What You Don’t Know About Penile Erections, But Should

Blog #158

Although there are many who don’t care to know the details of how things function as long as they are functioning well, there are others who are curious about what’s “under the hood.” This blog, largely excerpted from my new book: Male Pelvic Fitness: Optimizing Sexual and Urinary Health, is for those who are interested in what is actually happening down there one encounters a sexually stimulating situation.

A penis is a mechanical device and, as with any machine, it is important to understand how it works before it can be used and its operation mastered. In order to begin to understand how a penis works, the names and functions of its parts must first be learned.

The control center for erections is housed within the cerebral cortex (brain), of immense importance to our sexuality and what can be considered to be the “governor” of erections and sexuality. The brain can initiate erections in the absence of tactile (touch) stimulation and in direct response to sights, sounds or smells that are perceived as erotic. It might be a quick glance at someone who is smoldering hot, a sexy and sultry voice, or the alluring scent of perfume. The stimuli need not be external, as even a thought can initiate an erection.

There are several “centers” in our brain that integrate sexual functioning and erections. Studies have demonstrated that electrical stimulation of these brain centers can induce erections, and disease processes that involve these areas can cause ED. There are also several spinal cord erection centers and illnesses that involve these spinal areas can also cause ED.

The cerebral cortex is the site where the sensations of sexual arousal are experienced and processed. The brain then sends nerve signals to the erectile nerves, the cavernosal nerves, which are also stimulated by direct sensory contact, including foreplay or the act of sexual intercourse itself.

Bottom Line: The brain is the master control unit of sexuality.

There are three types of erections: psychogenic, reflex, and nocturnal. Psychogenic erections are on the basis of sights, sounds or smells. Nerve impulses travel from the brain to the spinal cord centers to the penis to produce an erection. Reflex erections occur in response to direct tactile stimulation of the penis. Nerve impulses travel from the penis to the spinal cord centers. Nocturnal erections have a unique mechanism controlled in thebrainstem and occur during REM (rapid eye movement) sleep. Most healthy men have 3-6 nocturnal erections while sleeping, each lasting about 10-15 minutes.

Bottom Line: Erections can originate from the following: erotic thoughts; stimulation of our senses of vision, hearing and/or smell; and direct touch. Nocturnal erections have a unique brainstem origin.

Who Knew? When the urinary bladder is full, it stimulates the same sacral nerves that are involved with penile sensation and erection. Because the bladder and the penis share a common nerve supply, when the bladder is distended, an erection may occur, often referred to in slang terms as a “piss hard-on.”

Who Knew? Some men ejaculate spontaneously during sleep while in REM sleep. These “wet dreams” or more formally, “nocturnal emissions,” demonstrate the central role that the brain plays with respect to both erections and ejaculation.

The erectile apparatus of the penis consists of three spongy erectile cylinders that run the length of the entire penis, both internally and externally: the solitary corpus spongiosumthat contains the urethra and forms the glans penis, and the paired erectile cylinders called the corpora cavernosathat are anchored internally to the pelvic bones and extend to the glans. These erectile cylinders communicate with each other and are enclosed in a fibrous sheath, the tunica albuginea. Erectile rigidity is on the basis of blood flowing into and being trapped in the penis.

Bottom Line: The penis obtains its bone-like rigidity (hence the term boner) by virtue of blood filling and inflating the spongy tissue within the three cylinders of the penis (corpora), similar to air inflating the tire of a car

Who Knew? It only takes 2 ounces or so of blood to inflate the average flaccid penis into a fully erect one.

Who Knew? The penis of many mammals has an “os penis,” a bone coursing through the penis to facilitate sexual intercourse by maintaining penile rigidity at all times.

Who Knew? When dogs copulate, the canine penis is not erect at the time of penetration. By virtue of the os penis, the penis can enter the vagina. After penetration, swelling of the erectile tissue at the penile base occurs. With vaginal contraction, the canine penis locks inside the female. The locking functions to decrease leakage of semen after ejaculation.

Who Knew? Pigs have a rather oddly shaped penis that twists into a corkscrew during erection, a shape that bears an uncanny resemblance to the coiled tail of the pig. Thrusting creates a motion that can best be described as semi-rotary. Not surprising, the female pig has a corkscrew-shaped cervix.

