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Key Words nociception, interoception, spinothalamic tract, insula, specificity
Abstract The issue of whether pain is represented by specific neural elements or by patterned activity within a convergent somatosensory subsystem has been debated for over a century. The gate control theory introduced in 1965 denied central specificity, and since then most authors have endorsed convergent wide-dynamic-range neurons. Recent functional and anatomical findings provide compelling support for a new perspective that views pain in humans as a homeostatic emotion that integrates both specific labeled lines and convergent somatic activity.
INTRODUCTION
Pain is an enigma. It differs from the classical senses (vision, hearing, touch, taste, and smell) because it is both a discriminative sensation and a graded motivation (or behavioral drive). It is a leading clinical complaint that can present mystifying symptoms, such as allodynia (sensitization to normally innocuous stimuli), referral from deep tissue to skin, radiation over wide regions, temporal augmentation ("windup"), persistent after-sensations, emotional variability, and hyperpathia (hysterical responses). It can attain intolerable intensity, but it can disappear in the heat of battle. It is a universal human experience that is commonly generalized to psychic suffering of any sort.
Accordingly, the nature of pain has always been controversial. In his book Pain, Sir Thomas Lewis (1942), a clinician who differentiated the distinct sensations of first (sharp) and second (burning) pain as well as pain of different tissue origins, pointedly abstained from giving a global definition of pain. The neural basis of pain has been fervently debated from two opposing views-specificity and convergence. The former view posits that pain is a distinct sensation represented by specialized elements both peripherally and centrally, consistent with the idea that the nervous system is evolutionarily and reproducibly well-organized. The latter posits that pain is an integrated, plastic state represented by a pattern of convergent somatosensory activity within a distributed network (a so-called neuromatrix). Historically, these "splitter" and "lumper" views have alternated in dominance.
One should expect that the solution to this puzzle involves both specificity and integration, as found in all other neural systems. A solution has been provided by recent neurobiological findings, which indicate that pain is not part of the exteroceptive somatosensory system that engenders touch, but rather is represented in an unforeseen, novel pathway in humans...