In the natural sciences, there has long been a continuous conceptual battle between the "lumpers" and the "splitters"—those who wish to lump together phenomena with similarities as variations of an overriding mechanism and those who wish to split observed events into multiple independent phenomena with their own unique mechanisms. Nowhere is this lumper versus splitter dichotomy more apparent than in the field of pain research. Some would extrapolate all findings related to one type of painful stimulus to all types of painful stimuli in all sites. Others would claim that there can be no generalization of pathways or function for any pains arising from different parts of the body. Obviously, there is a middle ground where general principles may apply to many systems, but there may be mechanisms specific to individual systems. Such is the case with visceral pain.
Clinically, visceral pain is common. It keeps gastroenterologists, cardiologists, urologists, gynecologists, general surgeons, and internists of all kinds busy on a daily basis in their attempt to diagnose and treat its causes. Until recently, our knowledge related to pain arising from the internal organs of the body was extrapolated from studies related to heating and poking the surface of the body, but studies in the last two decades gave evidence that this is an overextrapolation that contains many inaccuracies. There are differences in the clinical experience of visceral pain when compared with that of cutaneous pain, and these differences have been confirmed in psychophysical studies comparing the two types of pain. There are also clear differences in the neurobiology of visceral pain systems when compared with those of superficial pain systems. This chapter will present an overview of these differences with an emphasis on human studies, and will defer an in-depth description of basic science studies to subsequent chapters. This chapter builds on previous reviews of this topic (1-6), and many primary sources may be found in those other sites. The terms "superficial" and "cutaneous" are used interchangeably, and to avoid ambiguity, the term "somatic" is avoided, since pain arising in deep, nonvisceral somatic structures such as muscles and joints share many of the characteristics of pain arising from the internal organs of the body.
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