Small intestinal motility: normal and abnormal function.

This review outlines the properties and function of intestinal smooth muscle and the mechanisms that underlie contraction and relaxation. Both tonic and phasic (rhythmic) contraction are mediated by an increase in intracellular calcium. Phasic contraction is paced electrically by rhythmic changes in membrane potential (slow waves) which, upon reaching a threshold, lead to opening of membrane calcium channels and the entry of calcium into muscle cells; this inwardly directed calcium current or spike initiates a cascade of events resulting in contraction. Slow waves and spike potentials and, thus, phasic contraction, are influenced by neurotransmitters, hormones, and drugs. In circular muscle, these agents can also increase calcium by releasing it from intracellular stores, thus inducing tonic contraction. Ingestion of food initiates peristaltic propulsive activity which, in its rhythm, is superimposed on spontaneous phasic activity. The peristaltic reflex consists of two successive phases: relation of circular muscle distal to the distending bolus (descending relaxation) and contraction proximal to the bolus (ascending contraction). In-between meals, a different, slower pattern of muscle activity prevails, known as the migrating motor complex, which helps to maintain the lumen of the intestine free of contents. Improved understanding of normal muscle function is beginning to reflect itself in improved management of patients with motility disorders.