GI Tract Motility Flashcards
What is required for optimum functioning of the GI Tract?
- timing of mixing
- appropraite mixing
What is the function of incisor teeth?
cutting
What is the function of molar teeth?
grinding
What nerve is the majority of chewing innervated by?
5th cranial nerve
trigeminal
How does the brain stem nuclei control chewing?
- reticular areas for rhythmical chewing
- additional involvement from hypothalamus/amygdala and cerebral cortex
What are the main functions of chewing
- mixes food with saliva
- reduces sizes of food particles to facilitate swallowing
- mixes food components with digestive enzymes
- amylase and lipases
What acts as the main lubricant in the mouth?
mucin (glycoprotein) acts as a lubricant
What is required of the pharynx for swallowing?
requires pharynx to be a tract for propulsion of food; ensure food goes into the oesophagus not the lungs (respiration should not be compromised)
What are the 3 stages of swallowing?
- voluntary stage: initiates swallowing process
- pharyngeal stage: passage of food through pharynx into the oesophagus
- oesophgeal stage: involuntary transport of food from the pharynx to the stomach
What are the 4 main steps in swallowing?
- When ready for swallowing, food is voluntarily moved posteriorly into the pharynx
Next steps are almost automatic:
- The trachea is closed
- oesophagus is opened
- Fast-peristaltic wave intiiated by the nervous system of the pharnyx forces the bolus of food into the upper oesophagus
All in < 2 seconds
What intiates the pharyngeal stage of swallowing?
- primarily a reflex act
- voluntary movement of food to the back of the mouth
What elicits the swallowing reflex?
involuntary pharyngeal sensory receptors
What occurs following the activation of the involuntary pharyngeal sensory receptors in the swallowing reflex?
- next stages are initiated by neuronal areas of the reticular substance of the medulla and lower portion of the pons (deglutition centre)
- motor impulses from the swallowing center to the pharynx and upper oesophagus that cause swallowing are transmitted successively by the 5th, 9th, 10th, 12th cranial nerves (+ few superior cervical nerves)
What is the time frame of the pharyngeal stage of swallowing? And what is the implication of this?
occurs in < 6 seconds
interrupts respiration for a fraction on the usual respiratory cycle
What is the action of the swallowing centre on the respiration centre during eating?
Swallowing center specifically inhibits the respiratory center of the medulla during this time ie halts respiration
What is 1º peristalsis
- Simple continuation of peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal stage of swallowing.
- Wave passes from pharynx to stomach in ~ 8-10s.
What is 2º peristalsis?
- Result from distention of the oesophagus by retained food.
- Waves continue until all food has emptied into the stomach.
- Are initiated partly:
- by intrinsic neural circuits in myenteric nervous system
- by reflexes that begin in pharynx
What controls the peristaltic waves in upper/striated regions?
(upper 1/3)
controlled by skeletal nerve impulses from glossopharyngeal and vagus nerves
What controls the peristaltic waves in smooth muscle regions?
(lower 2/3)
controlled by vagus nerve that acts through connections with the oesophgeal myenteric nervous system
What does the relaxation wave precede?
peristalsis
What intiates the relaxation wave?
transmitted by myenteric inhibitory neurons
What occurs as a result of the relaxation wave?
Entire stomach becomes relaxed in preparation for food arrival
What functions as the oesophgeal sphincter?
last 3cm of oesophgeal circular muscle (gastroeophgeal sphincter)
What is the normal state of the oesophgeal sphincter?
tonically constricted
What is the functipn of ‘receptive relaxation’?
allows easy propulsion of food into the stomach
What are the 3 motor functions of the stomach?
- Storage of large quantities of food until it can be processed in the stomach, duodenum and lower intestinal tract;
- Mixing of food with gastric secretions until it forms a semifluid mixture (ie chyme);
- (Slow) emptying of chyme from stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.
How does food entering the stomach behave?
- forms concentric circles in orad portion of the stomach
- newest food lying cloestes to oesophgeal opening
- oldest food lying nearest the outer wall of the stomach
What is the chain reaction following food stretching the stomach
Food stretches stomach–> “vagovagal
refex” (stomach->brain stem-> back to the stomach)èêtone in stomach body muscular wall –> wall bulges progressively outward –> greater quantities food accommodated (up to a limit: ~0.8-1.5 litres.
