Gastrointestinal Function

This chapter discusses how proper function of the gastrointestinal tract is essential for the orderly digestion, absorption, and transport of food and residue. Digestion requires secretion of endogenous fluids from the salivary glands, stomach, pancreas, and small bowel to facilitate intraluminal breakdown of foods. Presentation of food to the mouth and olfactory stimulation trigger afferent nerves that stimulate secretory centers in the medulla. Gastric secretion of acid and pepsinogen follows stimulation of oral and gastric vagal afferents. Efferent vagal pathways synapse with submucous plexus neurons which innervate secretory cells via several important bioactive molecules including gastrin, histamine, and somatostatin. Cholecystokinin (CCK), which is released from the duodenal mucosal enteroendocrine cells after chemical stimulation by food, activates pancreatic enzyme secretion by stimulating vagal afferents. Gastrointestinal smooth muscle forms an electrical syncytium whereby the impulse that induces 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.