Then other parts of the brain leap into action. Some send signals back down to the body with certain instructions -- lubricate the vagina, stiffen the penis, pump blood harder, breathe faster.
The intensity builds to a crescendo, and just like a long-awaited sneeze, tension is released in an explosive rush. The heart rate doubles. In women, the uterus contracts rhythmically; in men, sperm-carrying semen is propelled out of the body.
And somehow, by mechanisms not yet understood, the brain perceives all this activity as a darn good feeling.
Such a signaling pathway would seem to rule out orgasms for anyone whose spinal cord is completely severed, because people with such injuries cannot feel the brush of a finger across the penis or clitoris.
But about two decades ago, anecdotal evidence started accumulating to the contrary. This was as a bit of a surprise to the medical profession, which for decades had told patients with damaged spinal cords to give up hope of a sex life. Researchers began to investigate.
One, Dr. Marca Sipski-Alexander, published studies in 2001 and 2006 reporting that about 50% of 45 men and 44% of 68 women -- all with varying locations and degrees of spinal cord injury -- had orgasms in the lab, with the help of adult videos and genital stimulation by hand or vibrator.
The findings show that the normal genitals-to-spine-to-brain route for an orgasm is not the only one. The best explanation may be that a touch unperceived by the brain can still be doing its work, says Alexander, a rehabilitation medicine professor at the University of Alabama at Birmingham School of Medicine.
Alexander thinks that an orgasm, like urination, is a reflex. Both functions can be controlled partly by willpower. But just as voiding your bladder doesn't require the say-so of your higher brain, she says, maybe orgasms don't either. Maybe all that's needed is some chit-chat between pelvis and spinal cord.
Some studies, mostly in animals, support this line of thought. In the brain stem and spinal cord, researchers have found hard-wired programs -- clusters of cells acting as primitive mini-brains of sorts -- that produce rhythmic movement without any higher brain input. These so-called central pattern generators are what let mollusks swim, rats crawl, tadpoles breathe and perhaps human males thrust their pelvises and ejaculate. Rat studies suggest that females, too, have these muscle-contracting proto-brains.
