lecture 24 - GI motility Flashcards
What are the factors that receptors in the wall of the GI tract respond to?
Stretch (via food in the lumen), change in composition - pH, osmolarity, amino acids, sugars, fats
What are the effectors of GI regulation?
Smooth muscle (contraction), glands (secretion)
What are the 2 types of GI regulation?
Hormonal and nervous (ENS and CNS)
What is the role of the parasympathetic nervous system in GI function?
Stimulates motility and secretion
What is the role of the sympathetic nervous system in GI function?
inhibits motility and secretion
How are the CNS and ENS connected?
The CNS modulates activity of enteric nervous system
What nervous system coordinates GI fucntion over long distances?
Central Nervous System
What component of the ENS regulates secretion?
Submucosal plexus
What component of the ENS regulates motility?
Myenteric plexus
What are the functional differences between the submucosal plexus and myenteric plexus?
Submucosal plexus regulates secretion, while the myenteric plexus regulates motility
What are the 2 key local motility reflexes of the GI system?
Peristalsis and segmentation
What are the 2 key functions of the CNS in GI function?
Integration of responses to external stimuli (senses, emotions, etc.) and response to internal stimuli, such as GI lumen contents via a long reflex path
What neurotransmitter is involved in sympathetic CNS control of the GI tract?
Noradrenaline
What neurotransmitter is involved in parasympathetic CNS control of the GI tract?
acetylcholine
What are the key types receptors that feedback to the ENS?
Stretch/mechano-, osmo- and chemo- receptors
What is the difference between reflex pathways in the ENS and CNS?
Short, local pathways in ENS, long pathways to CNS
What is the largest endocrine organ in the body?
The GI tract
What are the 2 types of hormonal regulation in the GI system?
Endocrine and paracrine
What is endocrine regulation?
Regulation by hormones that are secreted into the bloodstream for distant effect
What is paracrine regulation?
Secretion of hormones to effector cells that are adjacent to the secretory cells
What are the 4 critical hormones of GI regulation?
Gastrin, Gastric Inhibitory Peptide (GIP), Secretin, Cholecystokinin (CCK)
What does the GIP hormone stand for?
Gastric Inhibitory Peptide
What does the CCK hormone stand for?
Cholecystokinin
What are the 2 types of contractions in GI motility?
Tonic and phasic contractions
What are tonic GI contractions?
Sustained contractions for minutes to hours
What feature of the GI system often employ tonic contractions?
Sphincters - they are sustained shut/open for a long time
What are GI phasic contractions?
Waves of contraction and relaxation over seconds
What type of contraction is in peristalsis?
Phasic contractions
What is a motility pattern in the GI tract?
Any pattern of contraction or relaxation of GI tract smooth muscle
What is an example of relaxation being a GI motility pattern?
Storage in the stomach
What are movement/propulsion in terms of motility patterns?
Motility patterns that specifically move or propel GI contents along the tract.
What does it mean when the Gi tract is ‘spontaneously active’?
It contracts without external input
What is the frequency of contraction a property of?
The region of the tract
What is the frequency of contractions in the stomach?
3 per minute
What is the frequency of contractions in the duodenum?
12 per minute
What is the frequency of contractions in the ileum?
9 per minute
What is the strength of GI contractions regulated by?
Nervous and hormonal input
What is the motility pattern during the fasting state?
Migrating Motor Complex (MMC)
When is the Migrating Motor Complex employed?
During the fasting state
When does the Migrating Motor Complex begin?
4 hours after a meal
How often does the migrating motor complex repeat?
Every 2 hours until food is eaten again
What is the purpose of smooth muscle relaxation during the feeding state?
Allows the volume of the tract to increase without a change in pressure
Where does storage occur during the feeding state?
Stomach and colon
What is the motility pattern that occurs for propulsion/movement during feeding?
Peristalsis
What are the key steps in peristalsis?
Bolus of food arrives, circular muscles contract behind it, longitudinal muscles ahead of bolus contract, circular muscle contracts to force bolus forward
What muscles contract first in peristalsis?
Inner Circular muscles behind the bolus
What muscles contract second in peristalsis?
Longitudinal muscles ahead of the bolus
What is the segmentation motility pattern?
Intermittent ‘pinching’ of tract to mix chyme back and forth too expose it to absorbant surfaces
Is chewing conscious or voluntary?
Both - has voluntary skeletal muscle control but also reflex control of strength, frequency and rhythm
What is the purpose of chewing?
