Thus, a series of coordinated vascular events, controlled by autonomic nerves, leads to an erection: relaxation of the smooth muscle in the sinusoids in the corpus cavernosum; increased arterial inflow; and occlusion of the venous drainage from within the erectile spaces. But when the conscious or unconscious brain begins to be sexually stimulated, other chemicals are manufactured, which react with the penile erectile tissue (primarily smooth muscle cells) to set up the sequence of events that leads to an erection. In the penis, these chemicals normally keep the penile erectile tissue in the flaccid condition by keeping the smooth muscle cells contracted. Again, the left side of the diagram shows the flaccid penis, and the erect penis is on the right. Refer to it for a reminder of where these are located. This diagram shows the effect of nerve impulses on the endothelial cells and trabecular smooth muscle tissue within the two corpus cavernosum erectile cylinders of the penis. In this initial filling phase there is very little change in pressure within the two corpora cavernosa. The increased blood volume in the sinusoids initiates the venous occlusion process due to the stretching of the small veins below the tunica albuginea surface. Simply put, the penis is rapidly filling up with blood. The parasympathetic nerve stimulation now relaxes the smooth muscle of the small arteries and arterioles resulting in a four- to tenfold increase of arterial flow into the penis.

You can see the interplay of all these actions in the following summary of the phases of erection: When the penis is resting, the trabecular smooth muscle within the corpus cavernosum is contracted and arterial blood inflow is minimal, while venous outflow is quite rapid. These skeletal components play a role in the "rigid erection phase." In addition to the vascular components of an erection, there are skeletal-muscular components as well, which are controlled by the somatic (sensory and motor) nerves.