As stated before, perception depends on the intensity of the effective stimulus, so that the level of conscious sensation is related to the magnitude of the stimulus applied. However, the responses to gut stimuli depend also on the number of receptors activated, and specifically, visceral perception in humans is substantially modified by spatial summation phenomena (24,25). The area of stimulation in the intestine, that is, the extension exposed to a distending stimulus, determines the intensity of perception. Moreover, summation effects are similar whether adjacent or distant fields are stimulated, at least over the proximal half of the small bowel (25). These observations suggest that the intestine may tolerate circumscribed activation of sensory terminals without perception, but additional recruitment of afferents at other areas, even at distant sites of the gut, may induce symptoms.
The interaction of different types of stimuli in the gut also modifies conscious perception. For instance, transmucosal electrical nerve stimulation, even at a very low unperceived level, heightens perception of concomitant gut distension, and this sensitizing effect is not explained by changes in intestinal compliance (20,26,27).
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