There is no doubt that our sexual behavior is controlled by our brain. The brains of males and females are different and work differently when it comes to sex. But what exactly determines the difference in sexual behavior and traits between the genders?

Sigmund Freud believed that sex drive is the most powerful motivating force in our lives. Freud theorized that procreation of the species is an overriding priority for any organism. Human beings are driven to higher levels of growth and development due to sensual and sexual development. But where does our sex drive, which is such a powerful motivator of behavior, actually come from?

Sexual drive and desire is the result of an orchestration between our sensory systems (vision, hearing, taste, smell, and touch), the endocrine/hormonal system, and the autonomic nervous system, which is divided into two branches. The sympathetic system increases our physiological activity in response to an emergency and the parasympathetic system restores our physiological activity to normal level after an emergency has passed. Executive decision making processes are regulated by the right frontal lobe, the part of the brain behind the right side of the forehead.

Sex in regions of the brain

Sexual behavior is regulated in various part of the brain, including the hypothalamus. This structure is in our emotional control center which also regulates hunger, appetite, thirst, and body temperature. Basically, the hypothalamus is responsible for the short-term and long-term survival of our bodies, and thus the viability of the species. Without food or water, or without the ability to regulate our body temperature, we would not survive long. Without sex, there would be no procreation of our species. This area of the brain is associated with sexual desire.

The amygdala is near the hypothalamus, and is responsible for alerting us to changes in our environment detected by our senses. This part of the brain is also associated with sexual arousal. During sexual arousal, our bodies show the same signs as they would in a life-threatening emergency: muscle tension, increased heart rate, blood pressure, and respiration, perspiring, pupillary dilation, and tunnel vision.

The nucleus accumbens is the pleasure center of the brain. Anything that is pleasurable will activate the nucleus accumbens. Dopamine, a neurochemical messenger associated with pleasure, reward, and reinforcement, flows into this area, and gives this message: that was fun/ felt good/tasted good/smelled good, don’t forget, do it again. A similar mechanism has been found even in organisms as simple as nematodes, which will choose food over sex, and demonstrate the ability to remember past sexual encounters.

An individual can be sexually aroused by a variety of sensory input, such as seeing a beautiful member of opposite sex, the taste of their lover’s skin, the sound of their lover’s voice, a light touch, the smell of perfume, or experiencing these stimuli in one’s own imagination.

The sexual response cycle

The sexual response cycle is divided into four phases which correspond to various neurological processes:

• Desire starts with sensory input or cognitive processes.
• Excitement ensues, increasing activation of the sympathetic branch of the autonomic nervous system.
• Orgasm involves a peak activation of the sympathetic branch of the autonomic nervous system
• Resolution is activation of the parasympathetic branch of the autonomic nervous system.

In the desire phase, the amygdala has selected incoming sensory information as very critical and worth noticing. This leads to sympathetic nervous system activation, which peaks during orgasm. After orgasm, the parasympathetic nervous system is activated, and slows us down to normal again. Throughout this process, the nucleus accumbens is receiving dopamine, and sending the message that this is great, keep going, and don’t forget how much fun this was. This provides reinforcement to repeat the behavior. If sex was not so reinforcing, we would not be so motivated to do it, and again, there would be no procreation, and also less bonding and attachment between couples.

Differences between the genders

It is apparent, sometimes painfully so, that men and women perceive the world differently, process information differently, and have very different emotional responses to the same stimuli. Part of the reason for this is the difference in the male brain and female brain, particularly in the hypothalamus.

The hypothalamus is composed of dense neural centers, or nuclei. Several hypothalamic nuclei are sexually dimorphic, meaning there are apparent differences in their structure and function between genders. Most of the differences are in the neural connections, and neurotransmitter and hormonal sensitivity in particular areas. These structural and functional differences are manifested behaviorally by differences in sexual behaviors in men and women.

Men prefer the scent and appearance of women over other men, which is the start of male desire and initiating sexual behavior. If the sexually dimorphic nucleus is damaged, this sexual preference for females by men is reduced.

Researchers found an association between sexual orientation in males and a part of the hypothalamus called the suprachiasmatic nucleus (SCN). The suprachiasmatic nucleus in homosexual men is larger than in heterosexual men. Hypertrophied or enlarged SCN resulted in bisexual behavior in male rats as well. The part of the sexually dimorphic nucleus (SDN) known as the Third Interstitial Nucleus of the Anterior Hypothalamus (INAH 3) is nearly two times larger in heterosexual men than in homosexual men and heterosexual women.

Using Positron Emission Tomography (PET), scientists observed how hypothalamus responds to the scent of the hormone testosterone in male sweat, and the scent of the hormone estrogen in female urine. These studies showed that the hypothalamus of heterosexual men and homosexual women both activated in response to estrogen, while the hypothalamus of both homosexual men and heterosexual women activate in response to testosterone.

Sexual behaviors do not originate in the genitals or other erogenous parts. The signals that lead to arousal start there, but their destination is the brain.The male and female brains have small but critical differences in structure and function, which determine our individual sexuality.

References

Sammut, M., Cook, S., Nguyen, K., Felton, T., Hall, D., Emmons, S., Poole, R., & Barrios, A. (2015). Glia-derived neurons are required for sex-specific learning in C. elegans Nature, 526 (7573), 385-390 DOI: 10.1038/nature15700

Savic, I., Berglund, H., & Lindstrom, P. (2005). Brain response to putative pheromones in homosexual men Proceedings of the National Academy of Sciences, 102 (20), 7356-7361 DOI: 10.1073/pnas.0407998102

Stahl, S. (2010). Circuits of Sexual Desire in Hypoactive Sexual Desire Disorder The Journal of Clinical Psychiatry, 71 (05), 518-519 DOI: 10.4088/JCP.10bs06115whi

Swaab DF, & Hofman MA (1990). An enlarged suprachiasmatic nucleus in homosexual men. Brain research, 537 (1-2), 141-8 PMID: 2085769

Swaab DF, Slob AK, Houtsmuller EJ, Brand T, & Zhou JN (1995). Increased number of vasopressin neurons in the suprachiasmatic nucleus (SCN) of ‘bisexual’ adult male rats following perinatal treatment with the aromatase blocker ATD. Brain research. Developmental brain research, 85 (2), 273-9 PMID: 7600674

Image via LuckyKeeper / Shutterstock.