CB L3 - Gastric motility
what are the functions of the stomach?
- Storage for large volumes of food - Homogenizer fragmentation of food and mixing w/ gastric secretion -->Digestion - Controls emptying of gastric content into duodenum - SATIETY via DISTENSION (mechanoreceptor sensing) -propulsion (peristalsis)
what inhibits gastric emptying?
-CCK stimulated by fats -H+ in the duodenum -hypotonic or hyeprtonic solutions
what are some stimuli for vomiting?
-afferent fibers to brainstem -chemical stimuli in blood activate the trigger zone (area postrema) -sensation of nausea -stimulation of emetic center in medulla
what is found in feces?
-dietary fiber not processed in the colon -sloughed cells -dead bacteria -nutrients -sterocobilin -bile/chyme -water -gas
what is the stimulus for peristalsis?
-distension and H+ in the lumen, sensed by mechanoreceptors and chemoreceptors and cause the release of serotonin from ECL cells -afferent limb stimulates peristalsis -efferent limb causes contraction proximal to bolus w/ Ach and relaxation distal to bolus w/ NO/VIP -moves bolus along tract
what are the key signs to look for in feces?
-floating or greasy stools (steatorrhea) -undigested particles -watery or bloody stools (diarrhea)
what maintains continence?
-internal anal sphincter (involuntary) -external anal sphincter (voluntary) -puborectalis muscle
what are some stimuli for nausea?
-irritative impluses coming from GI tract -impuses that originate in lower brain w/ motion sickness -impuses from cerebral cortex
what influences antral contractions?
-neural: para increases, sym decreases -hormonal: gastrin and motilin increase, secretin and GIP decrease
what is the only part of the GI tract that has contractile tissue made of striated muscle?
-pharynx -upper 1/3 of esophagus -external anal sphincter everything else is smooth muscle
what are the three components to gastric motlity?
-proximal stomach motility -distal stomach motility -gastric emptying
what are the functions of gastric motility?
-receive food bolus -store food -mix with digestive juice -grind to smaller pieces -propulsion (empty) -clear food during interdigestive period
what is the proximal stomach's role in gastric motility?
-receptive relaxation -tonic contraction -no phasic contractions
what is the process of vomiting?
-reverse peristalsis in intestines -relaxation of pylorus and stomach -forced inspiration then inhibition of respiration -relaxation of LES -movement of larynx to close glottis, relaxing UES -forceful expulsion of vomitus involving abdominal musclese
what are the processes of the gastric phase?
-secretes gastric juice -motility: storage, mixing, propulsion -minimal protein digestion -minimal drug and alcohol absorption -releases gastrin and ghrelin
what are the functions of the distal stomach in gastric motility?
-slow waves (originate at greater curvature) -peristalsis -retropulsion
what is the purpose of the gastric phase of digestion?
-sterilize, mix food, begin digestion of protein, store food for timely delivery to duodenum and protect itself from damage -stimulated by food in the stomach
what is the mechanism of defecation?
-voluntary relaxation of external anal sphincter -increased intra-abdominal pressure (valsalva - straining) -relaxation of puborectalis -rectal contraction -internal anal sphincter relaxes followed by mass movement of colon -feces excretion
what is phase III of the migrating motor complex?
-when most forceful contractions are -regulated by motilin -ileocecal valve opens to allow undigested material to enter the colon
The gastrointestinal hormones have physiological effects that can be elicited at normal concentrations as well as pharmacological effects that require higher than normal concentrations. What is the physiological effect of the various hormones on gastric emptying?
22. E) CCK is the only gastrointestinal hormone that inhibits gastric emptying under normal physiological conditions. CCK inhibits gastric emptying by relaxing the orad stomach, which increases its compliance. When the compliance of the stomach is increased, the stomach can hold a larger volume of food without excess buildup of pressure in the lumen. None of the gastrointestinal hormones increases gastric emptying under physiological conditions; however, gastrin, secretin, and GLIP can inhibit gastric emptying when pharmacological doses are administered experimentally.
The spinal cord of a 60-year-old woman is severed at T6 in an automobile accident. She devises a method to distend the rectum to initiate the rectosphincteric reflex. Rectal distention causes which of the following responses in this woman?
