Brain scans of women reaching orgasm have cast new light on how the mind and nervous system work.

Had Meg Ryan not been faking it when she so memorably howled with pleasure while sitting in a café in When Harry Met Sally, various parts of her brain would have lit up, including the one which responds to psychoactive drugs such as cocaine. By scanning women's brains during sexual stimulation for the first time, scientists have found which parts of the mind and body are involved in orgasm. The discoveries may well lead to a breakthrough for women suffering from a number of sexual disorders, and lead to other medical breakthroughs, such as pain control for patients with spinal-cord injuries.

Scientists Barry R Komisaruk, Carlos Beyer-Flores and Beverly Whipple (co-author of the international bestseller The G Spot) embarked on the research after reports that women who had suffered complete spinal-cord injuries still felt sensations during intercourse or other sexual stimulus. Some even said they could achieve orgasms.

"This was very puzzling because the conventional view is that their spinal-cord injuries should have abolished all those sensations," says Professor Komisaruk of New Jersey's Rutgers University. "There was also a report of at least one such woman saying that she could feel her baby kicking when she was pregnant."

With support from the National Institute of Health, an agency of the US Department of Health and Human Services, they carried out a study on women with complete spinal injuries. The women were monitored and given tests relating to their pain thresholds during vaginal or cervical self-stimulation.

"When we tested these women I found that those who had a spinal-cord injury even at the highest level, in the upper back, could feel vaginal and cervical stimulation," said Professor Komisaruk. "Some of the women said they had orgasms."

The neuroscientist hypothesised that there was an alternative pathway from the vagina and cervix to the brain: the vagus nerve, which leads from the lowest part of the brainstem, the medulla, through the base of the skull, down the neck, into the chest cavity, through the diaphragm and into the abdominal cavity without using the spinal cord. The first evidence that the nerve goes to the pelvic region was found in 1990 with rats. At this stage, however, it was not known whether it reached the same region in humans.

The only way to test this hypothesis was to do brain imaging in these women and see if vaginal or cervical self-stimulation would activate the medulla known to link with the vagus nerve - he found it was true.

"The women were very happy to know that there was a scientific basis for the genital sensation they felt, because many of them were troubled by the fact that their doctors had told them that after their injury they couldn't possibly have any," he says. "Many of the women hadn't even tried to see if they could, and in the laboratory it was the first time in years since they had even tried. They were surprised that they did have genital sensation. It was very emotional and some of the women started crying with happiness. It was a very moving experience. Actually, some of us started crying with them because it was so dramatic."

There is still much to be learnt about the nervous system. Some nerve networks, especially on internal organs, including the vagina, cervix and uterus, are incredibly complex. It is not known which nerve endings should be preserved to keep sensation after a hysterectomy, for instance. But in the past two decades there has been a revolution in our understanding of neurotransmitters.

More than 50 neuropeptides (chemicals which modulate activity of neurons and can be released in various combinations, giving various "flavours" to neuronal responses) have been mapped in nerves that had never been known before. The mapping of the vagus nerve means it may be possible to bring back sensation in other internal organs in the pelvic region, such as the bladder.

Professor Komisaruk's brain imaging of an orgasm was a medical first. "One of the dramatic findings is that areas throughout the brain become activated during orgasm," he says. "There is tremendous utilisation of oxygen throughout the brain. Orgasm is good for the brain."

One part of the brain that was strongly activated was the nucleus accumbens, which other scientists have shown becomes activated by psychoactive drugs such as cocaine, nicotine and caffeine. Another two areas were the insula and anterior cingulate, which become active in response to pain. "It suggests there is some sort of inhibitory activity going on there, as orgasm and vaginal stimulation are strong pain-blocking stimuli." The third area of interest was the paraventricular nucleus, where the hormone oxytocin is produced. Oxytocin is released into the blood stream at orgasm and causes uterine contractions.

About 99 per cent of the brain remains a mystery to scientists. "There are an estimated 100 billion neurons in the human brain. It's been estimated that each neuron receives about 1,000 inputs from other neurons," he says. "As neuroscientists, we understand much less of the 'terrain' of the brain than astronomers know about the moon." The way in which neurons of the brain create conscious awareness is the "Holy Grail" of neuroscience.

Professor Komisaruk's new area of research involves showing women their brain activity in real time, to see if it can cure sexual problems - they stimulate themselves while lying in a scanner and watching a monitor. "Can women who don't orgasm voluntarily intensify the activity in the right parts of their brain? Can women who have persistent genital arousal voluntarily reduce that activity?" It is not as incredible as it sounds - people with chronic pain are able to reduce their suffering by looking at the parts of their brain that are responding to pain.

"There is also a lot of hope in this method of neural biofeedback," he adds. "It opens the door to possibilities of our relating to our own brains in ways which have not been thought possible. Can it help control addictions or depression? I see great potential."

'The Science of Orgasm' by Barry R Komisaruk, Carlos Beyer-Flores and Beverly Whipple is published by Johns Hopkins, priced £16.50