The mechanisms for erection are fairly complex: A sensory stimulus triggers the brain to send nerve impulses down through the spinal cord. These signals trigger the release of a chemical messenger that causes the vessels supplying blood to the penis to dilate. The rod-shaped spongy tissues (corpora cavernosa) in the penis then fill with blood and expand, pressing against the veins that normally allow blood to drain from the penis, thus producing an erection. Interference with any part of this process - whether psychological or physiological - may cause erectile dysfunction.
Although an occasional inability to achieve or maintain an erection is common and not a sign of a chronic problem, a doctor should be consulted if erectile dysfunction persists. Treatment depends upon the underlying cause of erectile dysfunction.