Nociception

Pain is an unpleasant sensory and emotional experience. The physiology is complex and is located largely in the brain. In normal circumstances pain is elicited in the presence of actual or threatened tissue damage. The nervous system relays information to the brain about that tissue damage. However that information itself does not constitute pain, and is not pain until that information reaches the brain. Stimuli that elicits pain is commonly termed noxious. The information about tissue damage is referred to as nociceptive information. Most of what we know about pain relates to the physiology of nociception.

Transduction

Transduction means the detection of stimuli that might elicit pain. There are two basic mechanisms of transduction - chemical and mechanical nociception.

Chemical Nociception

a free nerve ending is stimulated by chemicals. Once stimulated, action potentials are generated in that nerve. The cardinal noxious chemicals capable of triggered a nerve and eliciting pain are bradykinin, serotonin, hydrogen ions, and potassium ions. These chemicals are released by cell damage as well as through inflammation.

Prostaglandins are also released by cell damage, but aren't able by in large to elicit nociceptive activity in nerve fibres. Prostaglandins facilitate the excitatory effect of the other chemicals.

Substance P is another chemical involved in the process of nociception. This chemical is located inside the terminals of peripheral nerve fibres. When the nerve fibre is stimulated by other chemicals, substance P is released. In the periphery substance P is unable to directly stimulate these nerve fibres. It acts to promote vasodilation, which enhances inflammation, which thereby will secondarily increase the release of the above noxious chemicals and therefore secondarily enhance the stimulation of the nerve fibres.