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.

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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.

But orgasms are more than just muscular contractions. They feel good. So how do the brains of spinal-cord-injured people sense the pleasure? "I don't know. No one knows that yet," Alexander says.

An alternate route

Rutgers University's Komisaruk and retired Rutgers professor Beverly Whipple, coauthor of "The Science of Orgasm" and "The G Spot and Other Discoveries About Human Sexuality," believe they do know. But they don't think an orgasm is a reflex. Through studies of spinal-cord-injured women, they've found evidence of what appears to be a new orgasmic pathway, one that bypasses the spine completely.

The proposed detour makes use of a vast highway of nerves called the vagus nerve network. Like the vagabonds for which they were named, vagus nerves wander throughout the body. They start at the base of the brain, slide down the neck (but not the spinal cord) and stretch to all the major organs, and (at least in female rats) to the uterus and cervix. If vagus nerves reach human pelvises, genital signals could hopscotch over the spinal cord and still reach the brain.

Animal experiments support the idea. Female rats with intact vagus nerves but snipped genital nerves (cutting off their signals to the spinal cord) still respond to vaginal stimulation in their normal, albeit rodent-like, fashion: enlarged pupils, rapt attention and a tendency to ignore painful stimuli applied to their paws. But when the vagus nerves in the pelvises are also severed, all these sexual responses stop.

To investigate further, in a 2004 study, Komisaruk and Whipple worked with four women with shattered spinal cords. Each stimulated her cervix with a phallus while the researchers used fMRI scanning to measure brain activity.

Despite their severed spinal cords, all women reported feeling the touch of the stimulator, Whipple says. The sensation at the cervix was reaching the brain. What's more, in the fMRI scans their brains lighted up in an area where vagus nerve signals are processed. And three of the volunteers experienced an orgasm.

Komisaruk and Whipple have compared these brain images with those of women who are able to have orgasms by thought alone (who thus provide a clean brain image of a person reaching climax).

They found that orgasms elicit strong activity in the nucleus accumbens, the reward center, which also lights up in response to nicotine, chocolate, cocaine and music; in the cerebellum, which helps coordinate muscle tension; and parts of the hypothalamus, which releases oxytocin, the trust and social-bonding hormone.

Intriguingly, areas of the cortex that respond to pain also responded during orgasm. "Perhaps it's related to the fact that people often have pained expressions at the time of orgasm," Komisaruk says.

The amygdala, the brain's emotional center, and the hippocampus, which deals with memory, light up too. This helps explain a medical mystery: When epileptic seizures start in these areas, the electrical frenzy can triggers euphoric feelings called orgasmic auras.