Propulsion and Mixing of Food in the Alimentary Tract

Propulsion and Mixing of Food in the Alimentary Tract Motor Functions of The Stomach • • • Storage of large quantities of food Mixing of food with gastric secretions until it forms a semifluid mixture called chyme. Slow emptying of the chyme Propulsion and Mixing of Food in the Alimentary Tract Motor Functions of The Stomach  Storage Function Of The Stomach • As food enters the stomach, it forms concentric circles of the food in the orad portion of the stomach. Normally, when food stretches the stomach, a “vagovagal reflex” from the stomach to the brain stem and then back to the stomach reduces the tone in the muscular wall of the body of the stomach, accommodating greater quantities of (0.8 to 1.5 liters). • Propulsion and Mixing of Food in the Alimentary Tract Mixing And Propulsion Of Food In The Stomach • • • • As long as food is in the stomach, weak peristaltic constrictor waves, called mixing waves, begin in the mid to upper portions of the stomach wall and move toward the antrum about once every 15 to 20 seconds. These waves are initiated by the gut wall BER. Constrictor rings also play an important role in mixing the stomach contents. As each peristaltic wave approaches the pylorus, the pyloric muscle often contracts, which further impedes emptying through the pylorus. Propulsion and Mixing of Food in the Alimentary Tract Mixing And Propulsion Of Food In The Stomach • The moving peristaltic constrictive ring, combined with upstream squeezing action, called “retropulsion,” is an exceedingly important mixing mechanism in the stomach. Propulsion and Mixing of Food in the Alimentary Tract Hunger Contractions • • • • • These contractions are rhythmical peristaltic contractions in the body of the stomach. When the successive contractions become extremely strong, they often fuse to cause a continuing tetanic contraction that sometimes lasts for 2 to 3 minutes. Hunger contractions are most intense in young, healthy people. They are also greatly increased by the person’s having lower than normal levels of blood sugar. When hunger contractions occur in the stomach, the person sometimes experiences mild pain in the pit of the stomach, called hunger pangs. Hunger pangs usually do not begin until 12 - 24 hours after the last ingestion of food Propulsion and Mixing of Food in the Alimentary Tract Stomach Emptying • • •  Intense Antral Peristaltic Contractions During Stomach Emptying— “Pyloric Pump.” Antral systole Most of the time, the rhythmical stomach contractions (cause mixing of food and gastric secretions). About 20 percent of the time while food is in the stomach, the contractions become intense. Role of the Pylorus in Controlling Stomach Emptying Propulsion and Mixing of Food in the Alimentary Tract Regulation Of Stomach Emptying  Gastric Factors Promote Emptying • Effect of Gastric Food Volume (Stretching of the stomach wall elicit local myenteric reflexes in the wall that greatly accentuate activity of the pyloric pump and at the same time inhibit the pylorus). • Effect of the Hormone Gastrin • • Secretion of highly acidic gastric juice by the stomach glands. Mild to moderate stimulatory effects on motor functions in the body of the stomach. Enhance the activity of the pyloric pump. • Propulsion and Mixing of Food in the Alimentary Tract Regulation Of Stomach Emptying  Powerful Duodenal Factors Inhibit Stomach Emptying • Inhibitory Effect of Enterogastric Nervous Reflexes From the Duodenum These reflexes are mediated by three routes: Directly from the duodenum to the stomach through the ENS. Through extrinsic nerves that go to the prevertebral sympathetic ganglia and then back through inhibitory sympathetic nerve fibers to the stomach. Through the vagus nerves all the way to the brain stem, where they inhibit the normal excitatory signals transmitted to the stomach through the vagi. • • • • Propulsion and Mixing of Food in the Alimentary Tract Regulation Of Stomach Emptying  Enterogastric inhibitory reflexes include the following: • • • • • Distention of the duodenum The presence of any irritation of the duodenal mucosa Acidity of the duodenal chyme Osmolality of the chyme The presence of certain breakdown products in the chyme, especially breakdown products of proteins and, perhaps to a lesser extent, of fats.  