Where do mixing waves begin and how often do they occur?
begin in the mid/upper portions of the stomach walls, move toward antrum every ~15-20s
Initiated by gut basic electrical rhythm
What happens to mixing waves are you move through the stomach?
body –> antrum
waves become more intense and force the antral contents under higher and higher pressure towards the pylorus
Explain how the peristaltic waves are responsible for mixing stomach contents
antral contents are squeezed upstream through the peristaltic ring, towards body of stomach, not through pylorus
retropulsion = Important mixing mechanism
What is emptying of the stomach promoted by?
promoted by intense persitaltic contractions in stomach antrum
What is emptying of the stomach opposed by?
pylorus
degree of constriction is under the influence of the nervous system and hormonal signals from stomach and duodenum
Explain the gastric and duodenal regulating factors of the stomach
- increased stomach volume = increased gastric emptying
- Enterogastric inhibitory reflexes:
- Distention of duodenum
- Presence of any irritation of the duodenal mucosa
- Acidity/osmolality of duodenal chyme
- Presence of certain digestion products in chyme eg from proteins/fats
What are the hormonal regulating factors that contorl empyting the stomach?
– Stimulus mainly fats.
– CCK most potent hormone.
What are the 2 main movements of the small intestine?
- Mixing contractions – segmentation contractions
- Propulsive movements - peristalsis
Describe segmentatation contractions of the small intestine
chyme induced extension of GI wall → stretch → localised concentric contacted (spaced along intestine, short lasting) → segmentation
What is the frequency of segmentation determined by?
frequency of slow waves
When do segmentation contractions become weak?
when the excitatory activity of the enteric nervous system is blocked by atropine
Where do propulsive movements occurs?
any part of the small intestine
What is the rate of peristalsis?
move towards anus at 0.5-2cm/s
Faster in proximal SI; slower in terminal SI
How quickly do propulsive movements die out?
Weak and die out after 3-5cm. Rarely >10cm
Describe the net movement of chyme
SLOW (1cm/min)
Chyme pylorus to ileocecal valve = 3-5hr
What occurs to peristalsis activity after we eat a meal?
- Activity increased after a meal:
- Entry of chyme into duodemum → stretch of duodenum wall
- Gastroenteric reflex (from stomach distension via myenteric plexus)
- Gastrin, CCK, insulin, motolin, serotonin
- (Secretin, glucagon decrease motility)
What can cause a peristaltic rush? - powerful, rapid peristalsis
- mucosal irritation by infectious diarrhoea
- initiated by nervous reflexes
- autonomic/brain stem + intrinsic myenteric plexus reflexes
Describe the location and action of the ileocecal valve
Valve protrudes into lumen of cecum, is forcefully closed when excess pressure builds up in cecum and tries to push cecal contents backward against the valve lips.
What pressure can the ileocecal valve resist?
Usually can resist reverse pressure of 50-60cm water.
What is the normal contraction of the ileocecal sphincter
remains mildly constricted and slows emptying of ileal contents into the cecum.
What occurs at the ileocecal sphincter immediately after a meal?
Immediately after a meal, gastroileal reflex intensifies peristalsis in ileum ⇒ emptying of ileal contents into cecum.
What do reflexes from the caecum control?
– degree of contraction of ileocecal sphincter
– the intensity of peristalsis in the terminal ileum
What is the result of a distended caecum?
contraction of ileocecal sphincter intensified, ileal peristalsis is inhibited
=> greatly delay emptying of additional chyme into cecum from ileum.
What delays emptying of the caecum?
Irritant(s) in the cecum
eg inflamed appendix can cause intense spasm of the ileocecal sphincter and partial paralysis of the ileum ⇒ block emptying of ileum into cecum.
What are Cecum-to-ileocecal sphincter and cecum-ileum reflexes mediated by?
myenteric plexus and extrinsic autonomic nerves.
What are the mixing movements in the colon?
“Haustrations” ie bag-like bulges
Similar to small intestine segmentation
+
Longitudinal muscle strips contraction contract
How are the propulsive movements of the colon described?
mass movements
Give the 6 steps of modified peristalsis in the colon
- First, a constrictive ring occurs (usually in transverse colon) in response to distension.
- The ~20+cm of colon distal to constrictive ring lose haustrations and contract as a unit.
- Fecal material moves en masse down the colon.
- Contraction develops progressively more force for ~30s
- Relaxation occurs during next 2-3 mins.
- Another mass movement occurs, perhaps farther along the colon.
How long does a series of mass movements persist for?
~10-30mins
What is the function of gastrocolic and duodencolonic reflexes? and when do they occur?
Facilitate mass movements after meals.
- Result from distention of the stomach and duodenum.
- Occur either not at all or hardly at all when extrinsic autonomic nerves to the colon have been removed;
– . ̇. almost certainly are transmitted via autonomic nervous system.
• Irritation in the colon can also initiate intense mass movements.
– eg with ulcerated condition of colon mucosa (ulcerative colitis), mass movements that persist almost all the time can occur.
What is the intrinsic defecation reflex?
Mediated by rectal enteric NS
What is the action of the PSNS defecation reflex>
– “Fortifies” intrinsic reflex
– Involves sacral segments of spinal cord.
What is the normal content of faces in the rectum?
empty of faces
What prevents the loss of feacal matter?
constricted internal-external sphincter
