Scientists report that parts of a woman's brain switch off when she has an orgasm, including regions involved with emotion. Neuroanatomists from the University of Gronigen ran PET scans on women as they were resting, getting diddled by their partner's fingers, experiencing the "big O," or faking it. From New Scientist:
“At the moment of orgasm, women do not have any emotional feelings,” says Gert Holstege of the University of Groningen in the Netherlands....Link
As the women were stimulated, activity rose in one sensory part of the brain, called the primary somatosensory cortex, but fell in the amygdala and hippocampus, areas involved in alertness and anxiety. During orgasm, activity fell in many more areas of the brain, including the prefrontal cortex, compared with the resting state...
In one sense the findings appear to confirm what is already known, that women cannot enjoy sex unless they are relaxed and free from worries and distractions. "Fear and anxiety levels have to go down for orgasm. Everyone knows this but we can see it happening in the brain," (Holstege) explains.
From an evolutionary point of view, it could be that the brain switches off the emotions during sex because at such times the chance to produce offspring becomes more important than the survival risk to the individual. Holstege points to the extraordinary behaviour seen in some animals during the breeding season, such as March hares, when the urge to mate seems to override the usual fear of predators.