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Sense of pain learned by touching

May 25, 2004

The fact that a newborn baby can experience pain has previously been taken as evidence that pain reflexes are inborn, not learned. This is because the baby in the womb has been protected from everything that could cause pain and should therefore not have been able to learn what pain is. But according to a team of scientists at Lund University, Sweden, headed by Professor Jens Schouenborg, the tactile feeling of fetal movements in the womb is sufficient to initiate a process of learning in the undeveloped pain system.

"The system for tactile feeling needs only tiny stimuli: the skin reacts to extremely light pressure and contact. We have found that the tactile feeling is used to training the pain system, which normally reacts to stronger stimuli," says Alexandra Waldenstrom.

She recently defended her thesis, in which she has studied reactions to pain in newborn rats, using ultra-short pain-inducing pulses of heat. The experiments showed that the young rats started to learn how to withdraw their tails from noxious stimuli at the age of ten days. In terms of maturity this period in a young rat corresponds to the fetal stage in humans, roughly between weeks 10 and 30.

The fact that this learning is associated with touching was shown in further experiments in which the tails of the young rats were anesthetized so that they could sense touch but not feel pain. Here too, they learned to withdraw their tails. Other experiments indicated that the tactile feeling arising from the spontaneous twitching in rat pups during sleep led to learning of the pain-system.

This fine-tuning of the pain system proved to take place during a certain period during development. This may mean that learning about pain is limited to a certain period in human fetal development, and that disturbances during this period can lead to malfunction of the protective reflexes that make us instantly remove our hand from a hot burner on a stove, for example. Perhaps premature infants should therefore not be protected from touching, but rather should be exposed to touching similar to the tactile feeling they would have experienced in the womb.

Alexandra Waldenstroms thesis enhances our knowledge of the mechanisms behind the training of the pain system and the maturation of the spine. In the long term this increases the chances of developing better pain-relievers and perhaps even of repairing spinal damage.

Vetenskapsrådet (The Swedish Research Council)