Ch 63

Motor functions of the stomach

(1) Storage of large quantities of food until the food can be processed in the stomach, duodenum, and lower intestinal tract (2) Mixing of food with gastric secretions until it forms a semifluid chyme; (3) Slow emptying of the chyme from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.

The Functions of the Colon

(1) absorption of water and electrolytes from the chyme to form solid feces. (2) storage of fecal matter until it can be expelled.

The Chewing Reflex

-Bolus of food first initiates reflex inhibition of the muscles of mastication which allows the lower jaw to drop. -Drop initiates a stretch reflex of jaw muscles (rebound contraction) -Repeats

Ingestion of Food

-Hunger is the intrinsic desire for that food determines the amount of food a person eats. -Appetite determines what food the person wants to consume. -The mechanics of ingestion include mastication and swallowing.

Mastication (Chewing)

-Motor Branch of CN5. -stimulation of specific areas in the brain stem taste centers cause rhythmical chewing. -stimulation of areas in the hypothalamus, amygdala and the sensory cortex areas for taste and smell cause chewing. -Muscles in the jaw have a force of about 55 lbs incisors and 200 lbs molars.

Esophageal stage of swallowing ( 2 types of peristaltic movement)

1. Primary peristalsis is a continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus and passes all the way from the pharynx to the stomach in about 8 to 10 seconds. -If the primary peristaltic wave fails to move food into the stomach a secondary peristaltic waves result from distention of the esophagus by the retained food. 2. Secondary peristaltic waves are initiated by intrinsic neural circuits in the myenteric nervous system and by reflexes that begin in the pharynx that stimulates the vagal afferent fibers to the medulla that respond back to the esophagus through glossopharyngeal and vagal efferent fibers. •The musculature of the pharyngeal wall and upper third of the esophagus is striated muscle. the peristaltic waves in these regions are controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves. •In the lower two thirds of the esophagus, the musculature is smooth muscle and is controlled by the vagus nerves acting on the esophageal myenteric nervous system.

Defecation Reflexes

1. The rectal wall initiates afferent signals to the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus. 2. As the peristaltic wave approaches the anus, the internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus; if the external anal sphincter is also consciously, voluntarily relaxed at the same time, defecation occurs. 3. A parasympathetic defecation reflex that involves the sacral segments of the spinal cord. Defecation signals entering the spinal cord initiate other effects, such as taking a deep breath, closure of the glottis, and contraction of the abdominal wall muscles to force the fecal contents of the colon downward and at the same time cause the pelvic floor to relax downward and pull outward on the anal ring to evaginate the feces.

Swallowing (deglutition)

3 stages: (1) a voluntary stage, which initiates the swallowing process. (2) a pharyngeal stage, is involuntary allowing food passage through the pharynx into the esophagus. (3) an esophageal stage, is involuntary to transports food from the pharynx to the stomach.

Other Autonomic Reflexes That Affect Bowel Activity

Beside from the duodenocolic, gastrocolic, gastroileal, enterogastric, and defecation reflexes They are the peritoneointestinal reflex, renointestinal reflex, and vesicointestinal reflex. •The peritoneointestinal reflex results from irritation of the peritoneum; it strongly inhibits the excitatory enteric nerves and can cause intestinal paralysis, especially in patients with peritonitis. •The renointestinal and vesicointestinal reflexes inhibit intestinal activity during kidney or bladder irritation.

Storage Function of the Stomach

Food in the stomach, forms concentric circles in the orad portion with the newest food lying closest to the esophageal opening and the oldest food lying nearest the outer wall of the stomach. •Food stretches the stomach creating a "vagovagal reflex" that reduces the tone in the muscular wall and cause the stomach to bulge progressively outward, accommodating greater and greater quantities of food up to a limit in the completely relaxed stomach of 0.8 to 1.5 liters.

Regulation of Stomach Emptying

The more important control of stomach emptying resides in inhibitory feedback signals from the duodenum, including both enterogastric inhibitory nervous feedback reflexes and hormonal feedback by CCK.

Feedback inhibitory mechanisms slow the rate of emptying when

• (1) Too much chyme is already in the small intestine or •(2) The chyme is excessively acidic •(3) The chyme contains too much unprocessed protein or fat, is hypotonic or hypertonic, or is irritating.

voluntary stage of swallowing

•"voluntarily" squeezed or rolled a bolus posteriorly into the pharynx by pressure of the tongue upward and backward against the palate.

Prevention of Esophageal Reflux by Valvelike Closure of the Distal End of the Esophagus

•A valvelike mechanism of a short portion of the esophagus extending into the stomach. This prevents high intra-abdominal pressure from forcing stomach contents backward into the esophagus.

Mixing and Propulsion of Food in the Stomach Basic Electrical Rhythm of the Stomach Wall

•As long as food is in the stomach, weak 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. •Constrictor waves progress from the body of the stomach into the antrum progressively become extremely intense with powerful peristaltic action that force the antral contents under higher pressure to the pylorus •The moving peristaltic constrictive ring, combined with this upstream squeezing action, called "retropulsion," is an exceedingly important mixing mechanism in the stomach.

Propulsive Movements Peristalsis in the Small Intestine

•Chyme is propelled through the small intestine by peristaltic waves 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 to 5 centimeters, rarely farther than 10 centimeters, so forward movement of the chyme is very slow and averages 1 cm/min. •Average of 3 to 5 hours are required for passage of chyme from the pylorus to the ileocecal valve.