27. D) When feces enters the rectum, distention of the rectal wall initiates signals that spread through the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid colon, and rectum, all of which force feces toward the anus. At the same time the internal anal sphincter relaxes, allowing the feces to pass. In people with transected spinal cords, the defecation reflexes can cause automatic emptying of the bowel because the external anal sphincter is normally controlled by the conscious brain through signals transmitted in the spinal cord.
Vomiting is a complex process that requires coordination of numerous components by the vomiting center located in the medulla. Which of the following occurs during the vomiting act?
31. D) The act of vomiting is preceded by antiperistalsis that may begin as far down in the gastrointestinal tract as the ileum. Distention of the upper portions of the gastrointestinal tract (especially the duodenum) becomes the exciting factor that initiates the actual act of vomiting. At the onset of vomiting, strong contractions occur in the duodenum and stomach along with partial relaxation of the lower esophageal sphincter. From then on, a specific vomiting act ensues that involves (a) a deep breath, (b) relaxation of the upper esophageal sphincter, (c) closure of the glottis, and (d) strong contractions of the abdominal muscles and diaphragm.
A physiology experiment is conducted in an isolated rat small intestine. The intestine is bathed with all essential nutrients, ions, and gases in a glass dish maintained at a temperature of 37°C. The proximal jejunum is observed to contract at a frequency of five contractions per minute. A glass micropipette is then inserted into an interstitial cell of Cajal (pacemaker cell) at the same location in the jejunum, and a slow-wave frequency of 10 contractions per minute is recorded. Norepinephrine is then added to the bathing solution. Which of the following best describes the most likely slow-wave frequency and contraction frequency after treatment with norepinephrine (in occurrences per minute)?
40. B) Slow-wave frequency is not affected significantly by either the autonomic nervous system or hormones; it is relatively constant at any given location in the small intestine. When a slow wave reaches a threshold value, a calcium spike potential (action potential) occurs and calcium ions enter the smooth muscle cell, which causes it to contract. Norepinephrine hyperpolarizes smooth muscle cells in the intestine and thereby decreases the likelihood that the membrane potential can reach a threshold value. Therefore, norepinephrine does not affect the basal slow-wave frequency of 10 occurrences per minute but does lower the contraction frequency of the smooth muscle cells to 0 occurrences per minute in this problem.
A 65-year-old man eats a healthy meal. Approximately 40 minutes later the ileocecal sphincter relaxes and chyme moves into the cecum. Gastric distention leads to relaxation of the ileocecal sphincter by way of which reflex? A) Enterogastric B) Gastroileal C) Gastrocolic D) Intestino-intestinal E) Rectosphincteric
44. B) Relaxation of the ileocecal sphincter occurs with or shortly after eating. This reflex has been termed the gastroileal reflex. It is not clear whether the reflex is mediated by gastrointestinal hormones (gastrin and cholecystokinin) or extrinsic autonomic nerves to the intestine. Note that the gastroileal reflex is named with the origin of the reflex first (gastro) and the target of the reflex named second (ileal). This method of naming is characteristic of all the gastrointestinal reflexes. The enterogastric reflex involves signals from the colon and small intestine that inhibit gastric motility and gastric secretion. The gastrocolic reflex causes the colon to evacuate when the stomach is stretched. The intestino-intestinal reflex causes a bowel segment to relax when it is overstretched. The rectosphincteric reflex is also called the defecation reflex.
A newborn boy does not pass meconium within 48 hours of delivery. His abdomen is distended, and he begins vomiting. A suction biopsy of a distally narrowed segment of the colon shows a lack of ganglionic nerve cells. This newborn is at risk for developing which condition? A) Achalasia B) Enterocolitis C) Halitosis D) Pancreatitis E) Peptic ulcer
63. B) This infant has Hirschsprung's disease, which is characterized by a congenital absence of ganglion cells in the distal colon resulting in a functional obstruction. Prolonged fecal stasis can lead to enterocolitis (i.e., inflammation of the colon); full-thickness necrosis and perforation can occur in severe cases. In achalasia, the LES fails to relax during swallowing. Halitosis (bad breath) can occur in persons with Hirschsprung's disease, but this condition is not serious. Peptic ulcer and pancreatitis (inflammation of the pancreas) are not common in persons with Hirschsprung's disease.
What is the large intestine?
