Normal erectile function involves three synergistic and simultaneous processes: 1) neurologically mediated increase in penile arterial inflow, 2) relaxation of cavernosal smooth muscle, and 3) restriction of venous outflow from the penis.
Loss of the functional integrity of the endothelium and subsequent endothelial dysfunction plays an integral role in this issue. Pharmacological (Phosphodiesterase drugs such as Viagra) and endocrine (Testosterone) interventions help restore some function, but more is needed because of the 45% failure rate with these treatments. The incidence of ED dramatically increases in men who have endothelial dysfunction such as seen in middle age particularly those with diabetes mellitus, hypercholesterolemia, and cardiovascular disease. These diseases have in common a dysfunctional endothelium.
Penile erection is a psychoneurovascular phenomenon that depends upon cerebral stimulation, neural integrity, and a functional vascular system with healthy cavernosal tissues . It is the fit endothelium that produces the NO. Assuming the desire (libido), an intact nervous system and enough NO being produced from the endothelium, the penis will become erect and remain that way until it becomes refractory following ejaculation. Testosterone will encourage the brain if given sufficient visual and tactile clues. The intact nerves are needed not only for the brain to perceive touch, but to release the neurotransmitters to the penile arteries to dilate them. This dilatation to a large extent depends on the production of NO from the endothelium. If enough NO is released it will compensate for some lack of integrity of the neurovascularity. NO, then, is another therapy not generally used by doctors that is additive to testosterone and the phophodiesterase stimulators. Science has discovered several ways to increase NO production that will be discussed in another article.