GI Physiology | Motility Flashcards
GI contractions from what period of digestion is stronger?
Peristaltic contractions of the interdigestive period is stronger than the slower segmentation movements of the digestive period.
In the time frame of digestion, what GI viscera are involved with digestive periods? What of interdigestive periods? Quantify the average time spent in each organ.
Digestive periods - Stomach (2-4 hours) and Small Intestine (2-4 hours);
Interdigestive periods - Empty stomach (10-18 hours) with a migrating myoelectric complex (MMC) every 1.5 hours/cycle.
How long should a patient fast, before surgery to reduce the risk of food aspiration under general anesthesia? Why?
At least 8 hours (overnigth) since we want the stomach and small intestines to empty their contents before surgery.
What are 2 key features of GI smooth muscle that enables it to function properly>
- High elasticity - allows stretch with relatively low pressure increase.
- Electric control activity (ECA) - regulates smooth muscle contraction and relaxation
Walk through the step-wise physiological process at which food travels from oral cavity to the stomach.
- Stop breathing before swallowing.
- upper esophageal sphincter relaxes as tongue pushes bolus up.
- UES contracts.
- Pharynx contracts
- Slow esophageal peristaltic wave.
- LES relaes then contracts
- Fundus + body of stomach relaxes slightly = “Receptive Relaxation”
List and briefly describe the 4 anti-reflux mechanisms.
- High tone LES
- Secondary esophageal peristalsis
- Pinching of LES by diaphragm
- Reflexes - LES contracts in response to increases in gastric or abdominal pressure (i.e. sneezing or gagging)
In young infants, the only anti-reflux mechanism is the _______. Whereas, in pregnant women, the mechanism used is the _________.
Infants = pinching LES by the diaphragm; Pregnancy = secondary esophageal peristalsis
Why are pregnant woman more likely to experience heartburn compared to the non-gravid state?
Hormonal changes mess up the smooth muscle tone all over the body. The rising uterus also
What 2 disorders can results form problems with the LES? How do each of them present?
- Gastroesophageal Reflux disease (GERD) - insufficient contraction of LES; presents as heartburn
- Esophageal Achalasia - too much contraction of LES from lack of relaxation; presents as dysphagia
Name the 2 mechanisms for storage of food in the stomach.
Storage of food occurs in the upper stomach.
- Receptive relaxation - occurs during swallowing as the stomach relaxes slightly to receive bolus.
- Accomodation - stomach distends as volume increases (via stretching of highly folded rugae)
What are the 2 functions of the lower stomach.
Mixing and size reduction via peristaltic contractions of circular muscle are functions of the lower stomach.
What is the nexus for electric control activity (ECA) of the stomach?
The corpus pacemaker initiates 3.7 ECAs/min from the antrum of the stomach.
In the pylorus, stomach contents stimulate the secretion of certain factors. Glucose stimulates the release of ____. Fatty acids, AAs and proteins stimulates the release of _____. Acid H+ stimulates the release of _____.
Glucose causes the release of GIP. FFAs, AAs and proteins cause the release of CCK. H+ stimulates release of Somatostatin.
What 2 effects does the pyloric sphincter tone and duodenal contraction have in the control of gastric emptying?
Pyloric sphincter tone and duodenal contraction delay gastric emptying and inhibits the effects of gastrin.
Briefly describe the process of gastric emptying starting from food entering the stomach.
Food entering the stomach causes the upper stomach to stretch. This stimulates vagal afferents > CNS > vagal efferent > gastrin release > lower stomach peristalsis > promotes gastric emptying.
How does the interdigestive period contrast from the digestive period, in regards to gastric motility?
The interdigestive period has STRONGER peristaltic waves (MMCs) that serve a housekeeping function. This differs from the relaxation and storage in the digestive period. The latter period also involves peristaltic contractions that serve to mix, reduce the size and empty the bolus contents.
How do interdigestive MMCs compare to digestive motilities? List at least 3 major differences.
MMCs are intermittent, stronger peristaltic waves that remove undigestible materials and prevent bacterial growth in the small intestine. Digestive motilities are constant segmentations of moderate strength that serve to maximize digestion and absorption of nutrients.
Why is the pylorus closed during the digestive period?
This is to slow down gastric emptying while the stomach mixes, reduces and digests food contents. This ensures that gastric contents do not enter the small intestine too quickly and in bulk, allowing for optimal digestion and absorption.
What is the regulator of MMCs? What of digestive motilities?
MMCs are regulated by Motilin. Gastrin and other endocrines/paracrine hormones regulate digestive motilities.
What is the clinical consequence for unregulated gastric emptying?
Dumping Syndrome.
This muscle of the small intestine is involved with absorption and lymph flow.
Muscularis mucosae (villus muscle)
This muscle of the small intestine is important for intraluminal mixing and propulsion (via segmentation)
Muscularis externa - also has a housekeeping function by MMC
Describe how segmentation works in the small intestine.
Segmentation is the separation and recombination of chyme boluses. This slow movement mixes chyme with secretions and associates it with the cell membrane for digestion and absorption.
What are electric control activities? What is their unique feature?
ECAs are slow waves (basal electric rhythms) that originate from pacemaker cells and travel to SM cells through gap junctions. They occur spontaneously and constantly, whether we have contraction or not.
Describe how an ECA is generated in terms of ion channel activities.
Initiation of ECAs comes from activation of Ca and Na channels. K+ channel opens simultaneously because it’s voltage-dependent. Then once Ca and Na channels close, this leads to a repolarization.
