Motility of the GI Tract Flashcards
What are the characteristics of tonic contractions?
- Maintain a constant level of contraction w/o regular periods of relaxation
- located in orad stomach, lower esophageal sphincter, ileocecal sphincter, and internal anal sphincter
What are the characteristics of phasic contractions?
- periodic contraction followed by relaxation
- important for peristalsis
-located in esophagus, stomach (antrum), small intestine, and all tissues involved in mixing and propulsion
What are slow waves of the GI tract smooth muscle?
-depolarization and repolarization of the membrane potential
When does an action potential happen in the smooth muscle of the GI tract?
-when the depolarization moves the membrane potential to or above threshold
What causes larger contractions of smooth muscle in the GI tract?
-when there are more action potentials on top of the slow waves
What causes action potentials in GI tract smooth muscle?
- stretch
- Ach
- parasympathetics
What inhibits smooth muscle contraction in the GI tract?
- NE
- sympathetic stimulation
-decrease in GI motility d/t membrane potential being moved further away from threshold
Where are the interstitial cells of Cajal located?
-in the submucosal and myenteric plexus of GI tract
What is the role of Interstitial Cells of Cajal?
- generate and propagate slow waves
- spread signals via gap junctions
- drive the frequency of ctx
What are the two components of the enteric nervous system (ENS)?
- submucosal plexus
- -mainly controls GI secretions and local blood flow
- -also senses the lumen environment of small intestine
- myenteric plexus
- -mainly controls GI mvmts
What are the three phases of swallowing and their status as voluntary or involuntary?
- oral phase (voluntary)
- pharyngeal phase (involuntary)
- esophageal phase (involuntary)
What are the characteristics of the oral phase of swallowing?
- initiates swallowing process
- starts with the chewing process
What are the characteristics of the pharyngeal phase of swallowing?
- soft palate is pulled upward
- -epiglottis moves and UES relaxes
- peristaltic wave of ctx propels food down
-depression of respiratory system prevents aspiration
What are the characteristics of the esophageal phase of swallowing?
- longest phase
- consists of primary and secondary peristaltic waves
What are the characteristics of primary peristaltic waves during the esophageal phase of swallowing?
- controlled by the medulla
- cannot occur after a vagotomy
What are the characteristics of secondary peristaltic waves during the esophageal phase of swallowing?
- controlled by both medulla and ENS
- can occur after a vagotomy
What are the afferent and efferent pathways involved in the involuntary swallowing reflex?
- food in the pharynx is detected by CN IX and X
- -sends afferent info to swallowing ctr in medulla
- brainstem nuclei send efferent info back to pharynx
At rest, before swallowing, where are intraluminal pressures along the upper GI tract positive?
- UES
- LES
- fundus/orad stomach
At rest, before swallowing, where are intraluminal pressures along the upper GI tract negative?
-along the thoracic esophagus
How does the pressure of the upper esophageal sphincter change at the initiation of swallowing?
- pressure drops drastically to allow food to pass from pharynx to esophagus
- sphincter closes and pressure increases immediately after swallowing
What happens to the pressure along the thoracic esophagus during swallowing?
- peristaltic wave of ctx increases the pressure sequentially as food moves down the esophagus
- results in the propulsion of food downward
What happens to the pressure of the LES and the fundus/orad stomach during swallowing?
-LES and fundus/orad stomach pressures decrease and the structures relax to accommodate food bolus before it arrives
What is receptive relaxation?
- the distention of the lower esophagus by food produces relaxation of the LES and orad stomach
- orad stomach experiences decreased pressure and increased volume
What type of reflex is receptive relaxation?
vasovagal
What would happen to receptive relaxation after a vagotomy?
-a vagotomy would eliminate receptive relaxation
How does parasympathetic stimulation regulate gastric contractions?
- induces secretion of gastrin and motilin
- increases action potentials and force of ctx
How does sympathetic stimulation regulate gastric contractions?
- induces secretion of secretin and GIP (gastric inhibitory peptide)
- decreases action potentials and force of ctx
What are the characteristics of the migrating motor complex?
- periodic bursts of peristaltic ctx every 90 min mediated by motilin during fasting; inhibited during feeding
- cleansing mechanism for small intestine; prevents SIBO (small intestine bacterial overgrowth)
How would injury to the vagus N. affect gastric emptying?
gastric emptying would decrease
How would increased acid in the duodenum affect gastric emptying?
gastric emptying would decrease
How would the release of secretin affect gastric emptying?
secretin inhibits gastrin, so gastric emptying would decrease
How does presence of fats in the duodenum affect gastric emptying?
- gastric emptying would decrease
- presence of fats detected by I Cells would induce secretion of CCK from I Cells to inhibit stomach motility
What actions or characteristics of the duodenum would decrease gastric emptying time (increase gastric emptying)?
- increased diameter of duodenum
- decrease in segmenting ctx of proximal duodenum
What actions or characteristics of the duodenum would increase gastric emptying time (decrease gastric emptying)?
-segmentation ctx in duodenum
How do liquids empty out of the stomach versus solids?
liquids empty stomach faster
How does the tonicity of contents affect gastric emptying?
isotonic contents empty faster than hypo- or hypertonic contents
How does the size of particles affect gastric emptying?
particles must be reduced to 1 cubic mm or less
True or False: circular and longitudinal muscles of the small intestine work in opposition to complement eachother’s actions via reciprocal innervation
True
True or False: slow wave activity is always present in the small intestines
True
True or False: contractions only occur in the small intestines when spiked action potentials pass threshold
True
True or False: contractions only occur in the stomach when spiked action potentials pass threshold
False; the stomach does need action potentials to contract
-the stomach can produce weak contractions with subthreshold depolarization
What sets the frequency of contractions in the small intestines?
the frequency of slow waves
What is the frequency of slow waves in each section of small intestines?
duodenum - 12 per min
jejunum - 10 per min
ileum - 8 per min
What is the signal for the small intestines to contract?
muscle distention
What type of neuron senses the changes (such as muscle distention) in the lumen of the small intestine and what NTR does it respond to?
intrinsic primary afferent neuron (IPAN)
-responds to serotonin 5HT that is released by enterochromaffin cells
Where does the IPAN send its afferent signals?
