Lecture 2 - Motility Flashcards
What is the special type of motility of the interdigestive state? Describe it.
Migrating myoelectric complex (MMC) = housekeeping wavelike sequential contractions to sweep undigested material and bacteria into colon active from mid-stomach through terminal ileum
When does the interdigestive state occur?
3-4 hours after eating, when the food is past the upper GIT and in the ileum or lower
Describe the phases of the migrating myoelectric complex.
4 phases lasting a total of 75 to 120 minutes:
- Phases I, II, and IV have minor and disregulated contractions
- Phase III contractions last 10 min/cycle but these sweep material lower in the GIT
What is phase III of the MMC stimulated by?
Stimulated by motilin, secreted into blood from the M cells of the duodenum
What allows the enterocytes to be viable for longer?
MMC
Describe swallowing.
- Starts as voluntary action with the skeletal muscle in the mouth and pharynx
- Continues as involuntary action as soon as bolus enters the esophagus as the enteric and ANS systems take over
What are the 2 types of esophageal propulsions? Describe each.
- Primary esophageal peristalsis: caused by the medullary swallowing center, more of an extrinsic effect due to vagal innervation (ENS is also helping, but not main mechanism)
- Secondary esophageal peristalsis: more of an intrinsic effect due to local ENS (but vagus can help) stimulation of salivation and mucus secretions to lubricate stuck bolus of food and contractions
Can smooth muscle disorders in the GIT affect the enteric nerves? Example?
YUP
Achalasia: smooth muscle disorder in the lower esophageal sphincter which cannot relax making it hard for food to be propelled to stomach => esophagus dilates => very painful to eat and patients often become anorexic
Achalasia treatment? Side effect? Treatment for side effect?
No pharmacological treatment, but we can clip the esophageal sphincter to make it floppy
Side effect: GERD, which can be treated with H2 antagonist to lesser the HCl secretion in the stomach to avoid erosion of the esophagus
What does GERD stand for?
Gastroesophageal Reflux Disease
What is gastric accommodation? What to note?
As food enters the stomach it will expand and relax due to vagal stimulation causing VIP release so that intraluminal pressure will not increase until 1 L as entered the stomach
This also happens in the duodenum to a certain extent
Other name for gastric accommodation?
Receptive relaxation
Describe bariatric surgery.
Goal is to reduce the ability of the stomach to accommodate food
What is the antrum?
Narrowing of stomach proximal to the pyloris
What helps break down the chyme in the stomach?
Contractions of the stomach push the chyme against the antrum, so the chyme will retropulse and break down
What is the rate of gastric emptying dependent on? Describe this.
Depends on the types of nutrients in the stomach:
- Saline meal fastest: carbohydrates
- Acid meal: proteins need to be broken down by gastric proteases
- Oleate meal slowest: lipids because small intestine needs more time to get ready to emulsify these
How are contractions generated in the GIT? Is this similar to how contractions are generated in other areas of the body?
Different way than from other areas of the body:
- RMP is in the form of slow waves = basic electrical rhythm
- Various factors cause depolarization of smooth muscle cells
- At threshold (-40 mV) spike potentials are generated on top of the slow waves
- Ca++ influx in smooth muscle cells through VG channels to bind to calmodulin
- Contraction of smooth muscle cells
- Plateau due to slow movement of Ca++ and Na+
- Repolarization due to delayed increase in K+ conductance
What does the strength of GIT contractions depend on?
Depends on the max rate of contractions => depends on max number of spike potentials generated => depends on the basic electrical rhythm of that portion of the GIT
Are spike potentials action potentials?
YUP
Is the rate of slow waves aka the basic electrical rhythm the same throughout the GIT?
NOPE
Stomach: 3/min
Small intestine: 10-12/min
What causes depolarization of the smooth muscle cells of the GIT?
- Stretch
- ACh from parasympathetic NS
- Gastrin from duodenum and antrum
- Serotonin
- Substance P
- Motilin
Other name for substance P?
Tachykinin
What causes hyperpolarization of the smooth muscle cells of the GIT? Are these secreted locally or from extrinsic factors?
- NE
- Sympathetics
- VIP
- NO
Secreted LOCALLY by interneurons
What does VIP stand for?
Vasoactive intestinal peptide
2 types of motility in the small intestine? Can both be seen at different areas of the GIT? Which one is more prominent?
- Peristalsis
- Segmentation***
YUP
Describe peristalsis.
Circular muscle of the GIT will relax ahead of the bolus and contract behind it due to signals from cholinergic motor interneurons (contraction) and VIP motor interneurons (relaxation)
What happens during vomiting?
Reverse peristalsis
Is segmentation or peristalsis more under intrinsic control? What about extrinsic?
Segmentation: intrinsic
Peristalsis: extrinsic
Describe segmentation.
Contractions of CIRCULAR muscle are generated on either side of the bolus causing both mixing and propulsion and move down the GIT forming different pockets of bolus
Purpose of gallbladder?
It concentrates bile and then contracts to release bile into the duodenum to facilitate lipid absorption
Regulation of gallbladder?
- Vagal stimulation mildly relaxes the sphincter of Oddi from gallbladder into duodenum at the beginning of digestion
- CCK released in blood + vagal stimulation later in digestion stimulate gallbladder contraction and full relaxation of the sphincter of Oddi
What happens to fats in the duodenum?
