Various sexual stimuli (emotional, imaginative, visual, tactile, auditory, olfactory, etc) are received by the body and relayed to an area deep within the brain called the limbic system.

• Depending on the nature and extent of the stimuli, brain cells in the limbic system can become “excited” and send electrical signals into the spinal cord and eventually out to the penis via parasympathetic nerves to tell it to become erect.

• These nerves terminate at the single cell layer (endothelium) lining the inside of arteries which supply blood to the erectile tissue.

• There are three storage cylinders filled with spongy tissue (the paired corpora cavernosa and the single corpus spongiosum) which along with the urethra constitute the entirety of what’s inside the penis.

• The nerve signals cause the endothelial cells to make and release nitric oxide (NO).

• The NO diffuses into the network of smooth muscle cells which comprise the middle layer of the arterial wall.

• Inside of the muscle cells, NO activates an enzyme (guanylate cyclase) which transforms GTP (guanasine triphosphate) molecules into cGMP (cyclic guanasine monophophate) molecules.

• cGMP is the chemical that causes the smooth muscle cells to relax.

• cGMP is broken down fairly quickly by another enzyme called phosphodiesterase (PDE), but as long as nerve signals continue to arrive and cause release of NO and activation of guanylate cyclase, more cGMP is made.

• If enough cGMP is produced to relax the muscle layer sufficiently, the arterial wall expands outward and the opening (lumen) in the artery widens and more blood can flow through it.

• The arterial blood vessels carry the blood into “storage spaces” (sinusoids) inside of the spongy tissue.

• The increased blood flow allows the sinusoids to fill up and expand.

• Collagen and elastic fibers (tunica albuginea) like the “casing of a sausage” enclose the corpora and gives rigidity to the penis when they are stretched by the increased blood volume in the sinusoids. On average, a 3-fold increase in blood volume is needed to obtain a functional erection.

• But, like when trying to fill up a bathtub, there needs to be a way to plug the drainage system. Unlike arteries, veins don’t have a thick muscular layer that can contract and limit flow.

• The tiny, thin-walled veins that drain blood from the sinusoids come together to form a plexus of veins located between the peripheral sinusoids and the tunica albuginea before passing through the tunica on their way out of the penis.

• When the sinusoids expand and the tunica is stretched tight, the venous plexus and exiting veins are compressed.

• There is a marked reduction of blood leaving the corpora, blood volume in the corpora increases and the penis becomes increasingly firm. Pretty darn amazing!

• After ejaculation occurs or the limbic brain loses interest, parasympathetic nerve signals to the corpora stop and the Nitric Oxide – Cyclic GMP Pathway shuts down.

• The smooth muscle cells in the erectile tissue vasculature then return to their usual moderate state of contraction under the control of the sympathetic nervous system.

• Blood flow and trapping is decreased and the erect penis returns to its flaccid state.