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Fig. 1: Anatomy and mechanism of penile erection. The erection pathway can be triggered by direct genital stimulation and by auditory and visual stimulation, which act in concert to increase penile blood flow. The penis has a highly specialized anatomical structure that allows a massive increase in blood flow to be trapped within the inelastic layers surrounding the penis (the tunica albuginea), which causes rigidity and expansion of the cavernosal smooth muscle. Photo: Christine Kenney
Fig. 2: Cellular perspective of the erection pathway. The signal (nitric oxide) is released from nerve endings or from endothelial cells and activates a cascade reaction, which ultimately leads to an increased cellular concentration of cGMP (cyclic guanosine monophosphate). This second messenger molecule induces a series of events that lead to smooth-muscle relaxation through a reduction in the intracellular calcium ion concentration. The enzyme PDE-5 (phosphodiesterase type 5) reverses this effect by metabolizing the cGMP to GMP rapidly. The clinically important inhibitors of this enzyme (sildenafil, vardenafil and tadalafil) all act to promote smooth-muscle relaxation by their ability to allow cGMP to accumulate when nitric oxide is released, as is the case when sexual stimulation is present. Photo: Christine Kenney