GI Motility Of Esophagus And Stomach Flashcards
Motile functions of GIT
1)segmentation
2)propulsion (peristalsis)
3)reservoir function (storage)
Segmentation
-Large areas of the intestines contract their smooth muscle creating rings at multiple different points of the intestine
-This keeps mixing the chyme contents which helps the digestive process
-Helps the intestinal mucosa to come into contact with the substances
-Increases the absorption of nutrients
-Mostly in the small and large intestine
Propulsion (peristalsis)
The alternating waves of contraction and relaxation moving the GI content along the GIT
Reservoir function (storage)
-Holds GI content for long periods of time
-The large intestine holds a lot of the feces for large amounts of time
-The stomach can hold food for 4 to 6 hours
-The function is carried out by sphincters
2 types of contractions of smooth muscle cells
1) Rhythmic (alternating contraction and relaxation)
-Segmentation
-Propulsion
2)Tonic (sustained contraction) -Sphincters
Smooth muscle cells have the ability to depolarize…
-and to contract
-Resting potential of -80 mV
- Threshold potential of -55mV
-Interstitial cells of Cajal - pacemaker cells
-Spontaneously depolarize
-Generate action potentials
-Cause the smooth muscle of the GIT to contract
-They generate slow (subthreshold) waves
Stomach muscle cells have leakyCa2+ channels
-Always open
-Allows small amounts of Ca++ to enter the cell
-Slightly depolarize the cell
-When the threshold is approached
-Special K+ channels open
-K+ leaves the cell
-the cell becomes (-) charged
-starts to repolarize
Factors that allow cell to reach threshold
-Acetylcholine
-Released by the parasympathetic nervous system
-Vagus nerve
-Sacral (S2-S4) nerves
-GI Hormones
-Gastrin
-Cholecystokinin
-Secretin
-Motilin
-Stretching of the GI organ
These factors stimulate cations to float into the cell
-Makes the cell very positive
-Just enough to break the threshold
Sarcoplasmic reticulum of smooth muscle cell
-stimulated trough certain signaling processes
-There are a lot of Ca++ sequestered there by
-Calsequestrin
-Calreticulin
-When stimulated the sarcoplasmic reticulum
-Pushes Ca++ into the sarcoplasm
-There it helps to initiate the cross bridge formation
-By activating calmodulin
-Calmodulin activates different myosin light-chain kinases
-They phosphorylate the myosin head and generates contraction
When the charge of the cell…
passes the threshold it produces spike potentials
-Many cation stimuli, bring the depolarizing wave high above the threshold
-More spike potential is produced
-The force of the contraction is hig
Interstitial cells of Cajal
-connected to the other smooth muscle cells by gap junctions
-When the cell depolarizes
-Ca++ is released
-It can be pushed into the connected cells
Esophagus function in transportation
-GI contents form the pharynx down to the stomach
-Primary function is peristalsis
-The alternating waves of contraction and then relaxation
-Doesn’t do any segmentation and reservoir function
-Although there are a couple of sphincters
- Upper esophageal sphincter (UES)
-Primarily made up of the cricopharyngeal muscle
-Innervated by the vagus nerve
-Lower esophageal sphincter (LES)
-A.k.a. cardiac sphincter or gastroesophageal sphincter
When bolus stretches esophageal walls…
-Activates stretch receptors that have the ability to stimulate and inhibit myenteric plexus
-Ascending (stimulatory) neurons
-Descending (inhibiting) neurons
-The plexus is located between the two muscle layers the esophagus
-Outer longitudinal
-Inner circular
Stimulating the ascending neurons…
-to a certain point causes them to release specific chemicals
-To the circular muscle layer causing it to contract
-Acetylcholine
-Substance P
-To the longitudinal muscle layer causing it to relax
-Vasoactive intestinal peptide (VIP)
-Nitric oxide (NO)
-This closes up the lumen
Stimulating descending neurons
-to a certain point causes them to release specific chemicals
-To the circular muscle layer causing it to relax
-Vasoactive intestinal peptide (VIP)
-Nitric oxide (NO)
-To the longitudinal muscle layer causing it to contract
-Acetylcholine
-Substance P
-This opens up the lumen
3 functions of the stomach
-storage (reservoir)
-mixing (churning)
-emptying
Stomach acting as 2 individual organs
-The fundus and the upper part of the body:
-Reservoir function
-The lower part of the body and pylorus
-mixing and emptying
Reservoir function of stomach- cephalic phase
-Thought, sight, smell and taste of food have the ability to activate the efferent fibers of the vagus nerve
-Before the food reaches the stomach
-The efferent fibers of the vagus nerve go to the fundus of the stomach
-There they stimulate nerves to trigger the release of VIP and NO
-These substances cause the smooth muscle cells to relax
-The relaxation of the fundus and the upper part of the body of the stomach in response to the cephalic phase is called receptive relaxation
-The stomach starts relaxing and dilating prior to the food being in the stomach
-It is getting ready to receive the bolus
Reservoir function of stomach- gastric phase
-The bolus enters the stomach
-The volume starts increasing
-Causes distension and stretching within the walls
-Triggers reflex arks that release VIP and NO and promote even more relaxation of the smooth muscle
-Short reflex ark - a local reflex
-Triggers the release of VIP and NO
-Long reflex ark – activation of the afferent fibers of the vagus nerve which subsequently activate the efferent fibers
-Trigger the release of VIP and NO
-This relaxation is called adaptive relaxation
-The stomach can continue to occupy a large volume of content without increasing the intragastric pressure
-Both receptive and adaptive relaxation play a role in gastric accommodation
-The intragastric volume may increase but the intragastric pressure will remain constant to a limit
-Above 1.5 liters the pressure stats rising
-The bolus is high in partially digested proteins
-Increases the pH
-Activates particular cells – enteroendocrine G cells
-Located within the lower part of the stomach (body and antrum)
-These cells release specific chemicals:
-Gastrin
-Stimulates the SMCs to relax even more.
