Gastric motility Flashcards
Describe the basis of segmental motility
Rhythmic contraction
Involves anterograde and retrograde movements (back and forth).
Describe the basis of propulsive (peristaltic) motility
Rhythmic contraction
Moves GI content along the tract from oesophagus to anus.
Describe the basis of reservoir (storage) motility
Tonic contraction that maintains continence of substances.
Series of sphincters.
What is the function of segmental motility and where does it mainly occur?
Functions to mix chyme with digestive juices to promote digestion and absorption.
Occurs mainly in the small and large intestine
What is the function of reservoir motility?
Controls the movement of chyme through the distinct organs of the digestive system.
What is the name of the pacemaker cells that leads to contraction of smooth muscle?
Myenteric Interstitial cells of cajal
What are the stimuli that sent pacemaker cells above their threshold potential (3)?
Stretching/distention of walls
Hormones
ACh via PNS
How are the ICC-MY coupled to smooth muscle?
Gap junctions
How does excitation lead to contraction via ICC-MY?
Stimuli causes cations to leak into the cell - threshold potential reached
SR stimulated to release calcium as intracellular [Ca2+] rises (CICR)
Smooth muscle contraction
What is the name for the gut nervous system?
Enteric nervous system
What are the two plexi of the enteric nervous system?
Myenteric (Aurbach’s) plexus
Submucosal (Meissener’s) plexus
Where does the myenteric plexus lie?
Located between the longitudinal and circular muscle layers
Where does the submucosal plexus lie?
Situated between the circular muscle and mucosa.
Explain the organisation of the myenteric plexus
The neurones of the myenteric plexus run an ascending chain and a descending chain. The neurones synapse onto both layers of the muscularis.
What neurotransmitters cause contraction of the smooth muscle and which cause relaxation?
Contraction: ACh and Substance P
Relaxation: VIP and NO
What is the primary motile movement of the oesophagus?
Peristalsis, to move bolus from mouth to stomach
Explain the mechanism of peristalsis in the oesophagus
Above the bolus: nerves release ACh and substance P onto the inner circular layer causing contraction while they release NO/VIP onto the outer longitudinal layer causing relaxation.
Below the bolus: nerve releases NO/VIP onto the inner circular layer (causing relaxation), nerves release ACh and substance P onto the outer longitudinal layer which causes contraction pulling the oesophagus open allowing the bolus to move down.
What are the key motile movements of the stomach?
Acts as a storage reservoir for chyme (storing and emptying)
Important in mixing and churning food
In the cephalic phase, what motile effects does the stomach have in response to vagal innervation?
Vagus innervates the fundus of the stomach, activates neurones that release VIP and NO, causing relaxation of the smooth muscle of the fundus
Stomach expands to facilitate greater food intake - distention.
In the gastric phase what is a stimulus for motility changes?
Distention of stomach
Increase in the concentration of partially digested proteins
What does stomach distention cause?
Short reflex initiated, stretch receptors signal to cause release of NO and VIP, further distention of the fundus.
What does an increase in the concentration of partially digested proteins in the stomach cause?
Activation of G cells in antrum
G cells release gastrin -fundal relaxation
What effect does gastrin have on the fundus vs antrum?
Fundus relaxes
Antrum contracts
Where on the stomach do ICC-MY lie?
On the body of the stomach
What happens to the intensity of contractile activity as you near the plyorus of stomach?
Increases
What is the enterogastric reflex?
Receptors signal via the vagus to the CNS
Efferents return to the stomach, cause constriction of the pyloric sphincter and inhibition of gastrin release.
What triggers the enterogastric reflex?
Acid levels in the duodenum at a pH of 3–4 or in the stomach at a pH of 1.5
Levels or partially digested protein and fat rise
What does the enterogastric reflex protect?
Prevents expulsion of chyme when a lot already expelled
What is the MMC?
“migrating motility complex” is a wave of activation spreading caudally (towards the anus) from the stomach
When does the MMC occur?
During long gaps between meals
In anticipation of a meal
Frequently during a period of fasting or hunger
What is the understood role of the MMC?
