Erection takes place when structures inside the penis and along the urethra between a man’s legs are filled with blood and inflated. This occur upon signals from the brain and the spinal cord, and the signals are transmitted through nerves in the parasympatic nervous system, a part of the autonomous or involuntary nervous system.

HOW THE NERVOUS SYSTEM TRIGGERS AND CONTROLS THE ERECTION

The erection is triggered and controlled by the following events:

• Something stimulates sensorial bodies in the genital zone or other senses. Impulses are then sent from the senses through nerves and the spinal cord towards the brain, and reaches the upper areas of the brain. The brain then recognizes these impulses as something sexually arousing. Also thoughts originating in the brain itself may be recognized as something sexually arousing.

• The higher brain areas having recognized some arousing events then send impulses down to the limbic system at the lower area of the brain. It is the limbic system that actually produces the feeling of excitement.

• The limbic system sends signals down the spinal cord and out to the genital area through nerves called nervi erigentes - a part of the parasympatic nervous system

• The signals reaching the penis and the rest of the genital zone then trigger erection and engorgement of the area between the man’s legs.

• Stimuli to the penile or genital sensorial bodies do not need to reach the brain to result in an erection. Areas in the spinal cord will also recognize the stimuli and get excited. Then the spinal cord also by its own sends erectile impulses back to the genital area.

• The sensorial bodies in the genital area will also be stimulated mechanically by the erection. Since stimulation of these bodies trigger erection, the erection process is self enforcing. A beginning erection thus trigger even more erection.

THE ANATOMICAL BASE FOR ERECTION

In the penis lie three bodies consisting of a network of very elastic blood vessels having circular smooth muscles in their walls, called erectile bodies. There are two paired bodies lying at the upper side of the urethra (corpora cavernosa), and one lying around the urethra (corpus spongiosum).

The muscles in the vessel walls are able to constrict the volume of the vessels, or relax allowing the vessels to widen. Vessels leading blood to or from the erectile bodies also have the ability to constrict or relax in order to restrict or facilitate blood flow. The erectile chambers are not only confined to the penis, but continue in the area between the legs all the way backwards to the anal area.

WHAT HAPPENS IN THE GENITAL ZONE DURING AN ERECTION

The erection response consists of the following events in the genital zone:

• When the signals from the brain and the spinal cord reach the nerve ends in the penis and the genital zone, the chemical compound nitric oxide (NO) is released.

• Nitric oxide then spreads through the genital area and especially the penis. Nitric oxide then triggers the following reactions.

• Blood vessels leading blood to the erectile bodies relax. Then more blood flows into the penis.

• Blood vessels leading from the erectile bodies constrict, making it difficult for blood to leave the bodies.

• The smooth muscles around the vessels in the erectile bodies relax, allowing these vessels to widen.

• The blood going into the erectile bodies will then fill up in the vessels of the bodies and inflate the bodies to a much larger volume.

• The inflated bodies will get straight and hard. The growing erectile bodies will inflate the whole penis and make the penis rise.

• Since the erectile bodies continues backwards between the man’s legs, also this area swells and fixes the penis rigidly so that it does not sway from side to side when fully erected.