Human models

Stimuli which have been employed in experimental studies of visceral nociception in human subjects include electrical stimuli, chemical stimuli, thermal stimuli, ischemia, and mechanical stimuli. Electrical stimulation produces reports of pain in humans and has been used to evoke cerebral potentials, in order to assess visceral sensory pathways. Chemical stimuli have been applied topically, intravascularly, or via physiological pathways (e.g. excreted agents) in order to define the endogenous substances responsible for an altered sensitivity to mechanical or environmental stimuli (e.g. acidity of urine) which may occur spontaneously or secondary to inflammation. Thermal stimuli (hot or cold) have been administered using hot or cold solutions instilled into visceral lumens and utilized to test for normal sensation and function, but rarely have been sources of clinical pain.52 Ischemia of visceral structures has been produced by the occlusion of visceral vasculature. Experimentally and clinically, the effects of such occlusion are dependent upon collateral bloodflow and metabolic activity of the selected organ. Venous congestion has mixed ischemic and mechanical components and so could also be a source of pain. The most commonly utilized experimental visceral stimuli are mechanical stimuli, such as the probing and stretch of visceral structures or the distension of hollow organs using fluids or foreign bodies. Mechanical stimuli may mimic what is observed in certain pathological pain states (e.g. bowel obstruction) and the pattern of mechanical stimulation may be important as it has been proposed to be the source of pain in functional bowel disorders.

Due to the fact that the hollow organs of the gastrointestinal tract are readily accessible through natural orifices, the earliest clinical studies of visceral sensation used balloon distension of esophagus, stomach, small bowel, large bowel, and rectum as their visceral stimuli. The advantages of balloon distension of hollow organs are many, foremost being that balloon distension reproduces pathologically experienced pain in humans in terms of intensity, quality, and area to which the sensation is referred. Hollow organ distension at constant pressure produces sensations and responses that are reliably reproducible and easily controlled by the experimenter.

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