Emulsification and digestion
What happens to fats in the jejunum?
Micelle formation and absorption
What does CCK stand for?
Cholecystokinin
Motility in the large intestine? Describe each. Which is the predominant form of propulsion?
- *****Segmental propulsion = taneia coli contract to make sacs (haustrae) in the colon
- Mass movements = peristaltic contractions stimulated by GI hormones and vagus nerve in response to chyme in the upper GIT
Speed of segmental propulsion in large intestine?
SLOW
What are the taneia coli?
3 bands of longitudinal smooth muscle in the large intestine
Which 2 GI hormones stimulate mass movements in the colon?
- Gastrin
2. CCK
Timing of mass movements in the colon? What is this called? Describe it.
Gastrocolic reflex = occurs after feeding:
Chyme in the stomach stimulates colonic mass movements through the stimulation of the PNS and gastrin, both of which are stimulated very early in feeding => these both stimulate depolarization of slow waves lower in the GI tract (ileum, ascending and transverse colon), initiating peristaltic-type movements => chyme (and feces) move out of the lower GI tract
Where does the vagus innervation end?
Proximal 2/3rds of transverse colon = end of midgut
What typically stimulates the defecation reflex?
Mass movements in colon cause the haustrae to smooth out into a tube and the feces to descend from the colon to the rectum => rectal stretch => mechanoreceptors activate the ENS => signal to the brain with urge to defecate + relaxation of internal rectal sphincter => voluntary contraction of external rectal sphincter => over time the urge to defecate goes away as the rectum relaxes
Other name for defecation reflex?
Rectosphincteric reflex
What happens during defecation?
Increased abdominal pressure through the Valsalva maneuver
What is Hirschsprung’s disease? Treatment? Other name?
Congenital disorder in which the ganglion cells in the myenteric and submucosal plexus are lacking from a portion of the distal colon, rectum, and ALWAYS the internal anal sphincter => feces and gas accumulate proximal to the affected bowel segment causing tremendous distention/hypertrophy of the bowels => internal rectal sphincter cannot relax = mega colon
Treatment = surgery to remove that part of the colon and rectum, and reanastamosis with the external anal sphincter
What can constipation lead to?
Polyp formation and cancer
Usual age of diagnosis for Hirschsprung’s disease?
2 months
What are the interstitial cells of Cajal?
Specialized pacemaker cells located in the wall of the stomach, small intestine, and large intestine that generate the slow waves
Can the slow waves of the basic electric rhythm cause contractions of the GIT?
Only in the stomach
What sets the maximal possible rate of propulsion through the GIT?
The basic electric rhythm
Motility types in the mouth?
- Mastication
2. Deglutition = swallowing
3 parts of stomach?
- Fundus
- Corpus = body
- Antrum
What is special about the smooth muscle in the stomach?
Oblique muscle layer in addition to the longitudinal and circular layers found in the rest of the GIT
Other than in the stomach, where else does gastric accommodation occur?
Small intestine
What is colonic salvage?
Last absorption of Na+ and H2O by the colon through the haustrations that stay formed for long periods of time as they do not have villi
2 circumstances where retrograde peristalsis occurs?
- Vomiting
2. When the defecation reflex is voluntarily stopped
Rate of contractions during segmentation?
Same rate as slow waves
Which can be propagated over longer areas of intestine: peristalsis or segmentation?
Segmentation
What allows flow of bile into the duodenum?
- Vagal innervation
- CCK
- Peristalsis
What cells secrete HCl in the stomach?
Parietal cells
Where are the interstitial cells of Cajal located?
Between the longitudinal and circular smooth muscle layers
What is the ileogastric reflex? Purpose?
Chyme in the ileum => stimulates nerves that innervate the pylorus to increase sphincter tone => reduced amount of chyme leaving the stomach to empty into the duodenum
Purpose: this allows a little more time to clear the lower GI of chyme
What produces gastrin in the GIT? In response to what?
G cells of the antrum of the stomach and duodenum secrete gastrin in response to stretch and composition of chyme
Functions of gastrin?
- Enhance HCl secretion in stomach
2. Stimulate motility in the ileum and colon
When does the MMC stop?
As soon as food is in the stomach
How are the slow waves of the interstitial cells of Cajal generated? What regulates this?
The undulations reflect changes in the resting membrane potential caused by coupling of the muscle to the ICC and a PDGFR-α+ cell (Platelet-derived growth factor receptor), forming a SIP (smooth muscle, ICC, PDGFR-α+ cell) syncytium
The SIP syncytium is regulated by ion channels (primarily potassium and variations in Na+-K+ ATPase activity) and receptor-mediated processes
List the layers of the muscles of the stomach from outer to inner.
- Longitudinal
- Circular
- Oblique
If a slow wave has more spike potentials than another, what does that mean?
Force of contraction is stronger
Is motilin secreted and active during normal digestion?
NOPE, only during MMC
What is ileus?
Inability of a portion of the SMALL intestine to contract normally and move waste out of the body due to an issue with the ENS
Do mass movements ALWAYS stimulate a defecation reflex?
NOPE, might not move poop into rectum
When do the gastric contractions start?
As soon as food enters the stomach (?)
What is the MAIN purpose of the gastric contractions: propulsion to duodenum or mixing of the food? Why?
Mixing because rate of slow waves is very slow in the stomach (3/min)
Are mass movements fast or slow?
FAST
