Biology 202
2002 Second Paper
On Serendip

Tania Romero

Neurobiology of Human Sexuality
Human sexuality is a topic that has been taboo for many centuries. The orgasm, for example, is one aspect of sexuality that has been overlooked. In particular, the female orgasm is an important function to address because it's a subject that has been socially steeped in silence and shame, and is directly connected to female health in general. Not many people are willing to admit, that orgasms actually benefit a woman physically and psychologically in many ways (i.e. easing menstrual cramps to alleviate stress). One mystery about the female orgasm is why some women "fake" orgasms during sexual activity. One thing that is for sure, is that even though the male orgasm is reached quicker, the female orgasm is capable of repeated pleasure. Why do so many women have difficulty experiencing an orgasm and why do men climax so fast? Interestingly enough, most women will confess that their partners do not give them orgasm. Instead, they allow themselves to have orgasms. Not only that, but they can have a variety of different types of orgasms at the same time. (i.e. clitoral orgasms, G-spot orgasms, vaginal orgasms, ejaculatory orgasms, blended orgasms, etc). Men will comment for example, that the scent of a woman is what drives them crazy during sexual activity and makes them unable to hold the sensation for long. So do women have more control over physical pleasure in the brain than men? How do sexual problems happen in the brain?

There is a combination of things that come in effect when it comes to an orgasm in the brain. Neurotransmitters, and neuro-peptides for example, are heavily involved in the sexual response. To reach an orgasm, the central nervous system sends orders to the heart, so that it pumps faster, sending blood to oxygenate the increase of blood flow in muscles involved in sexual activity. Nitric oxide, serotonin, dopamine, epinephrine, and norepinephrine, are just a few of the neurotransmitters and neuro-peptides involved in sexual activity. Nitric oxide (NO) plays a critical role in both male and female sexuality. In penile erections, NO stimulates the release of guanylate cyclase, which, in turn, converts GTP to cGMP and produces relaxation of smooth muscles and increased blood flow into the penis. This information is used in the popular drug sildenafil (ViagraTM) that inhibits the metabolism of cGMP to prolong the effects of the erection. (1) Serotonin's role in sexual function is that of constriction of smooth muscles in the genitals, and peripheral nerve function. Epinephrine appears to be involved in maintaining the penis in a flaccid state. This increases the rate and force of the muscle's contractions during sexual activity. Conversely, in women, epinephrine has been shown to increase vaginal pulse amplitude. Norepinephrine is another neurotransmitter that mediates chemical communication in the sympathetic nervous system, a branch of the autonomic nervous system. Like other neurotransmitters, it is released at synaptic nerve endings to transmit the signal from a nerve cell to other cells. Levels in the brain of neropinephrine vary according to sexual arousal. They increase significantly with arousal and sexual activity in men as well as in women.

Hormone levels are also involved in sexual activity. In males, testosterone levels remain generally above the threshold required for sexual interest and activity. Thus increases testosterone above this threshold are believed to have additional influences on sexual interest or behavior. Interestingly, estrogen(the "female hormone") seems to have little impact on sexual desire on either males or females. Estrogen deficient women, however, can cause a decrease genital lubrication caused by the thinning of the vaginal epithelium. All factors can impair both the physiological and psychological aspects of sexual arousal. (2)

Other hormonal influences in sexual function include oxytocin, cortisol, pheromones, and prolactin. There is some evidence showing that oxytocin (produced by dopamine) levels increase during sexual arousal and orgasm in both men and women. (3) Also the combination of oxytocin and female hormones like estrogen in women, encourage an emotional attachment with a partner. In men, the bonding effect is muted, due to the male's higher testosterone levels. As a result, men form emotional ties slower, and are more likely to regard initial sexual contact as just sex. Studies of men and women with elevated levels of prolactin report decreased sexual interest, arousal, orgasm as well as mood disturbances such as anxiety and depression. Dysfunction in remaining erect has been described in men with both abnormally low and abnormally high prolactin levels. In women, high prolactin has been associated with infertility and decreased sexual activity. Animal studies also suggest that prolactin has an inhibitory influence on both male and female sexual behavior, although short-term prolactin elevations may play a role in some elements of sexual behavior in male rats.
Clearly, this molecule plays many roles in humans but the exact role is uncertain. It is known that prolactin shares a close connection with dopamine in the brain, particularly in the tubero-infundibular tract, which runs along the base of the hypothalamus and releases dopamine into the portal veins of the pituitary gland. Dopamine acts to inhibit the release of prolactin. This, in turn, affects dopamine release. Blockade of dopamine receptors in the tuberoinfundibular tract releases prolactin from the tonic inhibitor control of dopamine, allowing prolactin levels to rise. This self-regulation is critical to prolactin homeostasis, since any disturbance in the connection between the hypothalamus and the pituitary may lead to hyperprolactinemia (high prolactin levels). (4)

Knowing what happens in the brain during sexual arousal and orgasm can help to speculate why so many men and women have problems during sexual activity. There are various factors for both men and women that can cause sexual dysfunction such as depression(caused by low level of serotonin), everyday stress, etc. Understanding the sexual differences in both men and women, can also help differentiate male and female behaviors. But there is still on-going research being done on these issues because not much in known about how these chemicals work together in the brain to affect sexual behavior. There are speculations for example, that the olfactory sensors in men are highly sensitive to the "smell" of estrogen and in fact, cause quick arousal. But this kind of triggering in the "female" brain is less receptive. Perhaps this is why women have to "think" about having an orgasm in order to feel one. Whether women control their orgasm or not, is still questionable. But it is obvious that the levels and ways that these chemicals work together in the brain are different for men and women.

References

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1) Scientific American: News In Brief: Study Lays Bare the Physiology of Erection: March 19, 2002
2) Chan, John, Dellovade, Tammy L., Forrest, Douglas, Pfaff, Donald, W, The Two Thyroid Hormone receptor genes have opposite effects on estrogen-stimulated sex behaviors, Nature Neuroscience Vol 3 number 5 pp 472-475 2000.
3) Bartlik, Barbara, Gelenberg, Alan J., Legere, Regina, Villaluz, Jose, Sexual Dysfunction Secondary to Depressive Disorders The Journal of Gender-Specific Medicine, 2[2]:52-60 1999
4) Michele A. Kelly, Marcelo Rubinstein, Sylvia L. Asa, Ge Zhang , Carmen Saez , James R. Bunzow , Richard G. Allen, Robert Hnasko , Nira Ben-Jonathan , David K. Grandy , and Malcolm J. Low Pituitary Lactotroph Hyperplasia and Chronic Hyperprolactinemia in Dopamine D2 Receptor-Deficient Mice Neuron, Vol. 19, 103–113, July, 1997


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