Puberty

The endocrine system plays a key role, but by no means the only role, in triggering puberty. At the onset of puberty, a number of factors combine to cause a burst of gonadotropin releasing hormone (GnRH) from the hypothalamus. GnRH causes release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. FSH and LH, in turn, “switch on” gamete production. In males, spermatogenesis begins; in females, menstrual periods begin, the first of which is called menarche.

In both genders, puberty is also marked by the development of secondary sexual characteristics. Primary sexual characteristics are what differentiate little boys (penis, testes, scrotum, associated glands) from little girls (vulva, ovaries, uterine tubes, uterus, vagina, associated glands). In short, primary sexual characteristics can be visualized at birth and define biological gender.

Secondary sexual characteristics are those that differentiate little boys from men and little girls from women. They are gender-related but not necessarily gender-specific. For example, males are generally more muscular and have deeper voices than do females, but these things do not make a person “male.” In other words, there are some women who are more muscular and have a deeper voice than some men.

Male secondary sexual characteristics include body hair (face, chest, pubic, axillary, arms, legs), deeper voice, more prominent thyroid cartilage (Adam’s apple), more muscular, and larger stature.

Female secondary sexual characteristics include body hair (pubic, axillary, arms, legs), higher voice, larger breasts, and wider hips.

Androgens and estrogens also alter the structure of the brain. It’s no secret that men and women tend to think differently, although this wasn’t supported scientifically until the late 1990s, when a landmark study showed clearly that male and female brains are different both structurally and physiologically.

Puberty is a cataclysmic event. The brain is changing, the body is changing, and interpersonal relationships are changing. It’s true that an internal developmental clock participates in the process, but scientists have also shown that internal cues and external cues have an effect as well. Sex steroids act on the brain to alter behavior, and other neurotransmitter systems change as well.

Internal cues, external cues, and input from the internal developmental clock are all fed into an “integrator,” a sort of funnel that collects the disparate sources of information. If/when the integrator reaches a predetermined set-point, it directs the hypothalamus to ramp up GnRH synthesis. Unfortunately, we don’t know what functions as the integrator, we don’t understand the internal developmental clock, the internal cues, the external cues, or how much weight each of them carries, and we don’t know the predetermined set-point.

Remember that menarche is the term for the beginning of menstrual flow in a female. The cessation of ovulation (and, therefore, cessation of monthly menstrual cycles) is called menopause. Menarche is a relatively sudden event; it is usually apparent within a single menstrual cycle. On the other hand, menopause is a gradual process that takes many years to develop. Typically, at the beginning of menopause, ovulation becomes more and more erratic until it finally ceases. As the ovarian cycle becomes more unreliable, the characteristic hormonal changes of the ovarian cycle also begin varying more, and hormonal fluctuations occur that cause the characteristic symptoms of menopause.

The data represented in the Average Age at Menarche image refers to 5530 US women who participated in the 2015-2017 National Survey of Family Growth, conducted biennially by the National Center for Health Statistics. It represents the average age of menarche across all female participants. In actuality, age of menarche and age of menopause both vary significantly based on family genetics, race, ethnicity, geographical region, nutrition, and other factors.

Sexual Response

The study of the anatomy and physiology of human sexual response has a short history. It was only with the landmark research of Kinsey, Pomeroy, and Martin (1948) that it even began as a field of research. In the 1960s, William Masters and Virginia Johnson, first collaborators and later husband and wife, advanced the field greatly.

Some of the difficulties inherent in this field of study are obvious; others, not so much. For example, studying male sexual response is relatively easy, in both the stimulus and response. The factors which produce a male sexual response are well-understood, and the penile plesmythograph (an instrument for measuring penile erection) has been in use for decades.

Studying female sexual response is not at all simple. Masters and Johnson approached the female sexual response as if it operated, anatomically and physiologically, the same as the male sexual response. For example, instead of the plesmythograph, they used a glass capsule in the shape of a penis with a camera inside to visualize the female vagina during simulated intercourse. Turns out there is a lot more going on.

For both male and female, there are at least two components to the sexual response. The first, the reflexive component, is easiest to study and is well-understood. The second, the psychological component, is much less understood.

The reflexive component is primarily controlled by the autonomic nervous system and is wired through spinal nerves S2-S4 (sensory); S2-S4 (parasympathetic motor); and T10-L2 (sympathetic motor). Descending influences from the cortex are obviously important in this system, probably much more so in the female than in the male.

In males, activation of the sympathetic response (fight-or-flight) promotes flaccidity. Sympathetic neurotransmitters, like norepinephrine and epinephrine (acting through α-adrenoceptors) cause blood vessels supplying the penis to constrict, which prevents erection. This should make sense, as it’s pretty obvious a penile erection has no place in the fight-or-flight response.

On the other hand, parasympathetic activation promotes penile erection. The hypothalamus and limbic systems of the brain control the psychological aspects of penile erection. Stimulation of nerves leads to release of nitric oxide (NO), vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), and prostaglandin. Let’s take a closer look at NO.

