Gastrointestinal Function

This chapter discusses various functions of the gastrointestinal tract that is essential for the orderly digestion, absorption, and transportation of food and residue. The motor activity of the gut is one of the integrated functions that are essential for the normal assimilation of food. Gut motility facilitates the transportation of nutrients, brings together digestive enzymes and their substrates, temporarily stores content, particularly in the distal small bowel and right colon, for optimal absorption, and finally, excretes nondigestible residue by defecation in a well-coordinated function under voluntary control. The extrinsic autonomic nervous system (ANS) is critically important for almost all secretory and motor functions in the digestive tract. The function of the gastrointestinal smooth muscle is intimately controlled by the release of peptides and transmitters by the intrinsic (or enteric) nervous system; the modulation of the latter input arises in the extrinsic autonomic nerves, the craniospinal parasympathetic excitatory input, and the thoracolumbar sympathetic outflow, which is predominantly inhibitory to the gut but excitatory to the sphincter. Gastrointestinal smooth muscle forms an electrical syncytium whereby the impulse that induces the contraction of the first muscle cell results in efficient transmission to a sheet of sequentially linked cells in the transverse and longitudinal axes of the intestine. The pacemaker of the intestinal muscle syncytium is the network of interstitial cells of Cajal that serve to coordinate contraction circumferentially and longitudinally along the gut.