Neuroscientists generally classify pain as either:

  • Acute pain, caused by an injury to the body. It warns of potential damage that requires action by the brain, or

  • Chronic pain, that persists long after the trauma took place or even in the absence of a trauma.

We prefer to define pain as a warning that some condition in the body is causing harm to it. A particular nerve ending is sending a signal via the spinal cord to the thalamus and hypothalamus in the brain.

The relay of information involves several neural circuits within the central nervous system (CNS). The nerves from the thalamus relay the signal to various areas of the brain's somato-sensory cortex and limbic system. From there signals go back down through the spinal cord. The ones to the motor nerves will cause muscle tension.

Thalamic neural impulses descend to the midbrain, from where they synapse on ascending pathways in the medulla and spinal cord and inhibit ascending nerve signals. This produces pain relief (analgesia). Relief also comes from the pain-relieving neurotransmitters called endorphins, etc.

Pain signals can set off -autonomic- ANS pathways as they pass through the medulla, causing increased heart rate & blood pressure, rapid breathing and sweating. The reactions depend upon the pain intensity. They can be depressed from the neocortex via various descending pathways.

The influences of these pathways could be responsible for psychogenic pain (pain perception with no obvious physical cause). Thoughts, emotions and 'neural circuitry' can affect both ascending and descending pain pathways. See also "How stuff works". 

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