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Sex tames pain

Sex, it turns out, is a good pain killer. At Rutgers university, Dr. Barry Komisaruk tested the pain thresholds of women using a thumbsrew-like machine, which applied pressure to a subject’s fingertip. He discovered that the pain threshold of women increased 50 per cent when they stimulated their anterior vaginal wall. The amount of pain they could take was raised 100 per cent at orgasm.

This isn’t just a coincidence. For some animal species, pain is a vital part of sex, leading to the release of hormones necessary for impregnation. When lions mate – between 20 to 40 times per day when a female is in estrus – aggressive neck biting is the norm. The males also inflict pain with barbed penises, which cause the female to ovulate by painfully raking the vagina as they withdraw. Pain plays a similar role for rats, which, like lions, require multiple copulations to produce a litter.

For these species, the analgesic qualities of vaginal stimulation are necessary; if sex is an excruciating experience, females won’t want to do it. Komisaruk speculated that his research might explain why humans sometimes engage in painful or violent sexual practices.

“Sexual stimulation could take the edge off the aversive quality of pain and make it just arousing instead of aversive. That could be a basis for combining the two. I think there’s a very close contact between the pain and pleasure systems, which is why facial expressions look pained at orgasm,” he said.

Vaginal stimulation also reduces pain during childbirth. Although giving birth is legendarily painful, Komisaruk said that some women report painless, and even orgasmic feelings during childbirth.

“There are many women who say that childbirth was not painful, and in fact, there are many published reports that childbirth feels like an orgasm. After delivering one or two children it’s not necessarily painful, depending on how stressed and worried the mother is,” he said.

Stress can dampen the analgesic and orgasmic qualities of giving birth just as it can those of intercourse, and is part of the reason why painless childbirths are rare. In North America, anxiety about the painful aspects of childbirth prevents many mothers from entering labour with enough relaxation to benefit from innate painkilling mechanisms.

Stimulation of the anterior vaginal wall reduces pain by blocking the release of a chemical called substance P, which is a major player in pain perception. Stored in the spinal chord, it’s released in response to painful signals, and helps transmit them to the brain.