GI Motility Flashcards
What is the term used to refer to contraction and relaxation of walls and sphincters of GIT?
motility
What does motility accomplish?
- propel digesta
- mixing
- grinding
Contractile tissue of GIT is composed of?
- smooth muscle
- striated muscle in pharynx, upper 1/3 of esophagus, external anal sphincter
What is the smooth muscle type of the GIT?
unitary smooth muscle
Contraction of circular muscle does what?
Shortening of ring of smooth muscle (decrease diameter)
Contraction of longitudinal muscle does what?
Shortening in longitudinal direction (decrease length)
What are phasic contractions?
Periodic contractions followed by relaxation
Which organs have phasic contractions?
- esophagus
- gastric antrum
- small intestine
What do phasic contractions do?
mixing and propulsion
What are tonic contractions?
maintain a constant level of contraction or tone without regular relaxation
What regions of the GIT have tonic contractions?
- upper region of stomach
- lower esophagus
- ileocelcal and internal anal sphincters
What determines the movement of digesta?
slow waves of GIT
What are the three functions of chewing (mastication)?
- mix food with saliva to lubricate
- reduces size of food particles
- mixes ingested carbohydrates with salivary amylase
Chewing has an involuntary and voluntary component. What initiates the involuntary component?
initiated by food in mouth and mechanoreceptors in mouth relay to brainstem
Chewing has an involuntary and voluntary component. What does the voluntary do?
can override the reflex chewing of involuntary component at any time
What are the three phases of swallowing (deglutition)?
- oral phase
- pharyngeal phase
- esophageal phase
Reflex swallowing is controlled by what?
swallowing center in medulla
Is swallowing initially voluntary or involuntary?
voluntary
The pharyngeal phase of swallowing has four steps. Describe them.
- soft palate pulled up which narrows passage so bolus can not move to nasopharynx
- epiglottis covers opening to larynx, larynx moves against epiglottis and covers opening to trachea
- upper esophageal sphincter opens so food moves from pharynx to esophagus
- peristaltic wave propels bolus
The esophageal phase of swallowing is controlled by what?
swallowing reflex AND enteric nervous system
What is the receptor path for reflex swallowing?
receptors near pharynx detect and send info via vagus an gloospharyngeal nerves to swallowing center in medulla
Describe the oral phase of swallowing.
tongue forces bolus toward pharynx which has a lot of receptors and triggers involuntary swallowing to start
What is needed to propel bolus from pharynx to stomach?
esophageal motility
What are the four steps of esophageal motility?
-upper esophageal sphincter open to allow bolus into esophagus, then closes
- primary peristaltic contraction creates high pressure area behind bolus
- as peristaltic wave and bolus reach lower esophageal sphincter, it opens. we also get receptive relaxation of upper region in stomach
- secondary perstaltic wave clears esophagus and any remaining food
The pressure difference between the upper and low esophagus creates the need for what?
sphincters
What mediates the opening of the lower esophageal sphincter?
vagus nerve and VIP
What are the three components of gastric motility?
- relaxation of orad region in stomach
- contractions reduce bolus size and mix gastric secretions
- gastric emptying which is when chyme (digests) enters small intestine
Gastric emptying has what type of regulation?
hormonal regulation
What are the three muscle layers of the stomach?
- outer longitudinal
- middle circular
- inner oblique
What type of innervation is found in the stomach?
extrinsic and intrinsic
Describe receptive relaxation and its effects.
- relaxation of orad stomach caused by relaxation of lower esophageal sphincter
- reduces pressure, increases volume of stomach
- vagovagal reflex involved
What are the components of mixing and digestion?
- contractions, mixing in caudad region
- waves begin in middle of stomach body
- retropulsion (digesta propelled back)
- slow waves
- migrating myoelectric complexes mediated by motilin
Slow wave frequency cannot be altered by neural input or hormones, but strength can be. What can cause an increase in strength? Decrease?
- increase force via PSNS, gastrin, motilin
- decrease force via SNS, secretin, GIP
What are the three goals of small intestinal motility?
- mix chyme with enzymes and secretions
- expose nutrients to intestinal mucosa for absorption
- propel chyme into large intestine
In what part of the small intestine are slow waves more frequent?
duodenum
What is the parasympathetic innervation of small intestine and its effect?
via vagus nerve to increase contraction strength
What is the sympathetic innervation of small intestine and its effect?
via celiac and superior mesenteric ganglia to decrease contraction strength
What nervous system coordinates segmentation and peristaltic contractions of the small intestine?
enteric nervous system
What is the function of segmentation contraction in small intestine?
- mix chyme (digesta) and expose it to pancreatic enzymes
Describe segmentation contractions.
back and forth movement to mix, no forward movement
What is the function of peristaltic contractions in small intestine?
propel chyme toward large intestine
Describe peristaltic reflex for moving chyme.
bolus is sensed by enterochromaffin cells
Why do we need circular and longitudinal muscles working together for peristaltic contractions in the small intestine?
contraction is behind bolus, relaxed in front of bolus
List the events of vomiting reflex.
- reverse peristalsis in small intestine
- relaxation of stomach and pylorus
- increased abdominal pressure
- movement of larynx up, relaxation of lower esophageal sphincter, closure of glottis
- forceful expulsion
What causes vomiting reflex?
- information from vestibular system, back of throat, GIT and chemoreceptor trigger zone in 4th ventricle which does to vomiting center in medulla to trigger vomiting reflex
Describe retching.
upper esophageal sphincter remain closed, lower open, contents return to stomach when retch over
What are the sac like segments that help with mixing in a side to center fashion?
haustra
Segmentation contractions occur where?
cecum and proximal colon
What species do not have haustra?
- dogs
- rats
- carnivores
- ruminants
What species do have haustra?
- guinea pigs
- rabbits
- pigs
- humans
- monkeys
- horses
Haustra is associated with what type of contractions?
segmentation contractions
What species do not have a sigmoid colon?
- dog
- cat
- horse
In what species does the ascending colon form a spiral colon?
pigs
What allows contents to enter the cecum?
ileocecal sphincter opening
What is the gastrocolic reflex?
distension of stomach increases motility of colon
What are the mediators of the gastrocolic reflex?
CCK and gastrin
What is the purpose of mass movements in the large intestine?
move contents over long distances for water reabsorption and into rectum
What occurs for defecation?
- rectum fills and smooth muscle wall contracts
- internal anal sphincter relaxes
- external anal sphincter tonically contracted until defecation, then it relaxes for defecation to occur
What is a unique feature of GI smooth muscle?
slow waves in GIT
Describe slow waves of GIT.
- oscillating depolarization and repolarization of cell membrane potential of smooth muscle cells
- slow waves may or may not bring cell to threshold for AP to occur
What is the origin of slow waves in GIT?
interstitial cells of Cajal in myenteric plexus
What is the pacemaker for GI smooth muscle?
interstitial cells of cajal in myenteric plexus
What is the mechanism of the depolarizing phase and maintenance of plateau for slow waves?
opening of Ca channels maintain plateau
What is the mechanism of the repolarization phase for slow waves?
opening of K channels
