Men's Health: Analyzing the Three-Tier Neural Regulatory Centers for Sexual Function and the Mechanism of Sexual Arousal

Sexual function is regulated by a three-tiered neural center:

The first tier, also the primary center for sexual function, is located in the sacral segment of the spinal cord, also known as the spinal cord center.

Both male and female sex organs have numerous sensory nerve endings. It has been proven that although the female clitoris and the male penis differ greatly in size, the number of sensory nerve endings in both is almost equal. This explains the extremely high sexual sensitivity of the female clitoris. In the population, there is considerable individual variation in the number of sensory nerve endings in the sex organs, regardless of gender. This is one reason why the degree of sexual sensitivity varies from person to person. The primary center controlling sexual arousal and behavior is located in the spinal cord and is regulated by the sympathetic and parasympathetic nerves. Sympathetic nerves originating from the 1st, 2nd, and 3rd lumbar spinal cords reach the sex organs and nearby organs via the subenteroperitoneal ganglion. Parasympathetic nerves originating from the 2nd, 3rd, and 4th sacral spinal cords reach the sex organs via the pelvic nerves (erectile nerves), primarily controlling penile or clitoral activity and the secretion of accessory glands. The sacral segment of the spinal cord directly controls penile erection and ejaculation in men, clitoral erection in women, vaginal wall vascular congestion, and rhythmic contractions of the smooth muscles in the lower 1st and 3rd segments of the vaginal wall. In men, separate erection and ejaculation centers can be identified.

The second level, the higher centers of sexual function, are located in the subcortical centers of the hypothalamus and diencephalon. This is also the site of gonadotropin-releasing hormone (GnRH) production, and therefore has a close connection with endocrine function.

The third level, the highest centers of sexual function, are located in the limbic system of the cerebral cortex, particularly in the septum and related structures.

These three levels of centers and related neural centers are closely interconnected. Primary centers have relatively simple functions; stimulation of them produces a consistent and definite response. Higher centers regulate the activity of primary centers, modulating more complex physiological functions by promoting or inhibiting the activity of each primary center. Therefore, their responses are complex and variable, and the results of stimulating the limbic system vary considerably.

Taking men as an example, after receiving sexual stimulation from the sensory nerve endings of the penis, the stimulation travels through the pelvic nerves to the sacral region of the spinal cord, exciting the primary sexual center (spinal cord center). First, the erection center is excited, causing penile erection and secretion from accessory glands, enabling satisfactory penile insertion into the vagina for intercourse. At this point, the ejaculation center is not normally excited. Only during intercourse, due to the friction of the penis within the vagina, does sexual excitement gradually accumulate. When this excitement reaches the excitation threshold of the ejaculation center, it triggers a series of ejaculatory actions, including spasm of the bladder sphincter, paroxysmal contractions of the smooth muscles of the vas deferens and seminal vesicles, as well as peripheral sexual responses, leading to orgasm. In this process, the parasympathetic nervous system plays a dominant role.

After orgasm, the sexual center transitions from excitation to inhibition, and the parasympathetic nervous system also transitions from an excited state to an inhibited state, causing the erection to quickly subside. During orgasm, the sympathetic nervous system is also excited, manifesting as increased breathing and sweating. It's particularly important to note that in individuals or animals with severed spinal cords, although only the primary centers remain, stimulation of the genitals can occasionally induce ejaculation. However, the control of sexual function and behavior is far more complex than the aforementioned primary reflex arc process. In fact, the impulses from the sensory nerve endings of the sexual organs are usually not the initial source of excitation in the sexual center. The true initial source is often stimuli from vision, hearing, etc. In humans with a second signal system, trace reflexes (i.e., the reappearance of traces retained in memory) are also important sources of initial impulses; simply thinking about sex or even dreaming about sexual intercourse can induce erections and other male sexual behaviors.

Libido and Sexual Arousal

Libido refers to a sexual desire arising under certain sexual stimulation conditions, including the desire for contact and the desire to expel. The desire for contact refers to the strong desire of men or women to have physical contact with the opposite sex. This ranges from kissing, hugging, touching the body, or close contact, to direct genital contact. Functionally, "contact" enhances sexual arousal and leads to orgasm through accumulated arousal. The urge to ejaculate refers to the desire in men after sexual maturity, when the testes continuously produce sperm, filling the vas deferens with semen, increasing sexual sensitivity, and creating a special tension that leads to a strong desire to release it. Ejaculation is the ultimate goal of sexual behavior and a crucial factor in male libido, while women have a much stronger "desire for contact" than men. Libido primarily depends on three factors:

① the intensity of external sexual stimulation;

② the sensitivity to sexual stimulation;

③ the intensity of the sexual physiological response.

Sexual arousal refers to the process by which the sexual organs become sexually excited when the energy of sexual impulses accumulates to a certain level under appropriate sexual stimulation. Sexual arousal is formed by psychological and physiological conditioned reflexes and unconditioned reflexes. Unconditioned reflexes are instinctive behaviors determined by genetics and are self-taught, while conditioned reflexes are connected to human consciousness, which is subject to the brain's higher regulatory mechanisms. Animals primarily rely on lower senses like smell and touch for sexual desire, while in humans, vision and imagination play a crucial role in sexual arousal. Direct stimulation of erogenous zones is, of course, the easiest way to induce sexual arousal.

Sexual arousal begins by directing attention to sexual stimulation, exciting the reproductive organs and triggering a series of bodily reactions and changes, preparing the body for sexual activity. Sexual arousal is formed by psychological and physiological conditioned and unconditioned reflexes. Conditioned reflexes are linked to human consciousness, which is subject to the brain's higher regulatory mechanisms. Thinking primarily occurs and develops based on vision and hearing, and vision, hearing, and imagination play a significant role in sexual arousal. The degree of sexual sensitivity varies greatly among both men and women, and this is closely related to consciousness, beliefs, emotions, physical constitution, fatigue, and environment. Some people experience a strong sexual response to weak sexual stimulation, while others require stronger stimulation. The time required for sexual arousal also varies greatly, ranging from a few minutes to several hours for some.