Write a concise account on the regulation of exocrine pancreatic secretion.

Outline:

·        Structure of exocrine pancreas

·        Composition of pancreatic secretions

·        3 phases: cephalic, gastric, intestinal

·        Factors involved: CCK, secretin, acetylcholine

Essay:

            The human pancreas weighs less than 100g, yet each day it secretes 1 kg of pancreatic juice. The pancreas contains blind-ended tubules that are surrounded by polygonal acinar cells and are organized into lobules. The primary function of these lobules, or acini, is to secrete the enzyme component of pancreatic juice. The tiny ducts that drain the acini converge eventually to the common bile duct which drains the pancreatic secretion into the duodenum. The pancreas is supplied by branches of the vagus and postganglionic sympathetic nerves from the celiac and hepatic plexuses. Secretion of pancreatic juice is stimulated by parasympathetic activity and inhibited by sympathetic activity.

            The exocrine juice is composed of an aqueous component and an enzyme component. The aqueous component is rich in bicarbonate and helps to neutralize duodenal contents. The enzyme component contains enzymes for digesting carbohydrates, proteins, and fats. Both neural and hormonal signals control pancreatic exocrine secretion.

            The regulation of pancreatic secretions occur in three phases: cephalic, gastric and intestinal. The thought, sight, smell or taste of food stimulates the release of acetylcholine from cholinergic nerve endings. In addition, gastrin released from the mucosa of the gastric antrum in response to vagal impulses stimulates pancreatic secretion during this phase.

            Pancreatic secretion is enhanced during the gastric phase by distention and food breakdown products. Distention of the stomach elicits vagovagal reflexes induces the pancreas to secrete a small volume of pancreatic juice with high enzyme concentration. Food breakdown products cause the G cells of the antrum to release gastrin, which produces a low, high-enzyme pancreatic secretion. The cephalic and gastric phases of stimulating pancreatic secretion helps to prepare the small intestine for digestion of the incoming food.

            During the intestinal phase, the major stimulants of pancreatic secretion are the hormones CCK and secretin which are secreted from the mucosal cells of the duodenal and jejunal mucosa in response to the entrance of chyme into the small intestine. Secretin is released in response to a low pH of less than 4.5 caused by the presence of gastric acid in the intestine and it directly stimulates pancreatic ductular epithelial cells, increasing bicarbonate secretion to neutralize the acid. The secretion of CCK is stimulated by contact of the intestinal mucosa with the products of digestion, particularly the peptides, amino acids and fatty acids. CCK stimulates the secretion of a pancreatic juice rich in enzymes and augments the actions of secretin in producing secretion of an alkaline pancreatic juice. It also inhibits gastric emptying, exerts a trophic effect on the pancreas, increases the secretion of enterokinase and may enhance the motility of the small intestine and colon. Enteropancreatic vagovagal reflexes also enhance pancreatic secretion during this phase; vagotomy significantly reduces the secretion of pancreatic juice.