The penis can be thought of as an extension of the principal artery of our body, the aorta. In fact, one can think of the penis as a “dangling” aorta. The aorta gives rise to the common iliac artery, which gives rise to the internal iliac artery, which gives rise to the internal pudendal artery, which gives rise to the penile artery that divides into the dorsal artery (to the glans), the bulbourethral artery (to the corpus spongiosum) and the cavernous artery (which supplies the helicine arteries to the erectile tissues).

Bottom Line: The penis is essentially an extension of our blood vessels.

The corpora contain sinusoids(small sinuses that consist of spongy, vascular tissue) that have a very rich blood supply. In a sexually stimulating situation, the sinusoids of the corpora become engorged with blood, resulting in an erection.

Who Knew? Under the microscope, the tissues of the corpora appear virtually identical to the tissue of our nasal sinuses.

Vascular smooth muscle exists both in the penile arterial walls as well as the sinusoids. In the flaccid state, this smooth muscle is contracted, allowing only a minimal amount of arterial inflow, sufficient to meet the basic needs and nutritional demands of the penis. During this flaccid state, the sinusoids are closed while the venules (small veins that conduct blood out of the penis) remain open. In terms of oxygenation, the flaccid penis is filled with venous blood, while the erect penis is filled with arterial blood.

Bottom Line: The erectile tissue of the penis consists of sinuses, which under the circumstances of stimulation, become “congested” with blood.

Who Knew? The penis behaves as a vein when flaccid and an artery when erect.

Who Knew? Viagra, Levitra, Cialis and Stendra cause nasal stuffiness as a side effect since they act on the facial sinuses as much as they do on the sinuses of the corpora, causing congestion in both.

Who Knew? One of the treatments for priapism—an unwanted, persistent, painful erection—is an injection of neo-synephrine directly into the corpora. This is the very same medication that we squirt into our noses to relieve nasal congestion, aka nose drops.

Under the circumstance of erotic or tactile stimulation, the cavernosal nerverelays a chemical message to the cavernosal arteries to dilate (increase in diameter and pour in blood) and a message to the smooth muscle of the corpora to relax, allowing blood to fill the corpora. The cavernosal nerves release the neurotransmitter nitric oxide, the main chemical mediator of erections. The nitric oxide increases the release of another chemical known as cGMP (cyclic guanosine monophosphate)within the smooth muscle of the corpora. This causes relaxation of the smooth muscle of the small arteries and dilates the sinusoids within the erectile bodies. The sinusoids become “congested” with blood, causing engorgement and tumescence with an increase in penile length and girth. The small veins directly under the tunica albuginea (located between the tunica albuginea and the sinusoids) become compressed, reducing venous outflow. As the penis gets increasingly engorged, the tunica gets stretched to capacity, which occludes the small veins within the tunica itself, helping to maintain the erection.

Bottom Line: In an erotic situation, nerves to the penis release a chemical that increases blood flow to the penis, flooding blood into the sinusoids, filling the corpora and resulting in an erection. As the erection gets fuller, penile veins are compressed, limiting the exit of blood and enhancing the erection. The penis lengthens, thickens, and rises, resulting in quite the “proud soldier.

At this point, the blood pressure within the corpora cavernosa is about the same as arterial blood pressure, over 100 millimeters of mercury. Ultimately, with the contraction of the bulbocavernosus (BC) and ischiocavernosus (IC) muscles, the pressure inside the corpora cavernosa rises to way above systolic blood pressure (the numerator in our blood pressure reading), creating a rock-hard erection. The corpora cavernosa become rigid while the corpus spongiosum and glans become full and spongy plump. The blood pressure in the corpus spongisum and glans is only a fraction of that in the corpora cavernosum as a result of the tunical covering of the former being much thinner than that of the latter.

Bottom Line: PFM contractions, by further trapping blood in the penis, cause rock-hard erectile rigidity.

Who Knew? At the time of a fully rigid erection, the penile blood pressure is off the charts high. If our systemic blood pressure were this high, it would be considered an emergency situation—a hypertensive crisis—with the potential for a heart attack, stroke or rupturing of an aneurysm (weakness of the wall of an artery). Who knew that penile hypertension is what allows us to have bone-hard rigidity? The next time you have a rigid erection, tell your partner that you have penile hypertension and that you need “intervention.” See where that gets you!

The pudendal nerveprovides the nerve supply (sensation and contraction) to the ischiocavernosus (IC)and bulbocavernosus (BC) muscles. Contraction of the IC and BC muscles enhances penile rigidity and engorgement of the glans. The IC muscle contraction is primarily involved in generating a rock-hard erection while the BC muscle contraction maximizes engorgement of the corpus spongiosum and glans and is the motor of ejaculation, an event that occurs when the BC contracts rhythmically at the time of climax.

