Pain Theory today
Wilhelm Erb’s (1874) early pattern theory hypothesis, that a pain signal can be generated by intense enough stimulation of any sensory receptor, has been soundly disproved. The thin (A-delta and C) peripheral nerve fibers carry information regarding the state of the body to the spinal cord. Some of these thin fibers do not differentiate noxious from non-noxious stimuli, while others, nociceptors, respond only to painfully intense stimuli.
Regions of the cerebral cortex associated with pain.
Because the A-delta fiber is thinly sheathed in an electrically insulating material (myelin), it carries its signal faster (2.5–35 m/s) than the unmyelinated C fiber (0.5–2.0 m/s). Pain evoked by the (faster) A-delta fibers is described as sharp and is felt first. This is followed by a duller pain, often described as burning, carried by the C fibers.
A.D.Craig and colleagues have identified fibers dedicated to carrying A-delta fiber pain signals, and others dedicated to carrying C fiber pain signals up the spinal cord to the thalamus in the brain. Pain-related activity in the thalamus spreads to the insular cortex (thought to embody, among other things, the feeling that distinguishes pain from other homeostatic emotions such as itch and nausea) and anterior cingulate cortex (thought to embody, among other things, the motivational element of pain); and pain that is distinctly located also activates the primary and secondary somatosensory cortices.
Koji Inui and colleagues have recently shown that pain reduction due to non-noxious touch or vibration can result from activity within the cerebral cortex, with minimal contribution at the spinal level. Melzack and Casey’s 1968 picture of the dimensions of pain is as influential today as ever, firmly framing theory and guiding research in the functional neuroanatomy and psychology of pain.
Thousands of years ago, ancient peoples attributed pain to spirits and treated it with mysticism and incantations. Over the centuries, science has provided us with a remarkable ability to understand and control pain with medications, surgery, and other treatments. Today, scientists understand a great deal about the causes and mechanisms of pain, and research has produced dramatic improvements in the diagnosis and treatment of a number of painful disorders. For people who fight every day against the limitations imposed by pain, the work of NINDS-supported scientists holds the promise of an even greater understanding of pain in the coming years. Their research offers a powerful weapon in the battle to prolong and improve the lives of people with pain: hope.
