GI Mobility PDF

Chapter 27 GI mobility General patterns of motility: Peristalsis ○ Peristalsis is a reflex response that is initiated when the gut wall is stretched by the contents of the lumen, and it occurs in all parts of the GI tract from the esophagus to the rectum ○ The stretch initiates a circular contractio...

Chapter 27 GI mobility General patterns of motility: Peristalsis ○ Peristalsis is a reflex response that is initiated when the gut wall is stretched by the contents of the lumen, and it occurs in all parts of the GI tract from the esophagus to the rectum ○ The stretch initiates a circular contraction behind the stimulus and an area of relaxation in front of ○ The wave of contraction then moves in an oral to caudal direction, propelling the contents of the lumen forward at rates that vary from 2- 25 cm/s ○ Peristaltic activity can be increased or decreased by the autonomic input to the gut, but its occurrence is independent of extrinsic innervation. Indeed, progression of the contents is not blocked by removal and resuture of a segment of intestine in its original position and is blocked only if the segment is reverse before it is sewn back in place Segmentation and mixing ○ When the meal is present, the enteric nervous system promotes a motility pattern that is related to peristalsis but is designed to retard the movement of the intestinal contents along the length of the intestinal tract to provide time for digestion and absorption. ○ This motility pattern is known as segmentation and is provides for ample mixing of the intestinal contents (known as chyme) with the digestive juices ○ A segment of bowel contracts at both ends, and then a second contraction occurs in the center of the segment to force the chyme both backwards and forwards ○ Unlike peristalsis, therefore, retrograde movement of the chyme occurs routinely in the setting of segmentation ○ This mixing pattern persists for as long as nutrients remain in the lumen to be absorbed ○ It presumably reflects programmed activity of the bowel dictated by the enteric nervous system and can occur independent of the central input, although the latter can modulate it. Basic electrical activity and regulation of motility ○ Except in the esophagus and the proximal portion of the stomach, the smooth muscle of the GI tract has spontaneous rhythmic fluctuations in membrane potentials between about -65 and -45 mV ○ This basic electrical rhythm (BER) is initiated by the interstitial cells of Cajal, which are stellate mesenchymal pacemaker cells with smooth muscle-like features that send long multiply branched processes into the intestinal smooth muscle. ○ In the stomach and the small intestine, these cells are located in the outer circular muscle layer near the myenteric plexus; in the colon, they are at the submucosal border of the circular muscle layers ○ In the stomach and Small intestine, there is a descending gradient in pacemaker frequency, and as in the heart, the pacemaker with the highest frequency usually dominates. ○ Migrating motor complexes (MMCs). The 3 phases include a quiescent phase (phase 1), A phase consisting of small, irregular contractions that do not propagate (phase 2), and a phase of regular activity lasting about 5 minutes (phase 3) which sweeps along the length of the intestine ○ The entire cycle repeats every 90-100 minutes under fasting conditions ○ Note that the complexes are completely inhibited by a meal and resume 90-120 minutes later ○ Conversely, when a meal is ingested, secretion of motilin is suppressed (ingestion of food suppresses motilin release via mechanisms that have not yet been elucidated) and the MMC is abolished until digestion and absorption are complete Segments specific patterns of motility: Mouth and esophagus ○ In the mouth, food is mixed with saliva and propelled into the esophagus. Peristaltic waves in the esophagus move the food into the stomach Mastication ○ Chewing (mastication) breaks up large food particles and mixes the food with the secretions of the salivary glands ○ This wetting and homogenizing action aids swallowing and subsequent digestion. Large food particles can be digested, but they cause strong and often painful contractions of the esophageal musculature ○ Particles that are small tend to disperse in the absence of saliva and also make swelling difficult because they don’t form a bolus ○ The number of chews that is optimal depends on the food, but usually ranges from 20-25 ○ Edentulous patients are generally restricted to soft diet and have considerable difficulty eating dry food Swallowing ○ Swallowing (deglutition) is a reflex response that is triggered by afferent impulses in the trigeminal, glossopharyngeal and vagus nerves ○ These impulses are integrated in the nucleus of the tractus solitarius and nucleus ambiguus ○ The efferent fibers pass to the pharyngeal musculature and the tongue via the trigeminal, facial and hypoglossal nerves ○ Swallowing is initiated