Mixing Movements—"Haustrations."

•Circular constrictions occur in the large intestine about 2.5 centimeters with the teniae coli contraction. and cause the unstimulated portion of the large intestine to bulge outward into baglike sacs called haustrations. •Each haustration usually reaches peak intensity in about 30 seconds and then disappears in about 60 seconds. causing minor amount of forward propulsion of the colonic contents. This exposes the fecal material to the mucosal surface of the large intestine, and fluid and dissolved substances are progressively absorbed. • Only 80 to 200 milliliters of feces are expelled each day.

Types of Emptying

•Effect of Gastric Food Volume on Rate of Emptying •Effect of the Hormone Gastrin on Stomach Emptying •Powerful Duodenal Factors That Inhibit Stomach Emptying Inhibitory Effect of Enterogastric Nervous Reflexes from the Duodenum •Hormonal Feedback from the Duodenum Inhibits Gastric Emptying—Role of Fats and the Hormone Cholecystokinin

Control of Peristalsis by Nervous and Hormonal Signals

•Gastroenteric reflex is initiated by distention of the stomach and conducted principally through the myenteric plexus from the stomach down along the wall of the small intestine. •Several hormonal factors also affect peristalsis. They include gastrin, CCK, insulin, motilin, and serotonin, all of which enhance intestinal motility •Secretin and glucagon inhibit small intestinal motility.

Hunger Contractions

•Hunger contractions are rhythmical peristaltic contractions in the body of the stomach. they often occur when the stomach has been empty for several hours or more. •When the successive contractions become extremely strong, they fuse to cause a continuing tetanic contraction that sometimes lasts for 2 to 3 minutes that may cause mild stomach pain, called hunger pangs. •Hunger pangs usually do not begin until 12 to 24 hours after the last ingestion of food and in starvation, they reach their greatest intensity in 3 to 4 days and gradually weaken in succeeding days

Peristaltic Rush

•Intense irritation of the intestinal mucosa, as in cases of infectious diarrhea, can cause both powerful and rapid peristalsis, called the peristaltic rush. •The powerful peristaltic contractions travel long distances in the small intestine within minutes to the colon relieving the small intestine of irritative chyme and excessive distention.

Intense Antral Peristaltic Contractions During Stomach Emptying "Pyloric Pump."

•Intense peristaltic contractions often create 50 to 70 centimeters of water pressure, which is about six times as powerful as the usual mixing type of peristaltic waves. •These peristaltic waves, in addition to causing mixing in the stomach, also provide the "pyloric pump."

Function of the Ileocecal Valve

•It prevents backflow of fecal contents from the colon into the small intestine.

Defecation

•Most of the time, the rectum is empty of feces. • When a mass movement forces feces into the rectum, the desire for defecation occurs immediately, including reflex contraction of the rectum and relaxation of the anal sphincters. •Continual dribble of fecal matter through the anus is prevented by tonic constriction of: • (1) an internal anal sphincter, a several-centimeters-long thickening of the circular smooth muscle (2) an external anal sphincter, composed of striated voluntary muscle that both surrounds the internal sphincter and extends distal to it.

Propulsive Movements "Mass Movements."

•Much of the propulsion in the cecum and ascending colon results from the slow persistent haustral contractions,. It takes 8 to 15 hours to move the chyme from the ileocecal valve through the colon, •A mass movement is a modified peristalsis that starts with a constrictive ring occurring in response to a distended or irritated point in the colon. A series of mass movements usually persists for 10 to 30 minutes. When enough feces is pushed into the rectum, the desire for defecation is felt.

Pharyngeal stage of swallowing

•The bolus stimulates epithelial swallowing receptor areas all around the pharynx. •Impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions •The trachea is closed • The esophagus is opened •A fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper esophagus, the entire process occurring in less than 2 seconds.

Function of lower esophageal sphincter (gastroesophageal)

•The gastroesophageal sphincter normally remains tonically constricted with an intraluminal pressure of about 30 mm Hg. • "Receptive relaxation" occurs in the lower esophageal sphincter ahead of the peristaltic wave, which allows easy propulsion of the swallowed food into the stomach. •If the sphincter does not relax satisfactorily the resulting condition is called achalasia.

Nervous Initiation of the Pharyngeal Stage of swallowing

•The initiating the pharyngeal stage occurs due to tactile stimulation of the ring around the pharyngeal opening and the tonsillar pillars and are transmitted through the trigeminal and glossopharyngeal nerves into the medulla oblongata; tractus solitarius. •Swallowing is automatically initiated in orderly sequence by neuronal areas of the reticular substance of the medulla and lower portion of the pons known as the deglutition or swallowing center •The pharyngeal stage of swallowing is a reflex act initiated by voluntary movement of food into the back of the mouth, which excites involuntary pharyngeal sensory receptors to elicit the swallowing reflex

Mixing Contractions (Segmentation Contractions)

•When a portion of the small intestine becomes distended with chyme, stretching of the intestinal wall elicits localized concentric contractions spaced at intervals along the intestine and lasting a fraction of a minute. The contractions cause "segmentation" of the small intestine

Receptive Relaxation of the stomach

•When the esophageal peristaltic wave approaches toward the stomach, a wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the peristalsis. •When the wave reaches the lower end of the esophagus the entire stomach and the duodenum become relaxed to receive the food propelled during the swallowing act.