Pain control after surgery: a patient's guide, Agency for Health Care Policy and Research.

What is pain? Pain is an uncomfortable feeling that tells you something may be wrong in your body. Pain is your body's way of sending a warning to your brain. Your spinal cord and nerves provide the pathway for messages to travel to and from your brain and the other parts of your body. Receptor nerve cells in and beneath your skin sense heat, cold, light, touch, pressure, and pain. You have thousands of these receptor cells, most sense pain and the fewest sense cold. When there is an injury to your body--in this case surgery--these tiny cells send messages along nerves into your spinal cord and then up to your brain. Pain medicine blocks these messages or reduces their effect on your brain. Sometimes pain may be just a nuisance, like a mild headache. At other times, such as after an operation, pain that doesn't go away--even after you take pain medicine--may be a signal that there is a problem. After your operation, your nurses and doctors will ask you about your pain because they want you to be comfortable, but also because they want to know if something is wrong. Be sure to tell your doctors and nurses when you have pain.