The gastrointestinal tract must balance the ostensibly conflicting needs of nutrient assimilation and protection against potentially harmful pathogens, enterotoxins, and antigens. Furthermore, it must provide a functional barrier for the latter but allow nutrients, electrolytes, and water to cross the mucosa readily in order to meet the eventual metabolic demands of the individual (fig 1). Apparent in these conflicting needs is the ability to sense luminal contents in order to establish, through reflex and endocrine mechanisms, the appropriate motor and secretory activity either to facilitate uptake or to dilute and rapidly expel contents through diarrhoea and/or emesis.

Enteroendocrine cells may be secondary sense cells for sensory signal transduction. Their apical microvilli could detect the mechanical and chemical environment in the lumen, and in response to an appropriate stimulus would release mediators across the basolateral membrane. These mediators could then act as paracrine agents on neighbouring cells, including the terminals of afferent fibres situated below the mucosal epithelium or act in an endocrine fashion following diffusion into the systemic …