Autonomic control In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system (ANS) with minimal input from the central nervous system. Parasympathetic branches extend from the sacral plexus into the aeries supplying the erectile tissue; upon stimulation, these nerve branches release acetylcholine, which, in turn, causes release of nitric oxide from endothelial cells in the trabecular aeries. Nitric oxide diffuses to the smooth muscle of the aeries (called trabecular smooth muscle), acting as a vasodilating agent. The aeries dilate, filling the corpora spongiosum and cavernosa with blood. The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the ANS causes constriction of the penile aeries, forcing blood out of the erectile tissue. After ejaculation or cessation of stimulation, erection usually subsides, but the time taken may vary depending on the length and thickness of the penis. Voluntary and involuntary control The cerebral coex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord. The coex may suppress erection, even in the presence of mechanical stimulation, as may other psychological, emotional, and environmental factors.Ref: Ganong&;s review of medical physiology 23rd edition Page no: 405