What is the physiological mechanism of an erection? - Tadalafil and Erectile Dysfunction Basics

An erection is a neuro-vascular event in which  the degree of erection depends upon the balance  between arterial inflow and venous outflow  (Figure 1) [2]. Psychological or physical  sexual stimulation initiates the release of nitric  oxide—the ‘‘first messenger’’—from neurons  and endothelial cells in the penis. Nitric oxide  leads to increased generation of cyclic guanosine  monophosphate (cGMP). The cGMP,  which is an intracellular second messenger,  in turn activates protein kinase G. Protein  kinase G decreases calcium in the cytosol,  which ultimately leads to the relaxation of  the smooth muscle of the corpus cavernosum.  Arterial inflow increases and the venules  are compressed, impeding the outflow of  blood, which causes an erection by increased  blood trapping. 
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