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The controlled progression of contents along the gastrointestinal tract is an essential part of digestion. Different patterns of intestinal movements are involved in the physiological progression of contents along the digestive tract and are the result of the interplay between spontaneous activity of intestinal smooth muscle and enteric neural circuits (Costa & Furness, 1982;Huizinga et al. 1998). Almost one hundred years ago, Bayliss and Starling (1899) revealed the presence of polarized reflex pathways in the intestine and suggested that they were responsible for the propulsion of contents. The analysis of intestinal propulsion was significantly advanced by Trendelenburg in 1917 who showed that reproducible propulsive motor patterns could be triggered in isolated segments of guinea-pig ileum by liquid distension. This form of intestinal peristalsis elicited in vitro is dependent on the activation of enteric circuits as many investigators have demonstrated (Kosterlitz, 1968;Tonini et al. 1981;Waterman et al. 1994b).Slow distension of isolated segments of guinea-pig intestine by liquid infusion produces a neurally-mediated shortening of the longitudinal muscle (Kosterlitz & Robinson, 1959) and an increase in diameter coinciding with an inhibitory reflex mechanism involving nitric oxide (intestinal accommodation; Waterman et al. 1994a). This initial response to liquid distension has been named the 'preparatory phase' (Trendelenburg, 1917;Kosterlitz, 1968). At a threshold volume or intraluminal pressure, a contraction of the circular muscle occurs at the oral end and propagates aborally to empty the segment. This propulsive event is called the 'emptying phase' and involves the activation of different enteric neural pathways (Waterman & Costa, 1994;Waterman et al. 1994b). Despite the common description of this motor behaviour as the 'peristaltic reflex' (Kosterlitz, 1968), it has become apparent that there is a sequential activation of neural

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to 'sense' the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways.