Physiology-Intestinal Motility Flashcards
What is responsible for the “corkscrew” nucleus seen in smooth muscle?
Actin and myosin are connected to dense bodies within the sarcoplasm. This cause the smooth muscle cell to “wring out” when it contracts, twisting the nucleus.
What part of digestion is this layer in the wall responsible for?
This is the circular inner layer cut in cross-section, note that it is a tight spiral. It is responsible for compressing and mixing the content in the lumen.
Why is there so much electrical activity within the inner circular layer? How is the outer layer different?
There are many nexuses (gap junctions) between cells that creates an electrical syncytium. The outer longitudinal layer has fewer nexuses and is thus less electrically active.
What makes up sphincters? Tenia coli?
Sphincters = thickening of inner circular layer. Tenia coli = thickening of outer longitudinal layer.
Where do you find the myenteric plexus?
In the connective tissue between the inner circular layer and the outer longitudinal layer.
What neurons are found in the area indicated below?
This is the myenteric plexus. This is where postganglionic parasympathetic neurons are found (it is a ganglion in the target tissue). The enteric nervous system is also located in this layer. Blood vessels are also located here.
How is smooth muscle contraction initiated in the GI tube by the myenteric plexus?
Release of NTs from varicosities around the smooth muscle causes an action potential. Gap junctions between cells allow propagation of the signal because there are no nerve-muscle junctions.
How are nerves and hormones the gas pedal and brakes in the GI tube?
There is always a muscular tone present, the neurons and hormones are what determine how much tone is present.
What is responsible for the basal muscular tone present in the GI tube?
Interstitial cells of Cajal have a fluctuating Na/K ATPase pump. Consequently, there is a separation of charges across the sarcoplasm creating a resting potential of -60mV. In the inner circular layer, you see slow rhythmic changes in this potential between -45 mV and -75 mV.
What is the rate at which slow waves occur in the GI tube?
Basal electrical rhythm.
Where is basal electrical rhythm absent?
Esophagus, proximal stomach and most sphincters. It is present in most other places, despite varying gradients (cycles per minute shown below).
When do you get slow wave-induced spike potentials?
Once they cross the threshold of -40mV. These are true action potentials.
What determines how many spike potentials occur and thus determines how strong smooth muscle will contract?
How far above threshold the peak height of the slow wave is, the higher above the threshold the more spike potentials generated.
Why do spike potentials last 10-20 ms?
They use Ca-Na channels to generate action potentials, which are slower. Note that for these action potentials to occur there must be extracellular calcium present because smooth muscle cells do not have a sarcoplasmic reticulum.
What factors push the baseline resting potential closer to threshold?
Stretch, ACh, Substance P, Parasympathetics, GI hormones. Note that there is no change in frequency of waves.
What factors push the baseline resting potential away from threshold?
NE, EPI, NO, VIP, Sympathetics
What will change the basal electrical rhythm in the GI tube?
Nothing, these are set.
Do slow waves or spike waves allow Ca entry? Main role of slow waves?
Spike, this is why you don’t have contraction with slow waves. Slow waves determine timing, rate, strength and frequency of spike waves.
2 types of GI muscle contractions, where do they happen?
Phasic (distal stomach, small intestine and colon) and tonic (fundus, gall bladder and smooth muscle sphincters)
2 types of phasic contractions
Segmentation contraction: inner circular layer contracts to mix and move content according to the basal electrical rate. Perstaltic contractions: both inner circular and outer longitudinal layers contract to propel bolus forward.
What has large control over tonic muscle contractions in the GI system?
Autonomics. Neurotransmitters and hormones determine contraction and relaxation of sphincters that have tonic muscle contraction.
What are the excitatory neurotransmitters of the enteric nervous system? Inhibitory?
Excitatory: ACh and Substance P. Inhibitory: NO and VIP. Note that it is primarily excitatory to both muscle layers and can sometimes be inhibitor to the inner circular layer.
How does the parasympathetic nervous system change motility in the GI tract? How about the sympathetic nervous system?
PARASYMPATHETIC: increases tone, phasic contractions, intensity, rate of phasic contractions, velocity of conduction and inhibits sphincters. SYMPATHETIC: does the opposite.
