Answer
The two masses of erectile tissue that are more important in penile erection are the two dorsolateral right and left corpora cavernosa. The centrally located corpus spongiosum plays only a minor role in erection.
Work Step by Step
Different sensory stimuli--visual, tactile, auditory, olfactory or imagined can cause action potentials to be sent to sacral the portion of the spinal cord. The stimuli initiated cause the sympathetic fibers to release nitric oxide(NO). Nitric oxide causes the smooth muscle inside the blood vessels walls of the corpora cavernosa to relax; as a consequence, increased amount of blood flows into widened sinuses . The result is tumescence or filling, widening, lengthening and stiffening of the penis. This process is assisted by the fact that the increased pressure in the penis compresses the the veins and slows outflow of blood, thus maintaining the erection.
