Sexual arousal and the development of an erection are complex processes involving psychologic, endocrine, vascular, neurologic, and local anatomical circuits. Brain scans indicate that sexual arousal begins in the brain’s higher cortical areas that stimulate the medial preoptic and paraventricular nuclei in the hypothalamus.16 The signal travels down neural networks to activate the parasympathetic nerves lying in S2 through S4. The result is inhibition of adrenergic tone and nitric oxide release, which stimulates guanylate cyclase in penile smooth muscle. cGMP increases and the smooth muscle relaxes to allow increased arterial inflow and venous flow occlusion, ultimately producing an erection.
Metabolic syndrome disorders are associated with chronic inflammation and oxidative stress, creating endothelial dysfunction leading to erectile dysfunction.17 Cigarette exposure reduces the production of nitric oxide synthase in animal models.18
Erections are also androgen-dependent, as men with hypogonadism have a dramatic reduction in the frequency, amplitude, and rigidity of erections.19,20 Approximately one-third of men in the European Male Aging Study had low testosterone. Treatment with androgen therapy can improve libido and mild ED symptoms in hypogonadal men.21