Where does most parasympathetic innervation to the GI tract come from?
Vagus (stomach, esophagus, pancreas and some intestine). Pelvic splanchnics hit the hindgut.
Where does most sympathetic innervation to the GI tract come from?
T5-S1; celiac, superior mesenteric and inferior mesenteric ganglion.
What are the pacemaker cells in and surrounding the inner circular muscle layer?
Interstitial cells of Cajol (ICC). They are smooth muscle/fibroblast hybrid cells that link the enteric nervous system to smooth muscle. They catch the NT release from ENS varicosities. They transmit that signal to the smooth muscle through gap junctions.
What are short reflexes used for in the ENS?
Peristalsis and mixing between close segments in the GI tract. The signal stays within the muscle layer.
What are long reflexes used for in the ENS?
Emptying or secretions. These reflexes actually have to go back to the pre vertebral chain and come back
How does the stomach ensure that small particles are all that enters the intestine?
Contraction waves intensify as you approach the pyloric sphincter. As food approaches the sphincter, it also contracts and sends food back by retropulsion. This ensures good mixing of food down to smaller particles that can be digested by the intestine because retropulsion also causes food grinding by the contracted antrum.
What happens in the stomach once most of the food is digested into smaller particles?
Emptying contractions begin and go higher and higher up in the stomach as it completely clears itself of food.
What happens about 90 minutes after eating?
Migrating motility complex. This is a 15 minute period with 3 phases: 1) no contractions 2) mild contractions 3) strong contractions. This is all meant to clean out the entire stomach all the way down to the terminal lieum.
What happens when 90 minutes have passes since food ingestion in the small intestine?
It experiences a three phase migrating motility complex over 15 minutes to clear everything.
A patient had open abdominal surgery after a car accident. The next day she presents with nausea, vomiting, absent bowel sounds, constipation and decreased appetite. What is causing her symptoms?
Ileus. Irritation of the intestine during abdominal surgery causes release of VIP and NO, which are inflammatory substances and inhibitors of motility.
Where is chyme stalled until the next meal?
Ileocecal junction. It has both a sphincter and a valve. The sphincter is tonically active until a gastroileal reflex is sent. When food enters the small intestine, the reflex causes relaxation of the ileocecal sphincter. Distention in the area also helps relax the sphincter. Both of these things allow content to move into the cecum. Once the cecum is distended, the valve closes to prevent reflux of chyme back into the small intestine.
How doe these factors differ in the small vs. large intestine?
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What innervates the different regions of the large intestine?
Proximal colon: segmental contractions triggered by vagus. Distal colon: propulsive contractions triggered by pelvic nerves.
How does the BER changes as you move distally through the large intestine?
There is a reverse gradient in the proximal colon. In the most proximal part the gradient is slow and gets faster as you move towards the transverse colon. This sometimes results in reverse movement that causes increased transit time. In the rest of the large intestine, there is no gradient at all.
Most common and least common contractions seen in the large intestine?
MOST COMMON: Mixing movements (from one haustra to the next), migration movements (peristaltic movements from one haustra to a few down). LEAST COMMON: Mass movement (from one haustra to many haustra down)
Where do mass movements most commonly occur in the large intestine?
Distal colon. This is rare, but happens when the bowel is distended or irritated. A constrictive ring forms, haustra dissipate and a long segment contracts with increasing force of 30 seconds. Then there will be 2-3 minutes of relaxation before the next cycle. These cycles last about 10-30 minutes happen every 1/2 - 1 day, creating an urge to poop.
Fecal matter enters the rectum. What reflexes proceed from here?
Distention of the rectum activates rectosphincteric reflex. This causes relaxation of the internal sphincter and urge occurs as the external sphincter contracts. In the case that you do not relieve yourself, the rectum relaxes to accommodate feces, the internal sphincter regains tone and the external sphincter can relax until the next fecal entry.
What is this layer of the GI tube responsible for?
This is the outer longitudinal layer cut in cross-section, note that it is a loose spiral. It is responsible for propelling food down the tube by shortening.
