Formation of Sexual Desire (Libido)
Sexual desire is based on the accumulation of sex hormones—androgens and estrogens—in the systemic circulation. However, in humans, social factors as well as individual and societal experience play a key role in triggering sexual behavior. Desire is actualized when the characteristics of a partner match the template embedded within the action result acceptor (akseptor rezultata deystviya).
Leading neural centers responsible for libido are located in the hypothalamic region, which is directly targeted by circulating sex hormones. From the hypothalamus, excitation spreads via ascending pathways to other brain structures, ultimately reaching the cerebral cortex. Conditioned-reflex stimuli (olfactory, auditory, and visual) are of immense importance.
There are distinct gender differences in the formation of libido:
- In males: testosterone production occurs almost continuously. Prostatic secretion is an important factor: its accumulation in small doses stimulates testicular secretion, whereas excessive accumulation inhibits it.
- In females: desire depends on androgen levels and the degree of estrogen saturation. The physiological peak of libido occurs at the end of the follicular maturation phase and during ovulation, providing optimal conditions for conception.
It is worth noting that sexual behavior is not limited to the action of sex steroids alone; virtually all endocrine glands are recruited into this process.
Preparatory Stage: The Sexual Ritual
Actual sexual intercourse (the introduction of the erect penis into the vagina) is invariably preceded by a sexual ritual. Its primary physiological task is to activate mechanisms of libido and increase the excitability of the corresponding neural centers.
In humans, this ritual is closely linked to the mechanical stimulation of specific erotogenic zones. These include the lips, earlobes, breasts, and external genitalia. Adequate stimulation of these zones establishes a state of physiological sexual readiness:
- In males: penile turgor and rigidity develop;
- In females: swelling of the vaginal mucosa and labia minora occurs, alongside clitoral erection.
Neurophysiology of Erection, Friction, and Ejaculation
The executive mechanisms of sexual intercourse represent a cascade of reflex reactions integrated at the spinal cord level and controlled by the autonomic nervous system.
Erection Erection is mediated by a specific erection center, whose neurons are localized in the lower lumbar and upper sacral segments of the spinal cord (S3–S4 segments). Erection is predominantly a function of the parasympathetic division of the autonomic nervous system, whose impulses induce marked vasodilation of the corpora cavernosa of the penis.
Friction Rhythmic frictional movements lead to intensive stimulation of mechanoreceptors located on the glans penis, clitoris, and vaginal mucosa. Via the internal pudendal nerves, afferent impulses travel to spinal centers, causing an avalanche-like buildup of excitation that culminates in orgasm and ejaculation.
Ejaculation Ejaculation is a coordinated contraction of the smooth muscle elements of the vas deferens, seminal vesicles, prostate gland, and urethra. Unlike erection, this process is entirely controlled by sympathetic innervation. The spinal ejaculation center is located in the L2–L5 lumbar segments of the spinal cord.
Orgasm and Phases of the Sexual Cycle
Globally, sexual intercourse is divided into four sequential phases: excitement, plateau, orgasm, and resolution.
Orgasm represents a form of sensory saturation and manifests as an extremely pronounced emotional reaction. It sanctions the sexual act, integrates into the action result acceptor, and subsequently serves as a potent reinforcer for sexual behavior. In females, it is physiologically customary to distinguish between clitoral and vaginal types of orgasm.
The mechanism underlying the emotion of orgasm involves the ascending propagation of excitation from spinal centers to the hypothalamus and limbic system structures. From there, descending influences are formed that trigger muscular contractions and ejaculation.
Orgasm is accompanied by a massive autonomic response (the so-called "autonomic storm"):
| Parameter | Males | Females |
|---|---|---|
| Heart Rate | up to 150 bpm | approx. 110 bpm |
| Blood Pressure | up to 250/120 mm Hg | approx. 160/130 mm Hg |
| Respiration | sharply accelerated | accelerated |
Following the resolution phase, the excitability of all sexual centers plummets rapidly, concluding the physiological cycle of sexual intercourse.