Hormonal Feedback From the Duodenum  Cholecystokinin  Secretin  GIP Propulsion and Mixing of Food in the Alimentary Tract Regulation Of Stomach Emptying Food rich in carbohydrate leaves the stomach in a few hours. Protein-rich food leaves more slowly, and emptying is slowest after a meal containing Fat. Propulsion and Mixing of Food in the Alimentary Tract Movements Of The Small Intestine  • • • • • • MIXING CONTRACTIONS (SEGMENTATION) Stretching of the intestinal wall elicits localized concentric contractions spaced at intervals along the intestine and lasting a fraction of a minute. Divide the intestine into spaced segments that have the appearance of a chain of sausages. “Chop” the chyme 2 to 3 times per minute. The maximum frequency of the segmentation contractions is determined by the frequency of slow waves. Maximum frequency occurs only under extreme conditions of stimulation. These contractions are not effective without background excitation mainly from the myenteric nerve plexus.  Effect of Atropine? Propulsion and Mixing of Food in the Alimentary Tract Movements Of The Small Intestine  • • • • Propulsive Movements Chyme is propelled through the small intestine by peristaltic waves. These waves can occur in any part of the small intestine and move toward the anus at a velocity of 0.5 to 2.0 cm/sec (faster in the proximal intestine and slower in the terminal intestine). They are normally weak and usually die out after traveling only 3 - 5 cm. The waves rarely travel farther than 10 cm, so forward movement of the chyme is very slow (net movement along the small intestine normally averages only 1 cm/min). Propulsion and Mixing of Food in the Alimentary Tract Movements Of The Small Intestine Control of Peristalsis by Nervous and Hormonal Signals • Peristaltic activity of the small intestine is greatly increased after a meal. • Peristaltic activity is increased by gastroenteric reflex. • Gastrin, CCK, Insulin, Motilin, and Serotonin, enhance intestinal motility. • Secretin and glucagon inhibit small intestinal motility. • Propulsive Effect of the Segmentation Movements • Peristaltic Rush Irritation of the intestinal mucosa (severe cases of infectious diarrhea), can cause both powerful and rapid peristalsis. This is initiated by:  Nervous reflexes that involve the autonomic nervous system and brain stem.  Intrinsic enhancement of the myenteric plexus reflexes within the gut wall. Propulsion and Mixing of Food in the Alimentary Tract Ileocecal Valve • • • The valve usually can resist reverse pressure of at least 50 to 60 centimeters of water. Ileocecal sphincter normally remains mildly constricted and slows emptying of ileal contents into the cecum. After a meal, a gastroileal reflex intensifies peristalsis in the ileum. Feedback Control of the Ileocecal Sphincter. Distention of cecum, contraction of the ileocecal sphincter becomes intensified and ileal peristalsis is inhibited. Irritation of cecum delays emptying (inflamed appendix). Propulsion and Mixing of Food in the Alimentary Tract Movements Of The Colon  • • Principal functions of the colon: Absorption of water and electrolytes to form solid feces (proximal half of the colon) Storage of fecal matter until it can be expelled (distal half).  Mixing Movements “Haustrations” • large circular constrictions. At each of these constrictions, about 2.5 cm of the circular muscle contract. combined contractions of the circular and longitudinal strips of muscle cause the unstimulated portion of the large intestine to bulge outward into baglike sacs called haustrations. Propulsion and Mixing of Food in the Alimentary Tract Movements Of The Colon  • • • • • Propulsive Movements “Mass Movements” From the cecum to the sigmoid, mass movements can, for many minutes at a time, take over the propulsive role. These movements usually occur only 1 to 3 times each day, in many people especially for about 15 minutes during the first hour after eating breakfast. A constrictive ring occurs in response to a distended or irritated point in the colon, usually in the transverse colon. Then, rapidly, the 20 or more centimeters of colon distal to the constrictive ring lose their haustrations and instead contract as a unit. The contraction develops progressively more force for about 30 seconds, and relaxation occurs during the next 2 to 3 minutes. Initiation of Mass Movements by Gastrocolic and Duodenocolic Reflexes. 29