Their nervous systems are not equipped to detect painful information. Without the ability to detect painful events, you would continue to cause injury to yourself. For example, if you broke a bone in your arm, you might continue using the arm because it did not hurt. You could cause further injury to your arm. Events that cause reactions are called stimuli .
Stimuli are painful when they damage tissues or threaten to damage tissue.
Pain is nature's way of telling the brain about injury to the body. Nociception, then, is the response of the nervous system to painful stimulation. When the nociceptors detect a nociceptive stimulus, they send a message to the spinal cord. A famous theory concerning how pain works is called the Gate Control Theory devised by Patrick Wall and Ronald Melzack in 1965. This theory states that pain is a function of the balance between the information traveling into the spinal cord through large nerve fibers and information traveling into the spinal cord through small nerve fibers.