The mechanism of erection

As a sexual tool, the penis is remarkably complex and elegantly effective. If it's to enlarge and stiffen sufficiently to permit penetration, three different systems must play a part. The nervous system must receive and send signals, the vascular system must supply sufficient blood to fill the penile chambers, and the muscular system in those lower regions of the body must exert pressures allowing the erection to be created and maintained. Without all of these systems working together, a normal, functioning erection is not in the cards.

Erections begin as a result of the stimulation of the nervous system. This can occur in many different ways, but all of the communications are filtered through the brain. Even the direct sensory stimulation of the genital organs will send messages along the pudendal nerve and up the spine to the central nervous system message centers. Everything else gets there, too. Sexual feelings, verbal or visual communications with the loved one, touching and kissing, or, if a man is by himself, erotic fantasy, sexual imagery, anticipation, memory, or any other excitation of desire will each in its -turn stimulate various regions of the brain.

All these regions combine their messages in the hypothalamus, from whence the good news is sent southwards in a cascade of neurotransmitter signals. Certain parts of the message go directly down the spinal cord to the penis. But the autonomic nervous system, that which controls unconscious functions of the body such as breathing and digesting, also receives the message and transmits it through chemical signals that the entire body responds to. So an extraordinary variety of incoming stimuli - everything from the words, "I love you," to the gentle touch of a loving hand - combine to send a variety of neuronal signals towards the groin, all with an essentially similar message: "Attention, please!"

The penis is, of course, primed to respond to the signals the body is so eager to send it. A man's sexual organ is a pretty unusual piece of equipment, basically an inflatable, multi-chambered filling tank with a slender tube running down its center. No other part of either the male or the female body carries out at short notice such an extraordinary transformation as the penis.

Triggered by stimuli from the central nervous system and/or the peripheral nervous system, the muscular walls of the arteries dilate allowing more blood to flow into the genital region. As that blood arrives, the smooth muscles in the penis relax, so that