What type of ECAs stimulate muscle contractions in the GI? What determines this feature?
LARGE ECAs with spike potentials cause muscle contractions. Small ECAs do not. The size of these waves is determined by the availability of neuroendocrine cells.
The more stimulatory neuroendocrine cells there are, the more _______ there are to start muscle contractions.
Electric response activities (ERAs) with spike potentials
What neuroendocrine factors excite ECAs?
Acetylcholine (ACh) and gastrin modulate excitatory ECAs with spike potentials. These increase the number and strength of contractions by increasing the ration of ERAs:ECAs
What neuroendocrine factors inhibit ECAs?
Norepinephrine (NE) and nitric oxide modulate inhibitory ECAs.
The wide lumen of ahead of the peristaltic moving unit involves the contraction of the ___________ and the relaxation of the __________.
Wide lumen at head of MMC = Longitudinal muscle contraction; Circular muscle relaxation
The narrow lumen of trailing behind the peristaltic moving unit involves the relaxation of the ________ and the contraction of the _________.
Narrow lumen behind MMC = Longitudinal muscle relaxation; Circular muscle constriction (explains itself)
When a bolus brushes against the villi of the intestine, the proximal circular muscle _____ the bolus _______.
Proximal circular muscle BEHIND bolus CONTRACTS. From Stimulatory neuroendocrine factors.
When a bolus brushes against the villi of the intestine, the distal circular muscle ______ of the bolus ______.
Distal circular muscle AHEAD of the bolus RELAXES. Done by Inhibitory neuroendocrine factors.
During the digestive period, segmentation is _______ as ___ ___ propulsion occurs. This can result in some backflow of food although net motion is _______.
Digestive period is UNCOORDINATED as it involves short-distance propulsion. There is a NET FORWARD motion of bolus.
In the interdigestive period, MMCs are ______ as they perform ______ ____ propulsion.
Interdigestive period is COORDINATED with intermediate-distance propulsion.
Why is there a STRONG tonic then phasic contraction after pressure in the ileum drops from being distended?
These contractions occur to prevent a reflux from the cecum back into the ileum as wastes are transported ahead of the ileocecal sphincter.
Distension of distal ileum by chyme results in sphincter _______. Which allows _______.
Distal ileum stretch causes ileocecal sphincter to RELAX, enabling propulsion of chyme into large intestine.
Distension of proximal cecum by fecal matter causes the sphincter to _____ to prevent ____.
Stretching of proximal cecum causes CLOSURE of sphincter to prevent reflux.
Briefly walk through the motility of fecal matter in the large intestines in 5 steps.
- Segmenting NON-peristaltic contractions push matter up ascending colon.
- Water, electrolytes and vitamin K is absorbed in proximal colon.
- Some peristalsis pushes stool across transverse colon.
- RARE mass movement from peristaltic wave pushes tool to rectum.
- Awareness and voluntary process of defecation occurs when EAS opens.
Describe the process of defection in 5 steps. What is unique about this process pertaining to GI motility.
- Stool stretches rectum.
- Internal Anal Sphincter (IAS) reflexively relaxes.
- Stool enters anal canal.
- Signals to CNS make me aware of stool in canal.
- I can voluntarily control contraction or relaxation of EAS for kaka! Greater coordination of ENS and CNS
What is the significance of constipation as an inadequate mode of defecation?
Constipation, though it’s cause is not well known, is a common problem in our society. It is believed to be linked to a diet low in fiber, stress or colorectal cancer.
List 5 factors that can result in constipation.
- Low fiber diet
- Narcotics - reduce fluid secretion and increase sphincter tones
- Physical inactivity
- Antibiotics - loss of microbiome = lower ENS activities
- Old age - reduced food intake and metabolism
What is the gastroileal reflex?
This is a stimulatory, long-distance reflex where food entering an empty stomach causes relaxation of the ileo-cecal sphincter (distal) resulting in the ileum emptying.
Describe the gastrocolic (duodenocolic) reflex.
A stimulatory reflex where food entering the empty stomach or chyme in duodenum increases propulsive movement in the colon.
Briefly describe the 2 long-distance inhibitory reflexes.
- Duodenogastric reflex - distension of duodenum decreases gastric emptying.
- Ileogastric reflex - ileal distension leads to decreased gastric emptying.
What happens to stomach emptying if a gastroenterologist resects (takes out) the ileum?
We lose the ileogastric reflex so the stomach empties FASTER.
Which pathophysiological reflex can occur as a side effect of abdominal surgery? What is the clinical significance*?
Peritoneo-intestinal reflex (adynamic ileus) can occur which results in a generalized INHIBITION of muscle activity in the GI tract. This is why surgeons ask patients not to eat or drink after a major surgery. Material can back up in the GI tract allowing bacteria to ferment resulting in gas and bloating.
How would someone know that a post-surgical patient has regained GI motility after eating? Why?
By asking “Have you farted yet?” We always have gas diffused from gut since gut microbiome (bacteria) should still preside in the GI tract even after surgery.
What are the 3 phases of swallowing?
- Oral phase - voluntary; creating bolus
- Pharyngeal - involuntary; UES relaxes to allow peristatltic wave to pushes bolus back to pharynx
- Esophageal - involuntary primary peristaltic wave; starts at UES and pushes food to LES via VIP
What occurs during antral systole?
Pylorus is shut and antrum contracts to grind the food. Gastric emptying occurs BEFORE antral systole.
