The science of pain

How and why your body reacts to dangerous stimuli

Millions of sensitive nerves guard our tissues, listening for physical danger. These pain sensors, or nociceptors, detect temperature, pressure and chemical signals. They have a high threshold for activation and only send messages when the body is at risk of harm. If skin temperature rises above 40 degrees Celsius or dips below 15 degrees Celsius, thermal nociceptors start to fire. If pressure exceeds three kilograms per centimetre squared, or if the skin stretches or tears, mechano-nociceptors kick into action. And if cells become damaged and start leaking their contents, chemical nociceptors switch on. A rapid response to nociceptor activation is crucial. If you put your hand in a flame, your body needs to react in fractions of a second. Nociceptors send their signals to the spinal cord, which manages the first step of the response. It can process some of the information without the brain, triggering a rapid withdrawal reflex. This is the very simplest form of damage control, and even primitive animals sense and respond to harm in this way. But pain is more than just a reflex. As the hand pulls away from the fire, the signal from the nociceptors passes up the spinal cord towards the brainstem. In the brain it enters the cerebral cortex, responsible for cognition and consciousness. Processing here ties the incoming sensory signals to memory and emotion, producing the complex feeling of pain. The unpleasant experience that follows helps us to remember harmful activities and to avoid them in the future.