Three things cause a male penis to become erect.

1. The smooth muscle of the corpora cavernosa relax, allowing blood to fill the open spaces in this spongy tissue.
2. Arterial blood flow to the penis increases.
3. Out-flow of blood from the penis is restricted (venous congestion)

Nitric oxide (NO) promotes dilation of blood vessels throughout the body. For example, nitroglycerine, a precursor of NO, is used to relieve heart-related chest pain (angina) by dilating coronary blood vessels and increasing blood flow to the heart. NO has the same relaxing effect in the blood vessels in and around the penis. Increased NO promotes penile erection by stimulating all three of the above processes.

The popular drug sildenafil (Viagra™) works similarly to NO, producing greater smooth muscle relaxation, increased blood flow to the penis, and improved and prolonged penile erection. It should be apparent why men should not take nitrates at the same time they’re taking Viagra.

Female arousal is somewhat more complicated. In terms of the mechanics of arousal, the same mechanisms seen in the penis are also observable in the clitoris (blood filling erectile tissue).

However, trials aimed at improving female sexual response using NO agonists (like Viagra™) have been complete failures. Yes, NO agonists increase and prolong erection of clitoral tissues, but this does not equate with improved female sexual arousal. We conclude that, while there is little more to male sexual response than penile erection, in females, merely causing clitoral erection is not enough to produce sexual arousal.

Direct imaging of female genital tissues shows that two areas are consistently engorged in sexual arousal: the glans clitoris and the vestibular bulb. The role of the glans clitoris in sexual arousal has been established for some time and is not controversial. However, there is much more to the clitoris than what is visible in the vulvar area (the glans clitoris). The rest of the clitoris (crus clitoris and vestibular bulb) is internal and lies on either side of the vaginal entrance. Also, the vestibular bulb, the pubocervical fascia, and the paraurethral glands embedded in nearby tissue, are homologous to the corpus spongiosum in males, so embryonic origin is consistent with a similar role in female sexual arousal.

A third region that has been the source of some controversy in the medical literature is the so-called Gräfenberg spot (“G-spot”), a location in the anterior vaginal wall near the position of the pubocervical fascia. Some investigators argue that the tissue in this area may be sexually responsive as well, but consistent data has been difficult to gather. It is possible that there are significant individual differences in the sexual responsiveness of this area.

Clearly, there is much more to be learned about the human sexual response, particularly in the female.

Birth Control

Conception results from intercourse, period. However, the process is complicated, as we have seen. A couple unsuccessfully trying to conceive are not happy with the odds they face. On the opposite end of the spectrum, two individuals whose instincts have overwhelmed their good sense aren’t going to be pleased with their odds, either.

For those who do not want to become parents, or who have completed their family, several birth control options are available.

The most obvious and reliable method to prevent conception is to not have sex, ie abstinence. While this method has the advantage of being absolutely reliable, it remains unpopular for reasons that are beyond the scope of this course.

The next most reliable methods are surgical. These have the advantage of not depending on the user following instructions (many find it difficult to follow instructions at a time when it probably matters most). For males, the method of choice is vasectomy. The name derives from the old name of the ductus deferens (vas deferens). A segment of the ductus deferens is removed through a small incision in the superior scrotum and the cut ends of the duct are ligated (tied off). The procedure is repeated for the other ductus deferens via a second incision on the other side of the scrotum. This is a simple and effective surgery, which prevents transport of sperm from testes to urethra.

Ejaculation will still occur, but the accessory gland secretions will be absent any sperm. On very rare occasions, the procedure can produce nerve damage that prevents erection. It is also not easily reversible, as scar tissue forms in the cut tubes and one can’t simply reattach them.

Similarly, in females, the surgery of choice is a tubal ligation, which involves cutting the uterine tubes (Fallopian tubes) and ligating the cut ends to prevent sperm and secondary oocyte from ever meeting. Since tubal ligation requires abdominal surgery, it is somewhat more expensive and complicated than vasectomy. Like vasectomy, it not (reliably) reversible.

The remaining methods are less reliable, partly because they depend more on patient compliance. Hormonal methods are popular; however, there is a relatively complicated sequence to the oral contraceptives. For this reason, some women choose a subcutaneous implant of time-release hormone (Depo-Provera™). The failure rate of this method is less than 1/200 for each year that it’s used.

Intrauterine devices prevent the implantation of an embryo. Since they are placed by a physician and then do not need maintenance, their reliability is relatively high.

The remaining methods of contraception have unacceptably high rates of failure for people who cannot seem to follow instructions. For example, a male condom has a 6% failure rate per year if properly used (i.e. about 1 in 16 men become fathers if they have intercourse regularly using a condom). If the condom is not properly used, the failure rate is 14% per year (about 1 in 7).

The rhythm method depends on knowing when a woman will ovulate. This is fairly useful if women have regular cycles; many women do not. A slightly more reliable method is to measure characteristic changes that occur around ovulation (for example, a slight rise in body temperature). However, if intercourse occurs, and then two days later the woman ovulates, conception can occur.