GIT Motility Flashcards
What is the GIT?
a long muscular 5m tube
What does the GI system consist of?
- luminal GIT lined by mucous membranes
- associated accessory organs
What are the associated organs of the GI system?
- salivary glands
- hepatobiliary-pancreatic GI system
What does the hepatobiliary GI system consist of?
- liver
- gallbladder
- bile ducts
What are general functions of the GIT?
- supplying nutrients to the body for bodily functions
- maintaining homeostasis
- integration with other systems
- defence against exposure to the external environment
What is the digestive system responsible for?
- ingestion
- digestion
- absorption
- defecation
What does processing food involve?
- motility
- secretion
- membrane transport
What is electrical activity in GI smooth muscle controlled by?
interstitial cells of Cajal (pacemaker cells)
Where do ICCs form networks?
within the submucosal, intramuscular, and intermuscular layers throughout the GIT
What does the electricity of ICCs account for?
the self-excitable characteristics of the muscle
Which parts of the GIT do not have their own BER?
oesophagus and proximal stomach
What is BER?
the spontaneous depolarisation and repolarisation of ICCs
How does the ANS affect contractile force of the GIT?
- parasympathetic activity increases contractile force
- sympathetic activity decreases contractile force
What does the amplitude of slow waves determine?
the strength of muscle contraction
What is the amplitude of slow wave altered by?
release of neurotransmitters from enteric neurons
What do agents that cause relaxation in smooth muscle cause?
contraction in skeletal muscles
What is the oesophagus responsible for?
transporting food from the mouth to the stomach; it prevents retrograde movement of oesophageal or gastric contents
What is the oesophagus closed at both ends by?
- upper oesophageal sphincter – made of skeletal muscle controlled by swallowing centre
- lower oesophageal sphincter – made of cardiac muscle modulated by swallowing centre
Why must reflux be prevented?
gastric contents are damaging to the oesophageal epithelium
What does tonic contraction in the upper and lower oesophageal sphincters do?
keep their lumens partially or completely closed to prevent reflux
What does lower oesophageal sphincter tone do?
prevent or minimise gastro-oesophageal reflux or regurgitation
How does lower oesophageal sphincter tone increase and decrease respectively?
- increase by cholinergic agonist, alpha-adrenergic agonist, gastrin and substance P
- decrease by beta-adrenergic agonist, dopamine, CCK, nicotine, tea, coffee and cola
What is the interdigestive period?
period following digestion that begins after the GIT is cleared of food
What happens during the interdigestive period?
gastric motility clears undigested debris and sloughed epithelial cells
What does gastric motility do after a meal?
relax to accommodate ingested food with little change in the intra-gastric pressure and grinds and disperses the meal into fine particles
How are contents delivered to the duodenum?
at a rate that affords optimal mixing with pancreatic-biliary secretions and maximal contact with the brush border of enterocytes
What is the duodenum?
first part of the small intestine that connects to the stomach
What is reflex relaxation?
a response to gastric distention mediated by mechanoreceptors in the gastric wall
What is gastric adaptive relaxation mediated by?
the vago-vagal reflex arc (relaxes smooth muscle in the stomach wall) and volume
What is the adaptive relaxation reflex?
a reflex in which the fundus of the stomach dilates in response to small increases in intragastric pressure when food enters the stomach
What is receptive relaxation?
a reflex in which the gastric fundus dilates (stomach relaxes) when food passes down the pharynx and the oesophagus triggered by the cephalic phase of gastric secretion
What is feedback relaxation triggered by?
chyme in small intestine
What is chyme?
the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food
What does the pyloric sphincter do?
- regulate gastric emptying
- prevent duodenal-gastric reflux
What and where is the pyloric sphincter?
a muscular valve that opens to allow food to pass from the stomach to the duodenum
What is gastric emptying regulated by?
- inhibitory reflexes
- neural control
- hormonal control
- nature of food
What neural controls increase and decrease gastric emptying respectively?
- increases - anger and aggression
- decreases - pain, fear and depression
What hormones increase and decrease gastric emptying respectively?
