When a healthy man is aroused by erotic stimuli and the pro-erectile stimuli are sufficient for his brain's sex centers to initiate the development of an erection, the sex centers release the neurotransmitters dopamine and oxytocin. These overcome the antierectile effect of the neurotransmitters noradrenaline and serotonin, thereby inhibiting the sympathetic nervous system's usual vasoconstrictive action on the penile arteries. Dopamine and oxytocin further activate Onuf's nucleus, the sacral spinal cord's erectile center.
From there, parasympathetic nerves convey the neural impulses down to the penis via the cavernous nerves, causing the release of additional chemicals that are actively involved in producing an erection through penile vasodilation. Additional parasympathetic stimulation down through the pudendal nerve causes the ischiocavernous muscles surrounding the corpora cavernosa to contract, increasing the rigidity of the erection. The pudendal nerve also conveys sensations of sexual pleasure and orgasm from the penis back up to the brain.
In a reflexogenic erection, however, the process is somewhat different. Direct sexual stimulation sends neural impulses up through the penile dorsal nerve to the spinal erectile center; from there, stimulation travels back down to the penis via the parasympathetic and cavernous nerves, without being modulated by the brain.