Answer
The complex series of digestive secretions, and the motions of the organs of the digestive tract are under the tripartite control and regulation of hormones , paracines, and neural relexes that involve both the enteric and central nervous systems:
1. Some of the important hormones involved in digestive processes are gastrin, secretin, ghrelin, CCK and GIP. Histamine, prostaglandins, and 5-hydroxytryptamine(5-HT), which are also involved, are classed as paracrines.
Hormones are are secreted into the blood and affect parts of the digestive tract distant form the sites of production, but the targets of paracrines are tissues and cells close to sites of secretion.
The neural control is exerted by short and long reflexes. The short reflexes act through the myenteric plexus which is stimulated by stretch ( mechanical) or chemical stimuli. Stimulation of these reflexes causes contractions of adjacent regions of the muscularis externa, resulting in the muscular motions such as peristalsis or swallowing. The long reflexes are mediated through autonomic nerve fibers (mainly vagal parasympathetic fibers) that carry sensory stimuli to the CNS centers (nucleus tractus solitarius, and the dorsal vagal motor nulceus etc) in the brain stem ( midbrain, pons, medulla). From these centers efferent messages are relayed back to the digestive tract,
Work Step by Step
The long reflexes are important in promoting digestive tract motility and stimulating secretion of digestive fluids.
CCK : cholecystokinin
GIP: Gut inhibitory polypeptide peptide
Gastrin which is secreted by G cells of the duodenal mucosa increases increases stomach motility and stimulates gastric acid secretion. Ghrelin stimulates the appetite , and CCK stimulates secretion of pancreatic juice and bile release.
Histamine , which is secreted by mast cells, affects the secretion of gastric acid, and of intestinal chloride. Prostaglandins derive from subepithelial fibroblasts; they stimulate intestinal secretions and and intestinal vascular flow.