Allows ingestion by reducing the size of food. Mixes food with saliva to start digestion and allow for taste.
Is swallowing reflexive of voluntary?
Initiated at will, but proceeds reflexively.
What is the purpose of swallowing?
The rapid transfer of material from mouth to stomach
What are the 3 phases of chewing/swallowing?
Buccal, pharyngeal, oesophageal
What are the 4 functions of gastric/stomach motility?
Storage, mechanical digestion, mixing, controlled delivery to the duodenum
What part of the stomach is involved in storage?
Fundus and body - at the top of the stomach
What part of the stomach is involved in mechanical digestion and mixing?
Antrum - where there is the greatest amount of muscle
What part of the stomach is involved in the motility pattern associated with controlled delivery to the duodenum?
Pyloric sphincter
In the migrating motor complex, what is the ratio of uncoordinated and coordinated activity?
50 minutes of uncoordinated activity followed by 10 minutes of coordinated activity
What is the primary purpose of the migrating motor complex during the fasting state?
House keeping
What are the 3 functions of the migrating motor complex/fasting gastric motility?
Removes residual secretions, removes undigested material, promotes epithelial cell turnover
What is the epithelial cell turnover?
Every 5 days all epithelial cells in the stomach are replaced
What triggers a reflex for gastric relaxation?
Swallowing
What feedback loop causes gastric accomodation?
Stretch reflexes in the stomach tell the brain that the stomach has expanded, triggering a signal for the stomach to expand even further to accomodate.
What is the primary nerve involved in nervous regulation of gastric storage?
The vagus nerve
What characteristic of the stomach is unchanged, despite the increased volume during storage?
Pressure has minimal change
Where is peristalsis initiated in the stomach?
The greater curvature, then spreads to the antrum
How do peristalsis contractions in the stomach change over time after a meal?
first 60 minutes after a meal the contractions are gentle, and then get more intense until approx. 300 minutes
What motility pattern in used in the stomach to promote mixing and mechanical breakdown?
Retropulsion.
What is gastric retropulsion?
Combination of peristalsis and closure of the pyloric sphincter
What is the purpose of gastric mixing?
Combines chyme with acid and pepsin to promote digestion
What is the rate of gastric emptying dependent on?
The digestive capacity of the intestine
What motility patterns aid in gastric emptying?
Combination of peristalsis and periodic opening/relaxation of the pyloric sphincter
What feedback regulates gastric emptying?
Feedback from the duodenum
What 2 factors affect gastric emptying?
Size/volume of the meal, meal composition
What food component slows gastric emptying due to being difficult to digest in the stomach?
Fats
Why are fats difficult to digest in te stomach?
Fats require bile salts for emulsification, which are not produced in the stomach
What are the characteristics of the chyme that is emptied from the stomach into the duodenum?
Fatty, hypertonic, acidic
What is the nervous negative feedback loop that slows the release of chyme into the duodenum?
When chyme enters the duodenum is triggers chemoreceptors and stretch receptors that trigger the enterogastric reflex, which stimulates both enteric neurons (short reflex) and CNS centres (long reflex) to decrease the contractile force of the stomach and slow stomach emptying.
What is the endocrine negative feedback loop that slows the release of chyme into the duodenum?
Presence of chyme in duodenum causes duodenal enteroendocrine cells to secrete hormones/enterogastrones to decrease gastric emptying and the contractile force of the stomach
What are enterogastrones?
Hormones released by the duodenum in response to fatty chyme, and act to inhibit gastric motility
What are the 3 key functions of small intestine motility?
Mix chyme with pancreatic, biliary and intestinal secretions, control movement/propulsion, expose products of digestion to absorptive surfaces
What is the main motility pattern in the small intestine between meals?
Migrating motor complex
What is the purpose of the migrating motor complex in the small intestine?
Clearing unwanted secretions
What is the main motility pattern in the small intestine after a meal?
Segmentation, with some peristalsis
What is the function of segmentation in the small intestine after a meal?
Mixing, exposure to absorptive surfaces
What are the 2 key motility patterns of the large intestine?
Segmentation and mass movement (propulsion)
What is the function of the segmentation motility pattern in the large intestine?
Exposure of faeces to absorptive surfaces for absorption of fluid and salt
How often does mass movement from the large intestine occur?
1 to 2 times a day
What type of motility pattern is mass movement?
Peristaltic wave
What is the fucntion of mass movement in the large intestine?
Drives faeces into the rectum and initiates defecation.