by the voluntary action of collecting the oral contents on the tongue and propelling them backwards into the pharynx ○ This starts a wave of involuntary contraction in the pharyngeal muscles that pushes the material into the esophagus. Inhibition of respiration and glottic closure are part of the reflex response ○ A peristaltic ring contraction of the esophageal muscle forms behind the material, which is then swept down the esophagus at a speed of approximately 4 cm/s ○ When humans are in an upright position, liquids, and semisolid foods generally fall by gravity to hte lower esophagus ahead of the peristaltic wave ○ However, if ant food remains in the esophagus, it is cleared by a second wave of peristalsis. It is therefore possible to swallow food while standing on one’s head. The tongue pushes the food bolus to the back of the mother The soft palate elevates to prevent food from entering nasal passages The epiglottis covers the glottis to prevent food from entering the trachea and the upper esophageal sphincter relaxes Food descends into the esophagus Lower esophageal sphincter ○ Unlike the rest of the esophagus, the musculature of the gastroesophageal junction (lower esophageal sphincters; LES) is tonically active but relaxes on swallowing ○ The tonic activity of the LES between meals prevents reflux of gastric contents into the esophagus ○ The LES is made up of 3 components. ○ The esophageal smooth muscle is more prominent at the junction with the stomach (intrinsic sphincter) ○ Fibers at the crural portion of the diaphragm, a skeletal muscle, surround the esophagus at this point (extrinsic sphincter) and exert a pinchcock like action on the esophagus ○ In addition, the oblique or sling fibers of the stomach wall create a flap valve that helps close off the esophagogastric junction and prevent regurgitation when infragastric pressure rises. Motor disorders of the esophagus ○ Achalasia (literally: failure to relax) is a condition in which food accumulates in the esophagus and the organ can become massively dilated ○ It is due to increased resting LES tone and incomplete relaxation on swallowing ○ The Myenteric plexus of the esophagus is deficient at the LES and the release of NO and VIP is defective. The opposite condition is LES incompetence, which permits reflux of gastric contents into the esophagus (gastroesophageal reflux disease) ○ This common condition is the most frequent digestive disorder causing patients to seek care from a clinician ○ It causes heartburn and esophagitis and can lead to ulceration and stricture of the esophagus due to scarring ○ In severe cases, the intrinsic sphincter or the extrinsic sphincter, and sometimes both, are weak, but less severe cases are caused by intermittent periods of poorly understood decreases in the neural drive to both sphincters. Aerophagia and intestinal Gas ○ Some air is unavoidably swallowed in the process of eating and drinking (aerophagia) ○ Some of the swallowed air is regurgitated (belching) and some of the gasses it contains are absorbed, but much of it passes on to the colon ○ Here, some of the oxygen is absorbed, and hydrogen, hydrogen sulfide, carbon dioxide, and methane formed by the colonic bacteria from carbohydrates and other substances are added to it ○ It is then expelled as flatus ○ The smell is largely due to sulfides ○ The volume of gas normally found in human GI tract is about 200mL and the daily production is 500-1500mL ○ In some individuals, gas in the intestines causes cramps, borborygmi (rumbling noises), and abdominal discomfort Stomach ○ Food is stored in the stomach; mixed with acid, mucus and pepsin; and released at a controlled, steady rate into the duodenum Gastric motility and emptying ○ When food enters the stomach, the fundus and upper portion of the body relaxes and accommodate the food with little if any increase in pressure (receptive relaxation) ○ Peristalsis then begins in the lower portion of the body, mixing and grinding the food and permitting small, semiliquid portions of it to pass through the pylorus and enter the duodenum ○ Receptive relaxation is in part, vagally mediated and triggered by movement of the pharynx and esophagus ○ Intrinsic reflexes also lead to relaxation as the stomach wall is stretched ○ Peristaltic waves controlled by the gastric BER begin soon thereafter and sweep towards the pylorus ○ The contraction of the distal stomach caused by each wave is sometimes called antral systole and can last up to 10s ○ Waves occur 3-4 times a minute Consequences of gastric bypass surgery ○ Patients who are morbidly obese can undergo a surgical procedure in which the stomach is stapled or otherwise reduced so that most of it is bypasses, and thus the reservoir function of the stomach is lost, along with gastric signals, such as ghrelin, that trigger food ingestion ○ As a result of loss of the gastric reservoir, patients can only eat small meals ○ If larger meals are eaten, because of rapid