Bottom Line: The IC muscle generates a rock-hard erection while the BC muscle engorges the corpus spongiosum and glans and contracts rhythmically at the time of ejaculation.

The corpus spongiosum and glans essentially behave as an arterio-venous shunt (a connection between an artery and a vein) during an erection, until the point that the BC muscle compresses venous return sufficiently to cause full engorgement of the glans and corpus spongiosum. When fully engorged, the corpus spongiosum functions to “pressurize” the urethra, which will facilitate forceful ejaculation.

In summary, for an erection to occur, three events need to take place. There must be an increase in arterial blood flow to the corpora. Relaxation of vascular and corporal smooth muscle has to occur. Finally, venous outflow has to decrease in order to trap blood within the corpora. The seemingly simple process of getting an erection is actually an incredibly complex event requiring integrated functioning of the brain, nerves, blood vessels, and hormones. The thrust of the matter is that it is really nothing short of a stunning orchestration.

Bottom Line: For an erection to occur, penile blood flow has to increase, the smooth muscle in the walls of the penile blood vessels and sinuses must relax, and venous outflow needs to decrease, thus trapping blood in the penis.

After ejaculation, the enzyme PDE (phosphodiesterase)is released. This chemical can be considered the main chemical controller of flaccidity—this degrades cGMP, resulting in a return to the flaccid state by a reversal of the aforementioned chemical mechanisms. Viagra, Cialis, Levitra, and Stendra work by inhibiting PDE.

Bottom Line: After ejaculation, the penis becomes flaccid from a reversal of the chemical mechanisms that created the erection.


Andrew Siegel, MD www.AndrewSiegelMD.com

The aforementioned is largely excerpted from my new book: Male Pelvic Fitness: Optimizing Sexual and Urinary Health; available in e-book (Kindle, iBooks, Nook) and coming soon in paperback. www.MalePelvicFitness.com

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4 Responses to “What You Don’t Know About Penile Erections, But Should”

  1. Randy Beran Says:

    Dear Doctor Siegel,

    I went to a naturopath on the 22nd who without my consent manipulated my cervical spine pulling it to put in in place she said because it was out of place and since then I have not had a normal nocturnal erection. She said it was the C4 then said it was C4 or C3. I feel that on waking in the middle of the night that, in the past would have been a full erection, is flaccid instead. During this night I woke in the middle of the night and my penis felt a bit fat and it was flaccid. Prior to the 22nd my nocturnal erections were strong and I would wake in the middle of the night having at times an erectile feeling and it would be erect. Now it is flaccid instead.

    I have done some research to find out about this and at urologyteam.com/male-impotence-or-erectile-dysfunction it says that “spinal cord and brain injuries can cause impotence because they interrupt the transfer of nerve impulses from the brain to the penis. And at https:// healthdoc13.wordpress.com/tag/reflex-erections you say ” Nocturnal erections have a unique mechanism controlled in the brain stem.” At harvardprostateknowledge.org, Overcoming erectile dysfunction when you have prostate disease, it says “the psyche doesn’t affect nocturnal erections, which occur involuntarily during sleep.”

    When ever my prostate has been checked it has been fine. Why do you think this is this happening? Can you help me? I live in Marlton, New Jersey. Is there anything I can do to help myself? Do you know of a neurologist or spine specialist I should go to who knows about this to get help also? Please contact me as soon as you can.

    Thank you very much.

    Randy Beran

    • Our Greatest Wealth Is Health Says:

      Yes, neurological issues and spinal cord injury (SCI) can profoundly affect erectile function. Generally a SCI refers to a signifcant event, typically a vertebral fracture and not merely a cervical manipulation. You definitely should be checked out by a neurologist. I do not know any in the Marlton area, so I would seek the referral advice of an internist or family practitioner or call the county medical society for a referral.
      Best of luck and hopefully the situation will rapidly resolve.

      • Randy Beran Says:

        Thank you Doctor Siegel for writing back to me. You mention that nocturnal erections have a unique mechanism controlled in the brain stem. What is the unique mechanism in the brain stem known as?

        Thank you very much.

      • Our Greatest Wealth Is Health Says:

        Randy, there are many details lacking on sleep erections, but what is known is that they occur during REM sleep and are based in the pons of the brainstem. This driver of erections is different from tactile or erotic driven erections.

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