-Part of the adaptive relaxation
-Causes the antrum of the stomach to contract
-Helps emptying the stomach
Reservoir functions of stomach intestinal phase
-When the stomach empties the contents that goes into the small intestine is rich in H+, fat, peptides, carbohydrates, etc.
-These factors stimulate certain enteroendocrine cells to secrete
-Cholecystokinin
-In response to fat and partially digested proteins
-Secretin
-in response to acidic chyme
-Gastric inhibitory peptide/ glucose-dependent insulinotropic peptide (GIP)
-In response to carbohydrates
-All these play a crucial role in the relaxation of the stomach
-Mainly the fundus and the upper body
-Allow the stomach to store the food a little bit longer
-The duodenum prepares for the chyme contents
Mixing and emptying functions of the stomach
-In the mid part of the body there is high concentrations of pacemaker cells of Cajal
-Between the middle circular and the outer longitudinal muscle layer
-Have a basic electrical rhythm of about 3 to 5 action potentials per minute
-They generate and send out action potentials
-Cause the stomach smooth muscles to contract
-The contractions start off within the body of the
stomach
-Increase in intensity and force as they go down to the pylorus
-An action potential generates and spreads to the upper part of the body
-Causes a contraction
-Yanks some of the chyme stored in the fundus into the body to be mixed with gastric juices
-In the mid part of the body
-Pushes the chyme towards the pylorus
-At the level of pylorus, the contractions are almost occluding the lumen.
-The pylorus can be divided into three portions – distal, middle and proximal.
Mixing and emptying functions of the stomach: proximal portion
-When the chyme goes in the proximal aspect of the pyloric antrum this part of the pyloric antrum squeezes down and occludes the lumen behind
-Prevents the chyme from flowing back to the stomach
-Meanwhile, the parts distal to the proximal aspect of the antrum relax so the chyme from the proximal aspect can be pushed in
-This is propulsion
-In order for substances to pass through the pyloric canal they need to be less than 2mm in size
Mixing and emptying functions of stomach: middle portion
-The middle aspect contracts and pushes its contents onwards through the still relaxed distal aspect
-About 3-4ml is pushed into the duodenum
-The rest is pushed back into the stomach to continue mixing with the gastric juice and reduce the size of the particles
-This is both propulsion and mixing
Mixing and emptying functions of the stomach: distal portion
The distal/terminal aspect of the antrum contracts and closes off
-Because its right next to the pyloric sphincter which also contracts and closes off the lumen
-Any substances left in the terminal aspect can’t go into the duodenum and are pushed back into the stomach
-This is retropulsion
Stomach secretion phases
1) Cephalic phase causes a lot of contractility in the pylorus and a little bit of the body
-The chyme can leave
2)During the gastric phase the pyloric contractility is inhibited
-The chyme going in the duodenum would have negative effects
3)During the intestinal phase receptors pick up significant distension and activate the SNS
-The pyloric sphincter is stimulated, contracts and
closes off the pyloric canal
-Helps prevent particles entering the duodenum
-Called the enterogastric reflex (a sympathetic reflex)
Migrating motor complex (MMC)
-During the ‘fasting’ state (the interdigestive period)
-Stimulated by the hormone called motilin
-A peristaltic wave that starts in the body of the stomach and moves its way down the pylorus
-Tries to empty the contents of the stomach into the
duodenum.
-Relaxes the pyloric sphincter
-Particles larger than 2mm can pass through it
-The surface epithelium of the body is undergoing much damage from the gastric juices and hydrochloric acid
-The cells regenerate every 3 to 6 days
- Produces a lot cellular debris (desquamated cells)
-MMC helps clear the debris
