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Brain mechanisms of sexual behavior

Brain mechanisms of sexual behavior

In humans, the control of the sexual response comes partly from the cerebral cortex, but it is the spinal cord that coordinates this brain activity with the sensory information that comes from the genitals, generating a critical activity that mediates the sexual response of the genital structures.

Content

    • 0.1 Neural control of the reproductive organs
  • 1 Brain mechanisms of sexual behavior
    • 1.1 Brain and male sexual behavior
    • 1.2 Female brain and sexual behavior
  • 2 Neurochemical mechanisms for controlling sexual behavior
    • 2.1 Male sexual behavior
    • 2.2 Female sexual behavior
    • 2.3 Sexual orientation
    • 2.4 Brain and sexual orientation

Neural control of the reproductive organs

The external genitals, especially the clitoris and glans penis, are densely enervated by mechanoreceptors. Proper stimulation of these organs can cause erection; The penis and clitoris are the male and female erectile organs, respectively, which are formed by two corpora cavernosa surrounded by a mucosa.

Sensory pathway

The mechanoreceptors of the penis and clitoris send their axons to the dorsal root of the spinal cord, to arrive in the dorsal horn of the medulla and project for the dorsal columns towards the brain.

Sildenafil, a potent drug marketed under the name of Viagra, is used as a treatment for erectile dysfunction. Specifically, it is a selective 5-phosphodiesterase inhibitor present in the corpora cavernosa. This compound acts by increasing the effects of nitric oxide on erectile tissues.

Parasympathetic pathway

The erection is mainly controlled by the axons of the parasympathetic nervous system. In the sacral region of the spinal cord, parasympathetic neurons can be activated directly thanks to the axons of the genore mechanoreceptors and the descending axons of the brain. The increase in volume and erection of the penis and clitoris depend on blood flow; the parasympathetic system stimulates the release of acetylcholine, from vasoactive intestinal polypeptide and nitric oxide, directly to the erectile tissues. During intercourse, the parasympathetic system also stimulates the release of lubricating substances from the vaginal walls (Bartholin's glands to women) and from the bulbourethral gland (man).

Nice way

When sexual stimulation is quite intense, the neurons descended from the brain and axons from the penis and clitoris activate the sympathetic neurons located in the lumbar and thoracic segments of the spinal cord. In man, efferent axons sympathetic trigger the processes of emission and ejaculation of semen and, in women, sympathetic activation generates (along orgasm) strong muscular contractions, cervical duct dilation and increased motility of the uterus and fallopian tubes, facilitating the transport of sperm to the egg.

The neural mechanisms underlying the sensations and perceptions of the sexual climax are, today, unknown in both sexes.

In the sexual response there is a sensory activation of the mechanoreceptors of the sexual organs and a stimulation of parasympathetic and sympathetic eferences.

Brain mechanisms of sexual behavior

Brain and male sexual behavior

In the hypothalamus human, and also to other animal species, the medial preoptic area has been closely related to the brain control of male sexual behavior.

In the human hypothalamus, and also to other animal species, the medial preoptic area has been closely related to the encephalic control of male sexual behavior.

Different experimental studies have shown the following:

  • The electrical stimulation of medial preoptic area generates the activation of coupling behaviors.
  • His injury irreversibly suppresses the sexual behavior of males.

There are researchers who state that the disruptive effects of the lesion of the medial preoptic area on sexual behavior seem to be explained by an alteration of intercourse behaviors and not by an inhibition of sexual motivation; However, recent experimental works (Shimura et al., 1994) point in the opposite direction, since they have found an activation of the neurons of this brain area in anticipatory behaviors of the consummatoria.

The medial preoptic area receives information from the vomeronasal organ, by means of the projections of the bed nucleus of the terminal stria and the medial nucleus of the tonsil. Likewise, the medial preoptic area receives information regarding the sensory receptors of the external genitalia (mechanoreceptors), through the projections of the mesencephalic reticular formation and the medial tonsil.

For example, it has been found that the volume of the sexually dimorphic nucleus of the medial preoptic area is related to the sexual activity of male rats and that their injury reduces male sexual behavior.

The medial preoptic area exercises control over sexual behavior, through its projections in the tegmental field mesencephalic lateral; the interruption of this neural communication pathway alters male sexual behavior.

Both the olfactory main system and the accessory send their axons to the amygdala Medial It has been found that lesions of this structure alter some male sexual aspects. Likewise, the bilateral excision of the entire tonsil complex generates pictures of hypersexuality in primates.

Female sexual behavior and brain

In the same way as in males, the chemosensory information of the olfactory main system and of the accessory, as well as the somatosensory information, the genital mechanoreceptors reach the medial nucleus of the tonsil.

In females, the medial tonsil neurons project into the medial preoptic area and the ventromedial nucleus of the hypothalamus.

So, for example, both the injuries of the gray substance as the section the axons that communicate with the ventromedial hypothalamus produce a deterioration in sexual behavior.

