Erectile Dysfunction

How does an erection happen?

The simple explanation is that the penis smooth muscle tissue relaxes while the arteries open and the veins get compressed. This results in a 6 fold increase in blood flow to the penis and a "tumescent" (ie. hard) state. A more detailed explanation is provided below.
1. Pre-Initiation/Foreplay = D4 Erectogenic Receptors stimulated in the brain along with other erectogenic receptors resulting in positive psychogenic signals.
2. Initiation = Positive psychogenic (from the Medial preoptic area of the hypothalamus ) or somatic sensory (from the pudendal sensory nerve ) signals to the erection control nerves located in the spinal cord.
3. Sinosodal Relaxation = Increased activity of the Parasympathetic Nerve signals to the Smooth muscle tissue in the penis. Associated with decreased activity of sympathetic Nerve signals.
4. Arterial Dilation = Increased activity of the Parasympathetic Nerve signals to the Penile arterioles (ie. helicine arteries ) mediated through nitrous oxide, acetylcholine, and vasoactive intestinal polypeptide (VIP). Associated with decreased activity of sympathetic Nerve signals to the penile arterioles and venules.
5. Venous Occlusion = Expansion of the penis sinusoids causes compression of the subtunical veins and reduced venous outflow. When this is combined with the increased arterial flow from steps 2-3 there is a resulting ballooning and erection of the penis.