receptor-bearing cells that become activated by temperature, touch, movement, or chemical changes in their environment. Pain signals travel to the brain by one of three main pathways, described in Box 4.1. Pain may be acute—short lived and intense—or chronic, persisting for days to years. For acute pain, such as the discomfort that follows surgery, doctors typically prescribe opiates: narcotic drugs derived from, or chemically similar to, opium. For chronic pain, however, opiates rarely bring relief. Even when they are effective, opiates often cause nausea and sedation that become a burden to the long-term user. At the very least, people with chronic pain develop tolerance to opiates over months or years and so must continually increase their dosage. Clearly, better pain medications would be welcome. Might marijuana be a source of these sought-after drugs?

Box 4.1

Types of Pain

Pain signals arise and travel to the brain by one of three main pathways, each of which produces different pain sensations:

  • Somatic pain is the feeling most people imagine when they think about pain: a message sent by receptors located throughout the body whenever injury occurs. Somatic pain signals travel to the brain via peripheral nerves, and are typically experienced as a constant, dull ache in the injured region.

  • Visceral pain occurs when tissues or organs in the abdominal cavity become stretched or otherwise disturbed due to disease or injury. Pain signals issue from a specific class of receptors present in the gut, producing feelings of pressure deep within the abdomen. Visceral pain often seems to be coming from a different part of the body than its actual source, a phenomenon known as referred pain.

  • Neuropathic pain occurs when nerves themselves sustain injury. It is often experienced as a burning sensation that can occur in response to even a gentle touch. Neuropathic pain does not usually respond to narcotic painkillers, which relieve many other types of pain. Antidepressant or anticonvulsant drugs, as well as certain surgical procedures, may improve some cases of neuropathy.

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