“housekeeping” function, preventing stagnation of intestinal contents, clear out debris and therefore helping to maintain a healthy gut flora
What hormone is thought to underlie the MMC?
Motilin
What are the key motile functions of the small intestine ?
Key digestion and absorption role
Regulates gastric secretion and emptying
Important in movement of foodstuffs along the GI tract.
What type of motility increases the digestion and absorption in the SI?
Segmentation
How does segmentation increase digestion and absorptive function of the small intestine?
Increases the time that the chyme is in contact with the wall of the intestine to enhance absorption.
Allows better mixing of chyme with juices from biliary tree and pancreas, to increase digestion
What is the mechanism behind segmentation in the SI?
Short stretch reflex
When the bolus stretches the walls, constriction at this site is generated.
This pushes the bolus in its centre in opposite directions.
What things stimulate and inhibit motility in the small intestine?
Stimulated by distention, PNS and irritants
The SNS inhibits.
What is the gastroilial reflex?
Vagus long reflex, sends afferents to the CNS which innervates the illiocecal junction
Causes opening/relaxation of the iliocecal sphincter, and contraction of the distal ileum allowing the fecal matter to enter the ascending colon.
What triggers the gastroilial reflex?
Causes opening/relaxation of the iliocecal sphincter allowing the fecal matter to enter the ascending colon.
What type of contractions are haustral contractions and where do they occur?
Segmental contractions
Ascending and transverse colon
What drives the majority of the motility of the gut (smooth muscle ribbons)?
Tendinae coli
What is the function of haustral contractions?
Upregulates the absorption of water and electrolytes as increases contact of fecal matter with the wall of the large intestine.
Where do the peristaltic movements occur mainly in the large intestine?
Transverse and descending colon
What is colon peristalsis triggered and inhibited by?
Promoted by PNS, stretch and irritants, inhibited by SNS.
What is the gastrocolic reflex?
Vagus acts on colon
Releases ACh onto the large intestine leading to contraction
What stimulates the gastrocolic reflex?
Stimulated by gastrin.
Stretch receptor reflex
What prompts the urge to defacate?
Movement of faeces into the rectum, distention of the rectal wall
What are the two anal sphincters, what are they made of, what are they innervated by?
Internal anal sphincter: Smooth muscle, controlled by ANS
External anal sphincter: Striated muscle, under voluntary control
What parasympathetic nerve and roots innervate the ano-rectum?
S1-S4, pelvic nerves
What does parasympathetic innervation of the ano-rectum cause?
Promotes defecation: stimulation of rectal motility and relaxation of internal sphincter
What sympathetic nerve (plexi) and roots innervate the ano-rectum?
L1-L3, passes through the mesenteric and pelvic plexuses.
What does sympathetic innervation of the ano-rectum cause?
Promotes continence: inhibition of rectal smooth muscle and contraction of internal sphincter.
What somatic nerve innervates the external anal sphincter?
Pudendal
What is the short reflex that leads to defacation?
Distention of rectal wall
Stimulation of stretch receptors
Activation of myenteric plexus in sigmoid colon and rectum
Increased local peristalsis
What is the long reflex that leads to defacation?
Distention of rectal wall and sigmoid colon
Stimulation of stretch receptors
Stimulate sensory fibres which go to CNS
Stimulation of parasympathetic motor neurons in sacral spinal cord
Increased peristalsis of large intestine
Relaxation of internal anal sphincter (smooth muscle)
Does defecation require voluntary contraction, how does this interact with other nerve reflexes?
Yes
Cerebral cortex to ventral horns to synapse onto the pudendal nerve.
Can rectal stretch receptors trigger a somatic reflex without voluntary input? If so when?
Yes
If rectal pressure exceeds 55mmHg
What drug can cause constipation?
Atropine
Opiates
Antiemetic drugs are often
Dopaminergic antagonists
What does the defacation reflex include?
Short and long reflex
Mass movement to rectum
Relaxation of internal anal sphincter
What can cause gut relaxation?
Enkephalins
Relaxation of gut smooth muscle is caused by activation of the following receptors on the smooth muscle membrane
Alpha adrenoreceptors