The ventromedial nucleus of the hypothalamus sends its axons to the periacueductal gray substance of the midbrain. From this region, the information is sent to the reticular formation of the spinal bulb, to ultimately reach the spinal cord and be able to implement the efferent mechanisms that control intercourse behavior.

Neurochemical mechanisms for controlling sexual behavior

Male sexual behavior

Some hormones, such as gonadal steroids and neurohypophytic peptide hormones, can act on nerve cell receptors, altering the patterns of sexual behavior.

Gonadal Steroids

As we have seen in the previous nucleus, gonadal hormones have organizing and activating effects on the brain and sexual behavior. Specifically, androgens have activating effects of male sexual behavior with their performance on neuron receptors in the medial preoptic area.

Thus, for example, if a male rat has his testicles removed, his sexual behavior will be inhibited; however, normal patterns of sexual response can be restored by intracerebral administration of testosterone in the medial preoptic area.

Oxytocin

The oxytocin It has a regulatory role on the sexual behavior of mammals: intracerebral administration of oxytocin generates more vigorous and persistent sexual behavior in both sexes. Oxytocin can act in the brain by enhancing the induction of sexual desire: specifically, its neural effects are concentrated in the nucleus of the bed of the terminal stria, in the medial preoptic area and the ventromedial nucleus of the hypothalamus. This hormone also intervenes in the mechanisms of penile erection and ejaculation.

An example is the decrease in latency of ejaculation and an increase in the probability of penile erection, following the cerebral administration of oxytocin.

Vasopressin

Another neurohypophytic peptide hormone, vasopressin, is secreted by neurons of the medial tonsil and the nucleus of the terminal striatal bed. Intracerebral administration of antagonists of this hormone inhibits male sexual behavior in rats.

It has been verified that Testosterone administration induces recovery of brain vasopressin levels, minimized after testicular excision. This effect coincides with the recovery of sexual activity.

In addition to the neural control of hormones, some classic neurotransmitters are essential for male sexual behavior.

Dopamine

In 1995, Hull and colleagues found that neurons in the medial preoptic area released dopamine during sexual activity

For example, the administration of dopamine agonists in the medial preoptic area facilitates erection mechanisms and increases intercourse behavior..

Both gonadal steroids such as dopamine, oxytocin and vasopressin seem to have a very important role in male sexual behavior.

Female sexual behavior

Female sexual behavior depends on the secretion of estradiol and progesterone. The effects of estradiol and progesterone on female sexual behavior are carried out by activating the ventromedial nucleus receptors of the hypothalamus, by these hormones.

Intracerebral administration of these two hormones in the ventromedial nucleus of the hypothalamus activates the patterns of female sexual behavior, even in the absence of the gonads.

The mechanism of action of estradiol and progesterone is concatenated: estradiol increases cellular sensitivity to progesterone, increasing the number of receptors for it; in this way, an effect of amplification of the hormonal signal is generated.

Similarly, the peptide hormone Oxytocin is able to facilitate the sexual response in females, previously treated with estradiol and progesterone.

The mechanical stimulation of the receptors of the sexual organs generates a noradrenergic activation. Thus, the section of the noradrenergic pathways that project to the bone marrow or forebrain impair sexual behavior.

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Sexual orientation

The fact of finding sexual dimorphism in the human brain led us to think that there were probably some structural differences according to the sexual orientation of the subject.

We must keep in mind that Human sexual behavior is very complex and that the mechanisms of choice of a sexual partner are broad and influenced by educational and social factors.

Brain and sexual orientation

In 1989, Laura Allen and Roger Gorski, researchers at the University of California, found that interstitial nuclei 2 and 3 of the anterior hypothalamus were larger in males than in females.

In 1991 Simon Le Vay, a neuroanatomist who worked at the time, at the Salk Institute in San Diego, California, published an article in the journal Science describing that the interstitial nucleus 3 of the anterior hypothalamus seemed to show twice the size in the brains of straight men compared to men with a homosexual orientation. This same comparison made between brains of homosexual men and heterosexual women showed how the size of the nucleus in both groups was very similar.

On the other hand, Dick Swaab and collaborators, of the Brain Research Institute of Amsterdam, described structural differences in the suprachiasmatic nucleus of the hypothalamus, according to the sexual orientation of the subject: this nucleus had a larger number of cells in the hypothalamus of homosexual men than in heterosexual men.

Volume of the nucleus of the terminal stria bed in 4 population groups: heterosexual men, homosexual men, heterosexual women and transsexual women (men who have undergone surgery): Dick Swaab and collaborators found that the nucleus of the bed bed terminal stretch marks had more volume in men than in women, without presenting variations according to the sexual orientation of the subject. However, in transsexual women the volume of the nucleus was even lower than that of heterosexual women.

Thus, in the suprachiasmatic nucleus of the hypothalamus and in the interstitial nucleus 3 of the anterior hypothalamus, structural differences have been found according to the sexual orientation of the subject.

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