Painful memories

A young boy touches a hot oven plate. Heat sensors in his palm realise that the temperature exceeds a minimum safety threshold and activate sensory nerves in the hand. Within milliseconds, the information is passed on through the arm and the spinal cord to the brain. The brain interprets the incoming information as pain, registers that the hand is in danger of being burned and orders it to retract. Motor nerves in the spinal cord and the arm then transmit this order to the muscles that contract and pull the hand away from the oven plate. The hand is now safe, but the experience of pain caused by the heat remains. The child begins to cry and runs to his mother for consolation. He will remember this painful experience and, on the whole, steer clear of hot oven plates.Pain is good for us. Over millions of years, the nervous system has developed the ability to experience pain as a protective system to warn us of imminent dangers and to keep us out of trouble. But although this data‐processing system is sensitive, it is prone to errors: unlike motor nerves that cannot re‐grow once cut, sensory nerves are trickier. And if something goes haywire in the delicate wiring of the sensory nervous system, it can create problems of its own. Injured nerves can grow back erroneously or start firing erratically and thus produce the sensation of pain with no physical external influence. Or the processing of painful sensations in the brain is short‐circuited and results in a permanent sensation of pain. These feelings are what is known as chronic pain or, in some instances, neuropathic pain, reflecting that it is a disease state of the nervous system. Of course, there are other causes, such as inflammation or diseases of the …