The Science of Sex

by Edward


Over the years, I have invested many hours of my life indirectly studying the effects of psychogenic and afferent sexual arousal, but have always come up short in terms of tangible conclusions (pun not intended).  Deciding to learn more, I have searched through various sources and found information, which I think, may shed some well-needed light on the situation.  Although not elegant at times, under the surface the human body’s response to sexual stimulation is actually a complex cascade of chemical and biological reactions, both within the genital area as well as in the brain.  In this multi-part series, I hope to convey the main scientific concepts behind the sexual response without inundating you with jargon so that we may discount or confirm myths about sex as well as become more informed about our bodies in the act.

Part I: Male Arousal

Before researching for this article,  I had never fully understood how soft tissue could become rigid without a strong infrastructure.  There are no ligaments or bones in the penis, so how does it work?  The answer, it turns out, is blood.  But I am getting ahead of myself.

There are two sources of arousal.  One is from physical stimulation, and the other is psychogenic in origin.  Of course, the two often complement each other to a varying degree.  In either case, signals from the sensory nerves of the glans (head of the penis) and the brain (specifically the paraventricular nucleus of the hypothalamus) reach neurons in the sacral-lumbar spinal cord.

Here, parasympathetic nerves relay this signal from the spinal cord to the hypogastic nerves, the pelvic nerves, as well as the sympathetic nerves of the pelvic plexus.   In turn, the cavernous nerves are excited.  The cavernous nerves interact with the corpora cavernosa, which is the name given to the tissue that becomes engorged with blood during an erection.

The cavernous nerves release a variety of “first messengers”, including nitric oxide, acetylcholine, and vasoactive intestinal peptide, all of which will eventually lead to increased blood flow into the corpora cavernosa, creating an erection.  Nitric oxide is a vasodilator that acts on the smooth muscle cells of arteries, causing them to relax and therefore expand in diameter.  This expansion reduces the blood pressure in the lumen, allowing more blood to flow.  In the case of acetylcholine, a more complex reaction mechanism takes place which requires explanation (as well as sheds light as to how erectile dysfunction pills work).

When acetylcholine connects with its G-protein-linked receptor on an endothelial cell, the phosphoinositide signaling pathway begins, eventually leading to the production of nitric oxide.  Nitric oxide (NO) readily diffuses through the plasma membrane and into adjacent smooth muscle cells.  Once inside, NO, through a series of chemical reaction cascades, results in smooth muscle relaxation through interactions of the secondary messenger cGMP with muscular proteins.  Relaxation is short lived, as phosphodiesterase activity inside smooth muscle cells leads to the degradation of cGMP and subsequent contraction of smooth muscle cells.  Once again, because the smooth muscle cells are relaxed, the diameter of the vasculature in the corpora cavernosa increases, leading to increased blood flow.  Sildenifil (Viagra) inhibits the enzymes that degrade cGMP to GMP (phosphodiesterase-5), and thus temporarily increase the time vascular smooth muscles are relaxed.

When blood fills the corpora cavernosa, two things happen. First, the veins that usually drain blood from this tissue are compressed, decreasing the flow of blood from the penis.  Second, because the corpora cavernosa is encapsulated by the tunica albuginea (an inelastic, rigid sheath) the pressure increase from the increased flow of blood into the penis pushes the penis outward and up into full extension.  In addition, the “user” can make his erection stronger by voluntarily contracting the ishiocavernosus muscles.  When this muscle group contracts, it squeezes the base of the corpora cavernosa, momentarily preventing blood from coming in or going out, which effectively increases the blood pressure inside the penis.  This is the same muscle group activated during Kegel exercises.

As an interesting aside, the ishiocavernosus muscle is under voluntary control by the pudendal nerve.  The pudendal nerve also provides a thoroughfare for sensory signals from the glans to the brain.  Although the pudendal nerve is able to move inside the pudendal canal, long term increased pressure on the nerve can damage it, leading to unpleasant side effects, such as erectile dysfunction and genital numbness.  As such, these problems are often seen in long distance cyclists.

The manner in which an erection is produced is complicated, and I have most certainly left many aspects of arousal and erection out during this brief summary.  Concepts and ideas will become more complex as we explore female arousal, the effects of foreplay on the body, the physiology of orgasm, sexual disorders,  and the schemes by which the human body uses to become more evolutionarily fit.

Sources