Science of sex

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.

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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.