Any solid food not absorbed into the blood is sent to the large intestine, a wider tube. It is also called the colon. Waste is pushed out of the body from the large intestine.
Gastric emptying is tightly regulated to ensure that chyme enters the duodenum at an appropriate rate. Which event promotes gastric emptying under normal physiological conditions in a healthy person?
C) Gastric emptying is accomplished by coordinated activities of the stomach, pylorus, and small intestine. Conditions that favor gastric emptying include (a) increased tone of the orad stomach, which helps to push chyme toward the pylorus; (b) forceful peristaltic contractions in the stomach that move chyme toward the pylorus; (c) relaxation of the pylorus; which allows chyme to pass into the duodenum; and (d) absence of segmentation contractions in the intestine, which can otherwise impede the entry of chyme into the intestine
Migrating motility complexes (MMCs) occur about every 90 minutes between meals and are thought to be stimulated by the gastrointestinal hormone motilin. An absence of MMCs causes an increase in which of the following? A) Duodenal motility B) Gastric emptying C) Intestinal bacteria D) Mass movements E) Swallowing
C) MMCs (sometimes called interdigestive myoelectric complexes) are peristaltic waves of contraction that begin in the stomach and slowly migrate in an aboral direction along the entire small intestine to the colon. By sweeping undigested food residue from the stomach, through the small intestine, and into the colon, MMCs function to maintain low bacterial counts in the upper intestine. Bacterial overgrowth syndrome can occur when the normally low bacterial colonization in the upper gastrointestinal tract increases significantly. It should be clear that an absence of MMCs would decrease duodenal motility and gastric emptying. MMCs do not have a direct effect on mass movements and swallowing.
What causes Hirschsprung Disease?
Congenital failure of ganglion cells to descend into myenteric and submucosal plexus in the bowel wall to the distal colon
What is the gastroileal reflex?
Contents in the stomach stimulate peristalsis in the ileum and relaxation of the ileocecal sphincter, which delivers the contents to the large intestine. -stomach distended --> increased ileal motility
what is GERD?
Gastric content empty into the esophagus. patients may experience sore throat, hoarseness and contact ulcers. aka acid reflux
where do slow waves originate?
Interstitial cells of Cajal in myenteric plexus (in circular muscle of muscularis externa) -pacemaker for GI smooth muscle, they set the maximum frequency of contractions for each part of the GI tract
What is the puborectalis muscle?
It is the fibres of levator ani that arise more laterally from the pubic bone and the first part of the arcus tendineus. sligs around rectum to keep a sharp angle
what are the sphincter regions of the stomach?
LES and pyloric sphincter
what make the membrane more negative (and less likely to depolarize)?
NE, sympathetics weak or no contraction
what are the functional regions of the stomach?
Orad (proximal) and Caudad (distal)
what are slow waves?
Oscillating membrane potentials inherent to some GI smooth muscle -cyclic variation in resting membrane potential -aka basal electrical rhythm -occur spontaneously
What is irritable bowel syndrome?
Relapsing abdominal pain with bloating, flatulence, change in bowel habits (diarrhea, constipation) that improves with defecation Middle age females Related to disturbed intestinal motility No pathology Increased fiber may improve symptoms
what is receptive relaxation?
Relaxation of the orad region of the stomach in preparation for a food bolus -reflex when the fundus dilates when food is passing -allows for the stomach to accomodate large increases in volume w/o pressure increase is a vagovagal reflex that is abolished with vagotomy
how do laxatives work?
They increase cAMP to stimulate CFTR to release more Cl (and thus Na and water follow). stimulating bowel muscles with irritatns
what causes retching?
UES not relaxed
what is the migrating motor complex?
a pattern of slow contractions sweeping along the GI tract -occurs during interdigestive period to clear any residual contents to prepare for next meal
what is the function of the proximal half of the large intestine?
absorption
What is Barrett's esophagus?
change from squamous epithelium of the esophagus to columnar metaplasia from chronic irritation from reflux. risk? adenocarcinoma
What is Hirschsprung's disease?
congenital lack of ganglion cells in the distal colon, leading to uncoordinated peristalsis and decreased motility patients are at risk for toxic megacolon
what are the symptoms of hirschsprung's disease?
