Sensory transmission in the gastrointestinal tract

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.