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Impotence is the inability to maintain an erection of the penis for satisfactory sexual intercourse regardless of the capability of ejaculation.

Penile erection is managed by two different mechanisms. The first one is the reflex erection, which is achieved by directly touching the penile shaft and the second is the psychogenic erection, which is achieved by erotic stimuli. The first one uses the peripheral nerves and the lower parts of the spinal cord, whereas the second one uses the limbic[?] system of the brain. In both conditions an intact neurologic system is required for a successful and complete erection. Stimulation of penile shaft by the nervous system gives rise to the secretion of nitric oxide (NO), which causes the relaxation of smooth muscles of corpora cavernosa (the main erectile tissue of penis), subsequently giving rise to penile erection. Additionally, adequate levels of testosterone (produced by the testes) and an intact pituitary gland are required for the development of a healthy male erectile system. As can be understood from the mechanisms of a normal erection, impotence may develop due to hormonal deficiency, disorders of the neurologic system, lack of adequate penile blood supply or psychological problems.

A few causes of impotence may be iatrogenic (medically caused). Various medications intended to control blood pressure or to modify central nervous system response, may inhibit erection by denying blood supply or by altering nerve activity. Surgical intervention for a number of different conditions may remove anatomical structures necessary to erection, damage nerves, or impair blood supply.

Treatment depends on the cause. Testosterone supplements may be used for cases with hormonal deficiency. However, usually the cause is lack of adequate penile blood supply as a result of age-dependent damage of inner walls of blood vessels. Previously, medical substances were used to be directly injected into the erectile tissue of penile shaft to treat impotence. In some cases refractory to the medical treatment, a penile implant (penile prosthesis) could be advised. After the discovery of orally active agents that increase the efficacy of NO, which dilates the blood vessels of corpora cavernosa, more conservative methods were started to be used.

Sildenafil citrate (Viagra) is an oral agent that blocks specific phosphodiesterases. NO acts on blood vessels by increasing the levels of cyclic guanosine monophosphate, which is required for dilation of the vessels. Phosphodiesterases block this substance and reduce the effect of NO on erectile tissue. By using sildenafil, NOs effects are increased and a more sustained penile erection can be achieved. Other agents with less side effects and holding promise as an effective oral treatment for penile erectile dysfunction are currently being developed.

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