Gut stimuli may also induce reflex motor responses. Reflex responses to gut stimuli in humans can be investigated in the laboratory using different methods to measure gut motor activity. The gut generates both phasic, pulse contractions, and tonic, sustained contractions. Phasic activity can be recorded by measuring pressure changes within the gut using conventional manometry. Tonic contractions do not produce detectable changes in intraluminal pressure, and thus, evaluation of tonic activity requires a more sophisticated methodology. Changes in gut tone can be measured by means of the barostat, as changes in the volume of air within an intraluminal bag, maintained at a fixed pressure level by an electronic air pump (9-11). When the gut relaxes, the barostat injects air into the intraluminal bag to prevent a pressure fall, and when the gut contracts, the barostat withdraws air. Using this isobaric approach, a volume expansion reflects a relaxation, and a volume reduction a contraction. The barostat has proven particularly useful for studying reflex activity, because brief inhibitory reflexes may be missed by recording intermittent phasic activity (7,8).
In contrast to the uniformity of perception, the reflex responses to gut distension are quite heterogeneous, and some data indicate that perception and reflex responses are dissociable and probably mediated by different mechanisms (7,8). From a pathophysiological standpoint this finding may be very important, because it means that perception and reflex responses to gastrointestinal stimuli may be independently altered in some conditions. Indeed, despite that gross motor abnormalities cannot be detected in patients with functional gut disorders using conventional techniques, more refined studies on reflex activity indicate that the dysfunction in these patients involves not only sensory pathways, but also regulatory motor pathways.
Some reflex responses involving central mechanisms may be directly related to conscious perception. For instance, it has been shown that visceral perception produces a parallel inhibition of a somatic flexion reflex, and the latter has been used as an objective equivalent of perception (23).
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