-to interneurons that stimulate either excitatory motor neurons or inhibitory motor neurons
What NTR’s are released by excitatory motor neurons in the small intestine and what is the muscular response?
Ach (acetylcholine)
Substance P
-muscles contract
What NTR’s are released by inhibitory motor neurons in the small intestine and what is the muscle response?
- VIP (vasoactive intestinal peptide)
- NO (nitric oxide)
-muscle relaxation
What is the neural input that mediates the peristaltic reflex?
-peristaltic reflex is mediated by the ENS
- -in general: parasympathetics stimulate
- -in general: sympathetics inhibit
What hormones stimulate smooth muscle contractions in the small intestine?
-serotonin, prostaglandins, gastrin, CCK, motilin, insulin
What hormones inhibit smooth muscle contractions in the small intestine?
- epinephrine from adrenal glands
- secretin
- glucagon
What part of the brain coordinates the vomiting reflex?
-medulla
What is the neural input for the vomiting reflex?
- nerve impulses are transmitted via CN X and sympathetic afferents to multiple brainstem nuclei
(ex: vomiting center)
What is the chemoreceptor trigger zone?
- located in the wall of the 4th ventricle
- responds to apomorphine and morphine
- also sends signals to the vomiting center
Delineate the events of vomiting.
- reverse peristalsis in small intestine
- pylorus and stomach relaxation
- forced inspiration increases abdominal pressure
- LES relaxation and glottis closure
- forceful expulsion of gastric contents
What is the retrosphincteric reflex?
-as rectum fills with feces, the smooth muscles of the rectum contract and the internal anal sphincter relaxes
What are the characteristics of the movement of contents through the colon?
- occur over large distances 1-3x daily
- stimulate the defecation reflex
- a final mass mvmt propels fecal contents into rectum
What is the open/closed status of the external anal sphincter under normal conditions and what nerves control this?
- tonically closed under voluntary control
- somatic pudendal nerves
Delineate the events of defecation.
- external anal sphincter is relaxed voluntarily
- rectum smooth muscles contract
- Valsalva maneuver
- -exhaling against a closed glottis to increase abdominal pressure
How would a spinal cord injury affect defecation?
- loss of voluntary control of defecation
- voluntary cntrl of external anal sphincter is mediated by pathways within the spinal cord that lead to the cerebral cortex
Delineate the gastric motility.
- wave of ctx starts in midstomach
- ctx increase in strength toward pylorus
- ctx mix the gastric contents
- ctx periodically propel some contents into duodenum
- most contents are propelled back into stomach
What are segmentation contractions of the small intestine?
- segmentation ctx = mix chyme and expose it to pancreatic enzymes
- section of small intestine contracts, splitting chyme, then relaxes creating back and forth mvmts
True or False: the large intestines also have segmentation contractions
True; cecum and proximal colon have segmentation ctx that function to mix intestinal contents
-ctx of the teniae coli create haustra
How would the speed of mvmt through the colon affect the recovery of water and electrolytes?
- poor motility causes greater absorption
- -harder feces causes constipation
- excess motility causes decreased absorption
- -diarrhea and loose feces
What are the characteristics of the vasovagal reflex?
- generally stimulatory
- -increases motility, secretomotor, vasodilatory
- CN X carries both afferent (75%) and efferent
ex: receptive relaxation
What are the characteristics of the intestino-intestinal reflex?
- depends on extrinsic neural connections
- generally inhibitory
- -if an area of bowel is grossly distended, contractile activity in the rest of the bowel is inhibited
What are the characteristics of the enterogastric reflex?
-negative feedback from duodenum will slow down the rate of gastric emptying
What are the characteristics of the gastroenteric reflex?
-gastric distention relaxes the ileocecal sphincter
What are the characteristics of the gastrocolic and duodenocolic reflexes?
- distention of the stomach or the duodenum initiates the mass mvmts in the colon
- transmitted by the autonomic nervous system
Clinical Correlation: Gastroesophageal Reflux Disease (GERD)
- lower esophageal sphincter relaxes or weakens
- backwash of acid, pepsin and bile into esophagus
- -d/t large meal, heavy lifting, pregnancy
- persistent reflux and inflammation causes complications
- -GI bleeds, esophagitis, scar tissue, Barrett’s
Clinical Correlation: Achalasia
- impaired peristalsis d/t incomplete relaxation or LES
- elevated LES resting pressure
- -d/t decreased ganglion cells in myenteric plexus
- degeneration of inhibitory neurons that produce NO
- causes regurgitation, dysphagia, heartburn, CP
Clinical Correlation: Gastroparesis
- slow emptying of stomach or paralysis of stomach
- commonly caused by DM1 d/t neuropathy
- can be caused by CN X injury
- N/V, early feeling of fullness, weight loss, bloating
Clinical Correlation: Megacolon (Hirschsprung’s Disease)
- ganglion cells absent from part of colon
- low VIP levels lead to smooth muscle constriction
- -colon contents accumulate
- can be present at birth
- Tx: surgical colon resection
What are symptoms of Megacolon in a neonate?
- difficulty passing stool
- poor feeding
- jaundice
- swollen belly
- malnutrition
Megacolon is often associated with which genetic condition?
Down Syndrome