- increases - gastrin
- decreases - CCK, secretin and GIP
What is the rate of gastric emptying slower for?
- solid meals
- low volume meals
- fat
- proteins
- chyme with high acidity or high osmotic pressure
What factors modify gastric liquid emptying?
- water
- feedback from small intestine
- caloric density (decreases)
- formaldehyde and amino acids (decreases)
What factors modify emptying of digestible solids?
- size of ingested food
- levels of fats, triglycerides and monosaccharides
What is the vomit centre in the medulla activated by?
- afferent fibres
- irritation due to injury
- increases in intracranial pressure
What does activation of the vomit centre in the medulla cause?
projectile vomiting not accompanied by nausea
What is the chemoreceptor trigger zone in brainstem activated by?
- afferent nerves originated from the GIT (chemoreceptors in the stomach/duodenum)
- circulating vomitic agents (e.g. apomorphine or copper sulphate)
What does activation of the chemoreceptor trigger zone in the brainstem cause?
vomiting accompanied by nausea
What are the 3 types of intestinal motility?
- segmentation – back and forth movement caused by a local reflex
- peristaltic reflex – propels bolus along the entire length of the intestine
- migrating motor complex – housekeeping motility that inhibits migration of colonic bacteria into the distal ileum
What is the distal ileum?
last part of the small intestine
What are the functions of intestinal motility?
- process and absorb nutrients
- organise motor activities – mixing chyme with digestive juices and bile to facilitate digestion and absorption
What is the transit time of intestinal motility?
2-4 hours from one end to the other (last part of one meal leaves ileum as next meal enters stomach)
What does gastrin do?
stimulate motility in the ileum and promote relaxation of the ileocecal sphincter
When is the intestino-intestinal reflex activated and what does it do?
in response to severe distension or injury to any portion of the small intestine to inhibit motility in the rest of the small intestine
When is the ileogastric reflex activated and what does it do?
in response to distension of the ileum to inhibit gastric motility
What is the gastroileal reflex stimulated by?
the presence of food in the stomach and gastric peristalsis
What does initiation of the gastroileal reflex cause?
peristalsis in the ileum and the opening of the ileocecal valve, which allows the emptying of the ileal contents into the large intestine or colon i.e. increases motility in the ileum
What is the colon?
the longest part of the large intestine
What is the transit time of chyme through the large intestine?
- 56 hours for small magnetic spheres to travel from mouth to anus
- 43 hours just to transverse the large intestine
What are motility functions of the large intestine?
- storage – most excreted within 72 hours
- non-propulsive segmentation – mixing of colonic contents, slow progression of contents distally
- mass movements associated with defecation i.e. mass peristalsis
What does the slow transit time and vigorous mixing movements of the large intestine aid in?
- microbial digestion of complex carbohydrates to volatile short chain fatty acids
- reabsorption of water and electrolytes
What is defecation?
strong mass movements several times a day, usually after a meal due to the gastro/duodeno-colic reflex
What decreases mass movements?
opioids
What triggers the defecation reflex?
distention of the rectal wall which causes relaxation of the IAS and contraction of the EAS
Why is conscious effort required to relax the EAS?
it is skeletal muscle under voluntary control
What is the pressure limit of voluntary control of the EAS?
55mmHg
What is incontinence caused by?
sensory malfunction and incompetence of IAS due to surgical or mechanical factors
What can a disorder of neuromuscular mechanisms of the EAS and pelvic floor be caused by?
- surgical or mechanical trauma
- childbirth
- ageing
What is diarrhoea caused by?
- increased bowel motility in response to inflammation
- failure to absorb nutrient molecules effectively
- excess secretion by small intestinal mucosa
What does the drug used to treat diarrhoea do?
slow down motility and allow the gut to reabsorb fluid and nutrients; the fluid is borrowed from the blood
What is constipation caused by?
- inadequate fibre in the diet
- lack of exercise
- slow passage through large intestine leading to further compaction of faeces
- repeated voluntary inhibition of defecation reflex
How does lactose counter constipation effects?
it increases osmotic pressure and expands the lumen to allow increased motility
