NEURAL REFLEXES CONTROLLING INTESTINAL MICROCIRCULATION

It has long been established that neural reflexes are involved in the regulation of gastrointestinal vascular function, in particular the mu cosal hyperemia that follows food ingestion. However, more precise ide ntification of reflex pathways involved in the control of mucosal bloo d flow had not previously been forthcoming because of a lack of adequa te methods to examine resistance arterioles within the intestinal wall . Recent advances have employed novel in vitro preparations and videom icroscopic techniques to investigate the neural control of the gastroi ntestinal microvasculature and involvement of intrinsic and extrinsic vasodilatory neurons in mucosal reflexes. Vasoconstrictor innervation to submucosal arterioles is mediated solely by extrinsic sympathetic n erves that release ATP onto arteriolar P-2x-purinoceptors. Neurogenic vasodilation of submucosal arterioles occurs by release of acetylcholi ne and/or neuropeptides from intrinsic submucosal neurons as well as b y release of substance P and calcitonin gene-related peptide from extr insic sensory nerves. Both vasodilator pathways can be activated indep endently by mucosal stimulation, and both have afferent and efferent c omponents confined to the mucosa and submucosal neuronal plexus. We sp eculate that the intrinsic enteric cholinergic reflex pathways are inv olved in local physiological control of mucosal blood flow, whereas ex trinsic sensory reflex pathways are preferentially activated during in flammatory states.