The pain sensation originates as a chemical soup of signals emitted by pain-sensing nerve endings called nociceptors. These signals travel through the network of peripheral nerves that run throughout the body to the central nervous system, the brain, and spinal cord. There they gather in the dorsal horn, a site in the spinal cord that acts as a clearinghouse for pain messages. The signals are transmitted into the brain's thalamus, which sorts them and passes them on to the cerebral cortex, where the pain is actually ''felt.''

ACUTE PAIN

Sharp, immediate pain comes from an injury to tissue but can also be triggered by bodily malfunction or severe illness.

1. Nociceptors in the peripheral nerves sense the injury and in response, release chemical messengers.

2. The messengers travel through superfast nerve conduits to the spinal cord, where...

3. ...they are passed directly to the thalamus and into the cerebral cortex.

4. The brain precisely identifies the site of the injury and sends a message back down the spinal column telling the muscles to contract and block the pain. The process can take place in the instant that your finger touches a hot stove and pulls away.

CHRONIC PAIN

Persistent, debilitating pain, such as that from a bad back or diseases of the nerves themselves, takes a more circuitous route.

1. Pain signals enter the dorsal horn of the spinal cord

2. ...and transfer back and forth between interconnected nerves that modulate the pain message as it travels up the spinal cord

3. ...to the cerebral cortex, which assesses the damage and adjusts emotions and other bodily functions, such as breathing.

4. This slower pathway creates duller, more persistent pain. If the pain persists, the entire nervous system may be reprogrammed to create a lower threshold for pain.