What is the normal erectile process relative to erectile dysfunction (ED)?

Updated: Aug 08, 2018
  • Author: Edward David Kim, MD, FACS; Chief Editor: Edward David Kim, MD, FACS  more...
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Answer

Erections occur in response to tactile, olfactory, and visual stimuli. The ability to achieve and maintain a full erection depends not only on the penile portion of the process but also on the status of the peripheral nerves, the integrity of the vascular supply, and biochemical events within the corpora. The autonomic nervous system is involved in erection, orgasm, and tumescence. The parasympathetic nervous system is primarily involved in sustaining and maintaining an erection, which is derived from S2-S4 nerve roots.

Sexual stimulation causes the release of neurotransmitters from cavernosal nerve endings and relaxation factors from endothelial cells lining the sinusoids. NOS produces NO from L-arginine, and this, in turn, produces other muscle-relaxing chemicals, such as cGMP and cyclic adenosine monophosphate (cAMP), which work via calcium channel and protein kinase mechanisms (see the image below). This results in the relaxation of smooth muscle in the arteries and arterioles that supply the erectile tissue, producing a dramatic increase in penile blood flow.

Relaxation of the sinusoidal smooth muscle increases its compliance, facilitating rapid filling and expansion. The venules beneath the rigid tunica albuginea are compressed, resulting in near-total occlusion of venous outflow. These events produce an erection with an intracavernosal pressure of 100 mm Hg.

Additional sexual stimulation initiates the bulbocavernous reflex. The ischiocavernous muscles forcefully compress the base of the blood-filled corpora cavernosa, and the penis reaches full erection and hardness when intracavernosal pressure reaches 200 mm Hg or more. At this pressure, both inflow and outflow of blood temporarily cease.

Detumescence results from cessation of neurotransmitter release, breakdown of second messengers by phosphodiesterase, and sympathetic nerve excitation during ejaculation. Contraction of the trabecular smooth muscle reopens the venous channels, allowing the blood to be expelled and thereby resulting in flaccidity.


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