Human REproduction

Puberty begins with the initiation triggered by gonadotropin-releasing hormone (GnRH) from the hypothalamus. This process leads to the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland, which in turn stimulates increases in testosterone and estradiol. Several factors can influence the onset of puberty, including nutritional state, body fat, and activity levels, all of which play a role in triggering GnRH increases. Leptin, a hormone secreted by adipose cells, is essential for the onset of puberty, while exercise may inhibit GnRH secretion, potentially delaying puberty in active, slimmer individuals.

The Tanner stages of puberty outline the developmental changes that occur throughout this period. Stage 1 occurs between ages 9–10 for both females and males, marking the beginning of growth spurts before any sexual development. By Stage 2, fat deposition increases, height accelerates (approximately 7–8 cm per year), and females experience breast bud development along with minimal pubic hair growth, whereas males show early genital changes. Stage 3 is characterized by peak height increase (up to 8 cm per year), the appearance of coarse pubic hair, gynecomastia in males, voice changes, and nocturnal emissions. In Stage 4, menarche occurs for females around age 12.7 years, and adult-quality pubic hair begins to develop alongside further genitalia growth. By Stage 5, there are no further height increases after about 16–17 years, and individuals reach mature genital size with adult distribution of pubic hair, coinciding with the first ovulation in females.

There are notable growth differences between sexes during puberty; girls typically enter puberty earlier due to higher estrogen levels, which stimulate growth, while boys experience a delayed growth spurt attributed to increased testosterone levels. Estrogen may also influence linear bone growth in males. Puberty can present several issues, such as precocious puberty, which is defined as onset before age 8 in girls or age 9 in boys, and can be attributed to either central causes, where no cause is found, or peripheral causes linked to tumors. On the other hand, delayed puberty is marked by no development by age 14 and may require intervention if it doesn't resolve naturally.

Hormone replacement therapy (HRT) carries risks including heart disease, stroke, blood clots, and breast cancer, and recommendations emphasize individualized treatment based on symptoms and risks. HRT may present a favorable benefit/risk profile if prescribed before age 60, and an annual evaluation of the regimen is advised.

As men age, testosterone levels diminish, a phenomenon sometimes referred to as "andropause," although this rarely leads to a menopause-like shutdown. Erectile difficulties frequently arise from circulatory issues rather than low testosterone levels, and testosterone replacement therapy is indicated for confirmed hypogonadism (defined as levels below 300 ng/dL). Historical trends indicate that the average age of puberty onset has decreased significantly over time, from approximately 16.6 years in 1860 to about 10.5 years in recent decades. This earlier puberty in girls is linked to rising obesity rates, while the connection for boys remains less clear.

The ejaculation process in men comprises two distinct phases: the emission and expulsion phases. The emission phase involves the movement of sperm from the testes and epididymis to the urethra, controlled by sympathetic nerves. This phase includes contractions in the vas deferens, seminal vesicles, and prostate gland. During the expulsion phase, semen is ejected from the urethra through strong rhythmic contractions of pelvic muscles like the bulbospongiosus and is involuntary once initiated. Following ejaculation, the penis gradually loses its erection during the resolution phase, a time often accompanied by relaxation and drowsiness.

David Reed’s erotic stimulus pathway (ESP) theory presents a psychosocial model of sexual response that emphasizes emotional and cognitive factors. The theory includes four phases: seduction, sensation, surrender, and reflection. Seduction involves creating desire through behaviors like dressing attractively and sharing feelings, while sensation encompasses the experience of sexual stimulation through the senses, influenced by past experiences. The surrender phase corresponds to the experience of orgasm, and the reflection phase entails a review of the sexual experience's meaning and emotional impact, affecting future desires and behaviors.

Physiologically, vasocongestion describes the increased blood flow to reproductive organs during arousal, while myotonia refers to muscle tension during sexual response. The orgasmic platform forms during the plateau phase, characterized by engorgement of the outer vaginal area. The clitoral hood covers the clitoris, and stimulation often leads to orgasm, while Bartholin’s glands secrete lubrication during arousal, with any blockage potentially resulting in cyst formation.

Different types of orgasms exist, including clitoral orgasms, which involve the pudendal nerves, vaginal orgasms, arising from pelvic nerves and cervical stimulation, and blended orgasms that combine both clitoral and vaginal stimulation. Several models have been developed to describe sexual response, including the Masters and Johnson model, which consists of four phases (excitement, plateau, orgasm, and resolution), Kaplan’s triphasic model emphasizing sexual desire, excitement, and orgasm, and the Walen and Roth cognitive model concentrating on cognitive aspects. The Basson model incorporates emotional intimacy, relationship satisfaction, and sexual stimuli.

In men, the sexual response phases parallel those of women but follow a simpler structure: excitement, plateau, orgasm, and resolution. During the excitement phase, there is a rapid onset of erection due to vasocongestion, and in the plateau phase, a stable erection is maintained. The orgasm phase in men consists of two stages: emission, where semen is moved into the tract, and expulsion, where semen is released. The resolution phase features a refractory period, which lengthens with age, and prevents immediate re-stimulation.

The main muscles involved in male ejaculation include the bulbocavernosus muscle at the base of the penis, which rhythmically contracts during the expulsion phase to release semen, along with the vas deferens, epididymis, urethra, seminal vesicles, prostate, and ejaculatory ducts that contract to assist in the emission phase. The anal sphincter also contracts during ejaculation. Female ejaculation may involve the Skene glands, with contractions of the vagina, uterus, and anal sphincter noted during the orgasm phase.

In terms of sexuality and aging, sexual activity often persists into older age. Recent research challenges the notion that aging narrowly correlates with declining sexual activity, showing instead that older adults remain sexually active and often report that such activity is linked to better health and happiness. The pharmaceutical industry has made significant advances in treating sexual dysfunction, with medications like Viagra transforming the management of erectile function, presenting it as a modifiable quality linked to health. Consensual non-exclusive relationships among older adults are also associated with greater sexual frequency and happiness, indicating the potential rewards of non-traditional relationship styles in promoting healthy aging.