pain

Sense that gives an awareness of harmful effects on or in the body. It may be triggered by stimuli such as trauma, inflammation, and heat. Pain is transmitted by specialized nerves and also has psychological components controlled by higher centres in the brain. Drugs that control pain are known as painkillers or analgesics.

A pain message to the brain travels along the sensory nerves as electrical impulses. When these reach the gap between one nerve and another, biochemistry governs whether this gap is bridged and may also either increase or decrease the attention the message receives or modify its intensity in either direction. The main type of pain transmitter is known simply as ‘substance P’, a neuropeptide concentrated in a certain area of the spinal cord. Substance P has been found in fish, and there is also evidence that the same substances that cause pain in humans (for example, bee venom) cause a similar reaction in insects and arachnids (for instance, spiders).

Since the sensation of pain is transmitted by separate nerves from that of fine touch, it is possible in diseases such as syringomyelia to have no sense of pain in a limb, yet maintain a normal sense of touch. Such a desensitized limb is at great risk of infection from unnoticed cuts and abrasions.