constipation, constriction of aganglionic segment, dilation of segment proximal to affected segment, megacolon -fecal matter acumulates, pressure is high, and feces explodes out
what does depolarization of longitudinal muscle cause?
contraction in the longitudinal direction and a decrease in length and increase in diameter of that segment of the GI tract
what does depolarization of circular muscle cause?
contraction of a ring of smooth muscle and a decrease in diameter of that segment of the GI tract
what are short duration contractions?
contraction of inner circular smooth muscle, form haustra
what are long duration contractions?
contraction of tenia coil, shorten colonic segment
What is the orad region of the stomach?
corpus and fundus for mixing and storage
what is the caudad region of the stomach?
corpus, antrum, pylorus for mixing and propulsion
how are slow waves produced?
cyclic opening of Ca+2 channels for depolarization followed by opening of K+ channels for repolarization
where are slow waves highest?
duodenum
what is defecation?
elimination of feces, the motility of the rectum and anus -coordinated action of smooth and striated muscle
what are the three major activities of the large intestine?
haustrations, multiple haustral contractions, peristalsis
what are mass movements of the colon?
high amplitude propagating contractions -clear the colon 1-3 times per day -opoids can decrease intestinal motility
what is the gastrocolic reflex?
increased secretory and motor functions of the stomach result in increased colonic motility -long reflex involving ANS -stimulated by stomach distension -causes increased mass movement of the colon -mediated by CCK and gastrin -this causes a bowel movment in children and pupies
what is atonic colon?
lazy colon, lack of normal muscle tone of the colon causes chronic constipation
what are the three layers of stomach smooth muscle?
longitudinal, circular, oblique thickness increases from proximal to distal stomach
what is the LES?
lower esophageal sphincter relaxing LES and cardia allows food in and gas out (burp)
what does the large intestine do for long propulsion?
mass movement w/ peristalsis
What are haustrations?
mixing movements in the colon that serve as main propulsive movement in cecum and ascending colon
what activity does the large intestine do for short propulsion?
multiple haustral contraction
what are sphincters?
muscles that control the passage of material from one organ to the next -allow for different compartments for different functions -are tightly controlled to deliver contents -prevent backflow
does motility change in the colon?
no it is the same in fed vs fasting state, no MMC
what are phasic contractions?
periodic contractions followed by relaxation
where do phasic contractions occur in the GI tract?
posterior stomach and small intestine carried out by musclaris externa
what is circular muscle?
sphincter
where are slow waves lowest?
stomach
what is the function of the distal half of the large intestine?
storage
what make the membrane less negative (and more likely to depolarize)?
stretching, Ach, parasympathetics make a stronger contraction
what tonic contractions are present in the proximal stomach?
sustained contraction compresses contents toward the antrum to facilitate mixing and emptying mediated by vagus nerve
what are tonic contractions?
sustained contractions
how do you modulate slow waves?
the frequency can't be increased but you can change the likelihood of action potential by making the membrane more or less negative
how do opoids decrease intestinal mobility?
the highest # of recepors are in the stomach and proximal colon, so binding inhibits propulsive and secretory activities -cause constipation by increasing intestinal tone, stimulating nonpropulsive motility, and stimulating the pyloric and ileocecal sphincters
What is peristalsis?
the involuntary constriction and relaxation of the muscles of the intestine to propel chyme distally
what is the material that enters the large intestine?
the material not absorbed by the SI, aka feces material passed through ileocecal sphincter
What is the pyloric sphincter?
the opening through which chyme leaves the stomach and enters the small intestine aka the gastroduodenal junction -regulates gastric emptying -high muscular tone -closed during gastric phase
what is segmentation?
the turning of the food in the small intestine, mixes chyme in a back and forth manner -causes chyme to be broken up to smaller pieces -controlled by ENS -local movement dominant in small intestines -ring of circular muscle contracts but other side relaxes
where do tonic contractions occur in the GI tract?
upper region of the stomach (orad), lower esophageal ileocecal and internal sphincters
how does the GI tract generate motility?
using smooth muscle subunits linked by gap junctions the subunits fire in a tonic or phasic fashion
what is the mediator for gastric relaxation?
vasoactive intestinal peptide
what does the colon absorb?
water, electrolytes, and some vitamins from chyme and stores fecal matter until it is expelled
when is gastric emptying the fstest?
when the stomach contents are isotonic
