a normal penile erection requires desire, sensual stimulation, release of the luteinising hormone, arterial engorgement of the corpora cavernosa, and venous constriction, as well as an intact nervous system. therefore anything that interferes with this process can cause erectile dysfunction.
an inability of the brain to respond to sexual stimuli can interrupt the signals to the parasympathetic nervous system that release impulse that cause small arteries in the penis to dilate, resulting in an insuffiecient blood flow to the corpora carvenosa.