absorption of glucose from the intestine and the resultant hyperglycemia and abrupt rise in insulin secretion, hypoglycemic symptoms sometime develop about 2 hours after meal ○ This weakness, dizziness, and sweating after meals due in part to hypoglycemia, are part of the “dumping syndrome” a distressing syndrome that can also develop in patients whom portions of the stomach have been removed or the jejunum has been anastomosed to the stomach ○ Another cause of the symptoms is rapid entry of hypertonic meals into the intestine; this provokes the movement of so much water into the gut that significant hypovolemia and hypotension are produced Vomiting ○ The protective response of vomiting is an example of central regulation of gut motility functions ○ Vomiting starts with salivation and the sensation of nausea ○ Reverse peristalsis empties material from the upper part of the small intestine into the stomach ○ The glottis closes, preventing aspiration of vomitus into the trachea ○ The breath is held in mid inspiration ○ The muscles of the abdominal wall contract and because the chest is held in a fixed position, the contraction increases intraabdominal pressure ○ The LES and esophagus relax and the gastric contents are ejected ○ The “vomiting center” in the reticular formation of the medulla consists of various scattered groups of neurons in this region that control the different components of the vomiting act Ileus ○ When the intestines are traumatized, there is a direct inhibition of smooth muscle, which causes a decrease in intestinal motility. It is due in part to the activation of opioid receptors ○ When the peritoneum is irritated, reflex inhibition occurs due to increased discharge of noradrenergic fibers in the splanchnic nerves ○ Both types of inhibition in peristaltic activity in the small intestine, its contents are not propelled into the colon and it becomes irregularly distended by pockets of gas and fluid ○ Intestinal peristalsis returns in 6-8 hours followed by gastric peristalsis but colonic activity takes 2-3 days to return Colon ○ The colon serves as a reservoir for the residues of meals that cannot be digested or absorbed ○ Motility in this segment is likewise slowed to allow the colon to absorb water, Na+ and other minerals ○ By removing about 90% of the fluid, it converts about 1000-2000mL of isotonic chyme that enters it each day from the ileum to about 200 mL of semi solid feces ○ The large intestine reabsorbs water from ingested food and stores waste until it is eliminated. It includes the cecum, ascending, transverse, descending and sigmoid colon and rectum Motility of the colon ○ The ileum is linked to the colon by a structure known as the ileocecal valve which restricts reflux of colonic contents and particularly the larger numbers of commensal bacteria into the relatively sterile ileum ○ The portion of the ileum containing the ileocecal valve projects slightly into the cecum, so that increases in conic pressure squeeze it shut, whereas increases in ileal pressure opens it ○ It is normally closed ○ Each time a peristaltic wave reaches it, it opens briefly, permitting some of the ileal chyme to squirt into the cecum ○ When food leaves the stomach, the cecum relaxes and the passage of chyme through the ileocecal valve increases (gastroileal reflex) ○ This is presumably a vasovagal reflex Defecation ○ Distention of the rectum with feces initiates reflex contractions of its musculature and the desire to defecate ○ In humans, the sympathetic nerve supply to the internal (involuntary) anal sphincter is excitatory, whereas the parasympathetic supply is inhibitory ○ This sphincter relaxes when the rectum is distended ○ The nerve supply to the external anal sphincter, a skeletal muscle, comes from the pudendal nerve ○ The sphincter is maintained in a state of tonic contraction and moderate distention of the rectum increases the force of its contraction ○ The urge to defecate first occurs when rectal pressure increases about 18 mmHg ○ When this pressure reaches 55 mmHG, the external as well as the internal sphincter relaxes and there is reflex expulsion of the contents of the rectum. This is why reflex evacuation of the rectum can occur even in the setting of spinal injury ○ Before the pressure that relaxes the external anal sphincter is reaches, voluntary defecation can be initiated by straining ○ Normally, the angle between the anus and rectum is about 90-100 degrees and this, added to contraction of the puborectalis muscle inhibits defecation ○ With straining, the abdominal muscles contract, the pelvic floor is lowered 1-3cm, and the puborectalis muscle relaxes ○ The anorectal angle is straightened and when this is combined with relaxation of external anal sphincter, defecation occurs ○ Defecation is therefore a spinal reflex that can be voluntarily inhibited by keeping the external sphincter contracted or facilitated by relaxing the sphincter and contracting the abdominal muscles