Health A-Z

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Harvard Medical School

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What Is It?

You accidentally touch a hot stove. In a millisecond, you jerk your hand away. What just happened?

You have pain receptors throughout your body, both outside and within. These receptors send electrical messages through your spinal cord to the brain. You only become aware of pain after your brain receives and interprets these electrical messages. In some cases, such as when you touch a hot stove, the body can spring into defensive action even before the brain knows what is happening. That's because pain messages that reach the spinal cord can cause an automatic reflex response, making muscles near the source of the pain contract to get away from the pain.

Pain receptors and nerve pathways differ throughout the body. Therefore, the sensation of pain differs, too, depending on where the message comes from and how it travels. At times, the source of pain is difficult to locate. For example, some people feel the pain from a heart attack in the neck or jaw. People also differ in their ability to tolerate pain and how they respond to pain medication.

Pain is classified into two types:

Acute pain:

  • Usually has a clear source

  • Begins suddenly

  • Doesn't last a long time

  • Can increase heart and breathing rates and raise blood pressure

Acute pain is generally useful. It's a clear sign of danger. Examples include touching a hot surface, stubbing your toe or being cut.

Chronic pain:

  • Usually lasts a month or more, and could last years

  • Can come and go many times or remain constant

  • Can disturb sleep patterns, decrease appetite and cause depression

  • Often has little or no effect on blood pressure, heart or breathing rates

Chronic pain can outlive its usefulness; the message has been sent and received, but keeps being sent over and over. Examples include arthritis, cancer and back injuries.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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