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 social experience play a key role in triggering sexual behavior. Desire is realized at the moment when the characteristics of the partner match the image embedded in the action result acceptor.
The leading nerve centers responsible for libido are located in the hypothalamic region. Circulating sex hormones act directly on them. From the hypothalamus, excitation spreads via ascending pathways to other brain structures, reaching the cerebral cortex. Conditioned-reflex stimuli—olfactory, auditory, and visual—are of immense importance.
There are pronounced gender differences in the formation of libido:
- In men: the process of testosterone production is practically continuous. An important factor is the secretion of the prostate gland: its accumulation in small doses stimulates testicular secretion, while in excessively large amounts, it inhibits it.
- In women: desire depends on the level of androgens and the degree of estrogen saturation of the body. The physiological peak of libido occurs at the end of the follicular maturation phase and the period of ovulation, which provides optimal conditions for conception.
It should be noted that sexual behavior is not limited to the action of sex steroids alone—virtually all glands of the endocrine system are involved in this process.
Preparatory phase: sexual ritual
Sexual intercourse itself (insertion of the erect penis into the vagina) is always preceded by a sexual ritual. Its main physiological task is to activate the mechanisms of libido and increase the excitability of the corresponding nerve centers.
In humans, this ritual is closely linked to the mechanical stimulation of specific erogenous zones. These include the lips, earlobes, mammary glands, and external genitalia. Adequate stimulation of these zones creates a state of physiological sexual readiness:
- In men tension of the penis occurs;
- In women swelling of the vaginal mucosa, labia minora, and erection of the clitoris are observed.
Neurophysiology of erection, friction, and ejaculation
The executive mechanisms of sexual intercourse represent a cascade of reflex reactions closed at the spinal cord level and controlled by the autonomic nervous system.
Erection The process of erection is managed by a specific erection center, whose neurons are localized in the lower lumbar and upper sacral segments of the spinal cord (III–IV sacral segments). Erection is predominantly a function of the parasympathetic division of the autonomic nervous system, the impulses of which cause 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 the spinal centers, causing an avalanche-like buildup of excitation that ultimately culminates in orgasm and ejaculation.
Ejaculation Ejaculation is the coordinated contraction of 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 II–V lumbar segments of the spinal cord.
Orgasm and phases of the sexual cycle
Broadly, sexual intercourse is divided into four consecutive phases: excitement, plateau, orgasm, and resolution.
Orgasm is a form of sensory saturation and manifests as an extremely pronounced emotional reaction. It sanctions sexual intercourse, is integrated into the action result acceptor, and subsequently serves as a powerful stimulus for sexual behavior. In women, it is customary to physiologically distinguish clitoral and vaginal types of orgasm.
The mechanism of development of the orgasm emotion is associated with the ascending spread of excitation from the spinal centers to the hypothalamus and structures of the limbic system. From there, descending influences are formed that trigger muscle contractions and ejaculation.
Orgasm is accompanied by a powerful autonomic reaction (the so-called "autonomic storm"):
| Parameter | In men | In women |
|---|---|---|
| Pulse | up to 150 bpm | about 110 bpm |
| Blood pressure | up to 250/120 mm Hg | about 160/130 mm Hg |
| Respiration | sharply accelerated | accelerated |
After the resolution phase sets in, the excitability of all sexual centers drops rapidly, completing the physiological cycle of sexual intercourse.
Briefly: answers to topical questions
What is the biochemical and molecular mechanism of vasodilation of the corpora cavernosa during an erection?
Vasodilation of the corpora cavernosa during an erection is associated with the activation of parasympathetic innervation: parasympathetic fibers from segments S2–S4 as part of the pelvic splanchnic nerves go to the corpora cavernosa, and their activation causes vasodilation in the corpora cavernosa, leading to an erection.
At the biochemical level, the sources describe the guanylyl cyclase system for vascular tone regulation:
- nitric oxide (NO) is linked to cGMP formation in vascular smooth muscle cells;
- guanylyl cyclase converts GTP into cGMP;
- cGMP activates protein kinase G;
- protein kinase G phosphorylates the enzyme;
- the system participates in the regulation of vascular tone and causes relaxation of vascular smooth muscle.
It is also noted that erectile dysfunction correctors exert their effects through the generation of NO.
Which specific hypothalamic nuclei serve as the centers regulating sexual behavior in men and women?
In the cited sources, specific hypothalamic nuclei directly designated as centers for regulating sexual behavior in men and women are not specified.
The following is confirmed:
- the leading centers for the formation of sexual desire are located in the hypothalamic region;
- the hypothalamus controls sexual behavior;
- the hypophysiotropic area of the hypothalamus is represented by neurosecretory nuclei: ventromedial, dorsomedial, arcuate, supraoptic, and paraventricular;
- cells of these nuclei produce liberins and statins;
- GnRH stimulates the release of LH and FSH, and its secretion is specified for the arcuate, supraoptic, and paraventricular nuclei of the hypothalamus.
For memorizing autonomic innervation: "Parasympathetic – Raises (erection, vasodilation), Sympathetic – Shoots (ejaculation, smooth muscle contraction)."
Class and exam questions
In which segments of the spinal cord are the centers of erection and ejaculation located?
The erection center is located in the III–IV sacral segments (S3–S4), while the ejaculation center is located in the II–V lumbar segments (L2–L5).
What is the role of the action result acceptor in sexual behavior?
A specific image is embedded in the action result acceptor. Sexual desire is realized only when the parameters of the real partner match this image. The orgasm itself is also integrated into this acceptor, becoming a stimulus for future behavior.
How does prostatic secretion affect male libido?
The effect depends on the volume: small amounts of secretion excite testicular secretion and stimulate libido, whereas its excessive accumulation has an inhibitory effect.
What else to study on this topic
- Mechanisms of forming the partner's image in the action result acceptor
- The role of specific structures of the limbic system in the formation of the emotion of orgasm
- The participation of extragenital endocrine glands in the regulation of sexual behavior
- Neurochemical aspects of parasympathetic vasodilation during erection (the role of nitric oxide)
- Physiological differences between clitoral and vaginal orgasm
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