Assignment 1
From an anatomical and physiological perspective, describe the menstrual cycle. Pay special attention to the hormonal changes during the cycle. When is a woman most fertile? What are the physical and emotional changes that tend to occur during this cycle, and why do these occur? (15 marks)
Describe the biological and physiological aspects of the sexual response cycle as described by Masters and Johnson. Note: Many researchers and sexual health educators feel the response cycle is incomplete. As part of your response, include your thoughts regarding whether the response cycle accurately represents what happens to the human system during sexual activity. Also, comment on the reasons why many researchers and educators feel it is incomplete. (15 marks)

The Menstrual Cycle

LO 3.5 Describe the regulation and the phases of the menstrual cycle.

Menstruation is the bleeding that results from the shedding of the uterine lining (endometrium) when a reproductive cycle has not led to fertilization of an ovum. The human menstrual cycle averages 28 days in length. (See A World of Diversity: Historical and Cross-Cultural Perspectives on Menstruation.)

The cycle is regulated by the hormones estrogen and progesterone. It can be divided into four phases. The first phase, the proliferative phase, follows menstruation. During this phase, estrogen levels increase, causing the ripening of perhaps 10 to 20 ova (egg cells) within their follicles and the proliferation of endometrial tissue in the uterus. During the second phase of the cycle, estrogen reaches peak blood levels, and ovulation occurs. Normally only one ovum reaches maturity and is released by an ovary during ovulation.

ovulation

The process by which a mature ovarian follicle ruptures and releases an ovum.

The third phase of the cycle—the secretory, or luteal, phase—begins right after ovulation, and continues through the beginning of the next cycle. The term “luteal phase” derives from “corpus luteum,” the name given to a follicle that has released an ovum. The corpus luteum functions as an endocrine gland, producing large amounts of progesterone and estrogen. Progesterone causes the endometrium to thicken, so it can support an embryo if fertilization occurs. If the ovum goes unfertilized, the estrogen and progesterone levels plummet. These falloffs trigger the fourth phase, the menstrual phase, which leads to the beginning of a new cycle.

corpus luteum

An ovarian follicle that has released an ovum. The corpus luteum remains in existence only if pregnancy begins.

endocrine gland

A gland that secretes a hormone directly into the blood, rather than through a duct. Examples include the thyroid, adrenal, and pituitary glands, as well as the ovaries and testes.

Ovulation may not occur in every menstrual cycle. Anovulatory (“without ovulation”) cycles are most common in the years just after menarche. They may become frequent again in the years prior to menopause, but they may also occur irregularly among women in their 20s and 30s.

menarche

(“men-AR-kee”) The first menstrual period.

Although the menstrual cycle averages 28 days, variations are common from woman to woman, and from month to month in the same woman. Girls’ cycles are often irregular for a few years after menarche, but later assume reasonably regular patterns. Variations from cycle to cycle tend to occur during the proliferative phase that precedes ovulation. That is, menstruation tends to follow ovulation reliably by about 14 days. Variations of more than two days in the post-ovulation period are rare. Although hormones regulate the menstrual cycle, psychological factors can influence the secretion of hormones. Stress can delay or halt menstruation.
Regulation of the Menstrual Cycle

The menstrual cycle involves finely tuned relationships between structures in the brain—the hypothalamus and the pituitary gland—and the ovaries and uterus. All of these structures are parts of the endocrine system, which means they secrete chemicals called hormones directly into the bloodstream.

hypothalamus

A bundle of neural cell bodies involved in regulating body temperature, motivation, and emotion. It’s located near the centre of the brain.

pituitary gland

The gland that secretes growth hormone, prolactin, oxytocin, and other hormones.

hormone

A substance secreted by an endocrine gland to regulate various body functions.

Several hormones play important roles in sexual and reproductive functions. The gonads—the testes (or testicles) in the male and the ovaries in the female—secrete sex hormones directly into the bloodstream. The female gonads produce the sex hormones estrogen and progesterone. The male gonads produce the male sex hormone ­testosterone. Males and females also produce sex hormones characteristic of the other sex, but in relatively small amounts.

testes

The male gonads.

testosterone