Erection can be regarded as a complex neurovascular process that can be initiated by recruitment of penile afferent signals (reflexogenic erection) and by visual, acoustic, tactile, olfactory and imaginary stimuli (psychogenic erection). Several brain regions have been identified that are involved in the initiation of penile erection. The effect of testosterone on these central mechanisms is described in depth in Chapter 4.
At the penile level, the erection is determined by the contractile state of the smooth muscles. Contracted smooth muscle cells in the flaccid penis minimize the blood flow into the sinuses of the corpora cavernosa. With sexual stimulation, three hemodynamic factors are essential for achievement of erection with full tumescence and rigidity:
(1) Relaxation of cavernosal smooth muscle cells which leads to intracavernosal reduction of resistance,
(2) increased arterial inflow into the sinuses of the corpora cavernosa by relaxation of smooth muscles of the arterial vessels, and
(3) restriction ofvenous outflow by compression of intracavernosal and subtunical venous plexus (for review van Ahlen and Hertle 2000; Shabsigh and Anastasiadis 2003).
In normal physiology, nitric oxide seems to be the key factor for smooth muscle cell relaxation in the penis. Neurogenic nitric oxide is considered as the most important factor for immediate relaxation of penile vessels. Endothelial nitric oxide seems to be responsible for maintaining erection (Andersson 2003). Within the muscle cell, nitric oxide activates a soluble guanylyl cyclase, which raises the intracellular level of cyclic GMP (Shabsigh and Anastasiadis 2003). cGMP activates a specific protein kinase which finally leads to a decrease of intracellular cytosolic calcium concentration and relaxation of the smooth muscle cells. Various other signal transduction pathways, such as the Rho-kinase pathway, are involved in erectile function which have been reviewed elsewhere (e.g., Andersson 2003; Kandeel etal. 2001; Wingard et al. 2003). However, sufficient levels of nitric oxide as well as the integrity of the smooth muscles of the penile arteries and the corpora cavernosa seem to be the predominant physiological factors for erection.
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