Reflex erections are facilitated by the pudendal nerve, which conducts sensory afferents from the penis (dorsal nerve of the penis) and the perineal skin to the S2–S4 sacral spinal cord.
The penis is innervated through sympathetic and parasympathetic autonomic fibers.
Parasympathetic impulses are increased and sympathetic impulses are inhibited, resulting in a net parasympathetic gain, which results in erection.
There is a very rapid nine-fold increase in arterial blood flow.
This vascular surge is accomplished by relaxation of the smooth muscle within the media of the cavernosal arteries and within the erectile sinusoids of the corpora.
In the filling stage of the erection, postganglionic parasympathetic fibers from the cavernous nerves release nitric oxide (NO).
Contractions of the ischiocavernosus and bulbocavernosus muscles produce an increase in the intracavernous pressure.
Erection is maintained by the contraction of the ischiocavernosus and bulbospongiosus muscles.
This contraction maintains limited venous outflow, thus maintaining corpora volume and pressure.
After ejaculation, the venous blood flow returns to its preerection baseline and the penis drains, returning to the arterial-venous steady state.