GI tract motility Flashcards
What is the primary purpose of incisor teeth?
Cutting
What is the primary purpose of the molar teeth?
Grinding
What is the majority of chewing muscle innervated by?
5th cranial nerve
What part of the brain controls mastication?
Brain stem nuclei
- Reticular areas for rhythmical chewing
- Additional involvement from hypothalamus/amygdala/cerebral cortex
What acts as a lubricant in the mouth?
Mucin (glycoprotein)
What digestive enzymes is food mixed with?
- Carbohydrate with salivary amylase for carbohydrate digestion
- Fat with lipases for lipid digestion
Describe the 3 stages of swallowing/deglutition
- Has a voluntary stage: initiates the swallowing process;
- A pharyngeal stag: is involuntary passage of food through the pharynx into the oesophagus
- An oesophageal stage; is involuntary transport of food from the pharynx to the stomach
Desribe the 4 swallowing steps
- When ready for swallowing, food is voluntarily moved posteriorly into the pharynx
- Trachea is closed
- Oesophagus is opened
- A fast peristaltic wave initiated by the NS of the pharynx forces the bolus of food into the upper oesophagus
- All in < 2 seconds
How is the pharyngeal stage initiated?
Almost always voluntary movement of food into the back of the mouth
- Detected in ring area around the pharyngeal opening, excitation of involuntary pharyngeal sensory receptors to elicit the swallowing reflex
What nerves stimulate motor impulses from the swallowing centre to the pharynx and upper oesophagus?
5th, 9th, 10th, 12th cranial nerves (+ few superior cervical nerves)
How long is the pharyngeal stage?
< 6 seconds
- Interrupts respiration for fraction of usual respiratory cycle (swallowing centre specifically inhibits respiratory centre of medulla)
What are the 2 types of peristalsis?
- Primary
Continuation of peristaltic wave that begins in the pharynx and spreads into the oesophagus during the pharyngeal stage of swallowing - Secondary
- result from distention of the oesophagus by retained food
- Waves continue until all food has emptied into the stomach
- Are initiated partly by:
- Intrincic neural circuits in myenteric nervous system
- By reflexes that begin in pharynx
What is the upper oesophagus innervated by?
- Skeletal nerve impulses from the:
Glossopharyngeal, vagus nerve
What is the lower 2/3 of the oesophagus innervated by?
VAgus nerves that act through connections with the oesophageal myenteric nervous sytem
What wave precedes peristalsis and what is it transmitted via?
- Relaxation wave
Transmitted via myenteric inhibitory neurons
Relaxes gastroesophageal sphincter and entire stomach
What part of the oesophageal circular muscle acts as a sphincter?
Last 3cm
What are the motor functions of the stomach?
- Storage of large quantities of food
- Mixing foos with gastric secretions until it forms a semifluid mixture (chyme)
- Slow emptying of chyme from stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine
What does the food entering the stomach form in the orad portion of the stomach?
- Concentric circles
- Newest food lying closest to oesophageal opening
- Oldest food lying nearest to the outer wall of stomach
What is the ‘vagovagal’ reflex?
Food stretches stomach -> Stomach -> brain stem -> back to stomach ->
- Decreased tone in stomach body muscular wall
- Wall bulges progressively outward
- Greater quantities food accomodated (up to a limit ~0.8-1.5 litres)
What are mixing waves?
- Weak peristaltic constrictor waves
- Begin in the mid/upper portions of the stomach wall
- Move towards antrum every ~15-20s
- Initiated by gut wall basic electrical rhythm
Where are the mixing waves strongest?
- More intense in antrum
- Force the antral contents under higher and higher pressure towards pylorus
What triggers emptying of the stomach?
Stretching of stomach walls
What are the enterogastric inhibitory reflexes?
- Distention of duodenum
- Presence of any irritation of the duodenal mucosa
- Acidity/osmolarity of duodenal chyme
- Presence of certain digestion products in chyme eg from proteins/fats
What is the effect of increased stomach volume on gasric emptying?
Increased gastric emptying
What are the hormonal regulating factors ?
- Stimulus mainly fats
- CCK most potent hormone
What does chyme induce on the GI wall?
Induces extension of GI wall -> stretch -> localised concenteric contractions (spaced along intestine, short lasting) -> segmentation
What is the frequency of segmentation determined by?
Frequency of slow waves
When do segmental contractions become weak?
- When excitatory activity of the enteric nervous system is blocked by atropine
What is the main source of background excitation that cause segmental contractions?
The myenteric nerve plexus
What is the highest slow wave frequency?
~ 12 per min
What is the speed of peristalsis in the small intestine?
0.5-2cm/s
Becomes slower as it moves down SI
How long does it take usually for chyme to pass from the pyloris to ileocecal valve?
3-5 hrs
What is the net movement of chyme in the SI?
1cm/min
Why is peristalsis increased after a meal?
- Entry of chyme into duodenum -> stretch of duodenum wall
- Gastroenteric reflex (from stomach distension via myenteric plexxus)
- Gastrin, CCK, insulin, motolin, serotonin
Secretin, glucagon decrease motility
What can cause peristaltic rush?
- Mucosal irritation with infectious diarrhea
- Initiated by nervous reflexes
Autonomic/brainstem + intrinsic myenteric plexus reflexes
How is the ileocecal valve closed?
When excess pressure builds up in cecu and tries to push cecal contents backwards against the valve lips
Immediately after a meal what happens to the gastroileal reflex?
Intensifies peristalsis in ileum -> emptying of ileal contents into cecum
What does pressure and chemical irritation do to the ileocecal sphincter?
Relaxes it and excites peristalsis
How much chyme enters the caecum after absorption each day?
1.5 - 2 L each day
What do reflexes from the cecum control?
- Degree of contraction of ileocecal sphincter
- Intensity of peristalsis in the terminal ileum
What does a distended cecum lead to?
Contraction of ileocecal sphincter intensified, ileal peristalsis is inhibited
- Greatly delay emptying of additional chyme into cecum from ileum
What effect can irrant(s) in the cecum have to emptying?
- Delays emptying
- Inflammed 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 via?
Myenteric plexus and extrinsic autonomic nerves
What si the intrinsic defecation reflex mediated by?
Rectal enteric NS
What are the 2 defecation reflexes?
- Intrinsic Mediated by rectal enteric NS - Parasympathetic defecation reflex Fortifies intrinsic reflex Involves sacral segments of spinal cord
What reflexes facilitate mass movements after meals?
- Gastrocolic and duodenocolic reflexes
- Result from distention of the stomach and duodenum
What happens to the gastrocolic and duodenocolic reflexes when extrinsic autonomic nerves to the colon are removed?
- They occur not at all or hardly at all
- Almost certainly are transmitted via autonomic nervous system
Waht can irritation of the colon initiate?
Mass movements
What does a distended caecum result in?
Contractions of ileocecal sphincter becoming intensified, ileal peristalsis inhibited
What are the main functions of the colon?
- To absorb water and electrolytes from chyme
- Store fecal matter unti lit is ready to be expelled
What are Haustrations?
- Mixing movements in the colon (bag-like buldges)
- Similar to smal intestine segmenttion contractions can restrict colon to occlusion
- Longitudinal muscle strips can also contract
How much water is lost in the feces?
80 - 200 ml
What are propulsive ‘mass movements’ 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 do the series of mass movements persist for?
~10-30 min then caese but return
When is the desire for defecation felt?
When faeces enters into the rectum
