Pain signalling in humans as rapid as touch signals

A recent study discovered that pain signals in the body can travel as fast as touch signals.

In monkeys and many other mammals, part of the pain-signalling system can conduct nerve signals just as fast as the system that signals touch. The scientists speculated whether such a system is also present in humans, reported the study published in the journal of Science Advances.

"The ability to feel pain is vital to our survival, so why should our pain-signalling system be so much slower than the system used for touch, and so much slower than it could be?" asked Saad Nagi, principal research engineer.

To answer this, the scientists used a technique that allowed them to detect the signals in the nerve fibres from a single nerve cell. They examined 100 healthy volunteers and looked for nerve cells that conducted signals as rapidly as the nerve cells that detect touch, but that had the properties of pain receptors, otherwise known as nociceptors.

Pain receptors are characterised by the ability to detect noxious stimuli, such as pinching and abrasion of the skin, while not reacting to light touch. The researchers found that 12 per cent of thickly myelinated nerve cells had the same properties as pain receptors, and in these nerve cells, the conduction speed was as high as in touch-sensitive nerve cells.

The next step of the scientists' research was to determine the function of these ultrafast pain receptors. By applying short electrical pulses through the measurement electrodes, they could stimulate individual nerve cells. The volunteers described that they experienced sharp or pinprick pain.

"When we activated an individual nerve cell, it caused a perception of pain, so we conclude that these nerve cells are connected to pain centres in the brain," said Saad Nagi.

The research team also investigated patients with various rare neurological conditions. One group of people had, as adults, acquired nerve damage that led to the thickly myelinated nerve fibres being destroyed, while the small fibres were spared. These patients cannot detect light touch.

The scientists predicted that the loss of myelinated nerve fibres should also affect the rapidly conducting pain system they had identified. It turned out that these people had an impaired ability to experience mechanical pain.

Examination of patients with two other rare neurological conditions gave similar results. These results may be highly significant for pain research, and for the diagnosis and care of patients with pain.

"It's becoming evident that thickly myelinated nerve fibres contribute to the experience of pain when it has a mechanical cause. Our results challenge the textbook description of a rapid system for signalling touch and a slower system for signalling pain. We suggest that pain can be signalled just as rapidly as touch," said Saad Nagi.