GI Motility Flashcards
What parts of the GI system are composed of striated muscle?
- Mouth
- Pharynx
- Upper esophagus
- External Anal Sphincter
What are the 2 types of contractions of GI smooth muscles?
- Phasic Contractions
2. Tonic Contractions
What are phasic contractions?
- Regularly repeated contractions
2. Cause mixing & propelling of GI contents
Where do phasic contractions happen?
Stomach & Small Intestine
What are Tonic Contractions?
- Continous contractions
2. Only Relax under neural stimulation
Where do tonic contractions happen?
Upper stomach & the sphincters
What type of cell generates slow waves?
Interstitial cells of Cajal (ICC) - have gap junctions
Electrical activity in the GI tract is located where?
Between the long & circular muscle layers (mid stomach to rectum)
What two enzymes/hormones are released from the PNS and act on the myenteric plexus causing the depolarization of slow waves & contraction?
- Ach
2. Substance P
What other 2 hormones can also depolarize slow waves and cause contractions?
- Cholecystokinin (CCK)
2. Gastrin
The parasympathetic NS enhances motility & secretions through which 2 plexus’s?
- Myenteric
2. Submucosal
The sympathetic NS reduces motility & secretions through which nervous system?
The enteric NS
How does the sympathetic NS reduce motility & secretions?
Inhibitory motor neurons get stimulated and release VIP (vasoactive intestinal peptide) and NO –> hyperpolarize the slow waves –> relaxes smooth muscle
What is peristalsis?
Sequential contraction of muscles in response to stretching of gut wall
What does this describe? ‘Long periods of rest with short periods of waves of contractions’
Migrating Myoelectric Complex (MMC)
Where does the MMC start and continue its short periods of contractions to?
Starts in the mid-stomach & continues to the terminal ileum during each cycle
What is the function of the MMC?
- Function is “housekeeping”
- Undigested material and bacteria swept out of stomach and sm. intestine into colon,
- Protects delicate upper GI tract from damage, and gets bacteria in colon in a drier environment which is less hospitable to the bacteria
Out of the 4 phases of MMC which one is the most important?
- Phase III
- Mo cells of the small intestine release motilin into circulation
- Motilin causes strong sequential contractions
What are the 3 stages of swallowing?
- Oral Stage/Buccal Phase
- Pharyngeal Stage
- Esophageal Stage
What happens during the oral stage of swallowing?
- Bolus of food moved from back of mouth into pharyngeal region
- Touch receptors in this area stimulate swallowing
What happens during the pharyngeal stage of swallowing?
- Bolus moves to back of pharynx
- Larynx moves towards epiglottis
- Upper esophageal sphincter (UES) relaxes
What happens during the Esophageal stage of swallowing?
- Bolus enters esophagus where peristalsis propels food down the GI tract through lower esophageal sphincter (LES) into stomach
- LES normally has high resting muscle tone
What is receptive relaxation?
- As bolus enters stomach, relaxation increases stomach size to accommodate meal called receptive relaxation
- Vagal mediation thru release of VIP (vasoactive intestinal peptide)
As the stomach fills, what stimulates contraction?
- Stretch
2. Chemical Contents
As chyme mixes how to contents separate? (CHO, Large Chunks, & fats)
- CHO in lower antrum
- Large chunks in body of stomach
- Most fats float on top
As waves of peristalsis move food through the antrum of the stomach what happens?
- Pyloric Sphincter Relaxes (Under vagal nerve stimulation)
2. Chyme enters into the duodenum
An upper GI bleed is considered to be everything above what structure?
The ligament of Treitz
Name the 2 types of propulsion?
- Peristalsis
2. Segmentation
What is peristalsis?
- Contraction behind bolus & relaxation ahead of chyme
2. Both excitatory and inhibitory motor neurons stimulated simultaneously
What is segmentation?
- Formation of moving pockets of chyme
- Accomplished by close constriction of circular muscle
- Contracts in middle of bolus –> spreads chyme proximally & distally while mixing food
What is the most frequent type of contractions in the small intestine?
Segmentation
CCK is secreted into the blood in response to what?
(as chyme enters the duodenum) In response to glucose & fats
What causes contraction of the gallbladder?
CCK & Vagal N Stimulation
What are the 2 specialized forms of propulsion through the large intestine?
- Segmental propulsion SLOW
2. Mass movements
The ileocecal valve which regulates amount of chyme entering the colon is controlled by?
- Enteric nerves
- CCK
- Gastrin
- ANS
How often do mass movements occur in the large intestine?
1-3 times daily
What are mass movements in the large intestine stimulated by?
- PNS
- CCK
- Gastrin
What is the defecation reflex?
When feces moves into rectum, rectal distension sensed by mechanoreceptors –> signal myenteric nerves to relax internal anal sphincter and initiate peristalsis –> feces into rectum
Which nerves initiate stronger contractions during the defecation reflex?
Pelvic Nerves
At what age is the defecation reflex fully developed?
2-3 yrs of age
How is motility in the GI tract regulated?
- Enteric nerves
- ANS
- Hormones
What do enteric nerves respond to?
luminal receptors that sense chyme
Which 3 hormones increase pyloric tone?
- GIP
- CCK
- Secretin
What does Gastrin do?
Stimulates gastric motility (THE EXCEPTION)
Which 2 hormones relax the pylorus?
- Gastrin
2. PNS
What is Achalasia?
- Loss of ENS in lower part of esophagus
a. Myenteric plexus of the esophagus is deficient at the LES - Aganglionic region not able to
a. Stimulate contractions
b. Transduce signal to relax LES - Results in tight LES –> food & fluid can take hours to leave esophagus
- Over time, leads to esophageal dilation and damage to esophagus
What is Hirschsprung’s Disease?
- aka megacolon or aganglionic megacolon
- Congenital disease that involves loss of ENS in distal colon
- Always involves internal anal sphincter (IAS)*
Name some stimuli that can initiate the vomiting reflex?
- Irritant in stomach or small intestine
- Head injury
- Abnormal stimulation of vestibular organs
- Systemic irritant sensed by chemoreceptor trigger zone in brain located in 4th ventricle
Name some receptors found in the chemo receptor trigger zone?
- Dopamine
- Serotonin
- Opiod
- Substance P
