube
• Early embryo does not enter uterus for
several days – until the hormonal conditions
are right for implantation
Uterus
• Uterus has a muscular layer (myometrium) and an
epithelial lining (endometrium)
• Endometrium responds to estrogen in the follicular
phase by growth and to progesterone in the luteal
phase by becoming secretory in preparation for
implantation
• Menstruation – if pregnancy does not occur,
progesterone levels fall and the endometrium dies
and is discharged through the vagina
• The cycle begins again with FSH stimulation
Pregnancy
• When fertilization occurs, the embryo
implants in the endometrium and secrets
hCG (human chorionic gonadotropin)
• hCG stimulate the corpus luteum to produce
progesterone to maintain pregnancy
• Placenta eventually produces progesterone
to maintain pregnancy
Lactation
• After delivery, prolactin is secreted to
stimulate milk production
• Follicular development can be suppressed
which results in amenorrhea by suckling
and this is lactational amenorrhea
• Pituitary tumors that secrete prolactin can
also cause amenorrhea