Control of smooth muscle in the GI tract Flashcards
What are the types of muscle in the GIT
Smooth (involuntary) muscle in most regions
Skeletal (voluntary) muscle in the pharynx top third of the oesophagus, external anal sphincter
What are the functions of the GIT
digestion
absorption
secretion
motility
What are the two types of smooth muscle?
Phasic - rapid relaxation and contraction of the muscle
Tonic - muscle remains contracted or relaxed for a long duration of time
what are the interstitial cells of CAJAL (ICC)
Pacemaker cells which have a spontaneous oscillating membrane potential. These cells are electrically coupled to the smooth muscle cells through gap junctions.
What can cause stimulation of electrical activity of smooth muscles in the GIT? (depolarisation)
stretch
acetylcholine
parasympathetics
Initiates smooth muscle contraction
What can cause hyperpolarisation of smooth muscles in the GIT?
Noradrenaline
sympathetics
Initiates smooth muscle relaxation
What is the role of Ca2+ ions in smooth muscle contraction?
Ca ions bind to calmodulin which activates myosin light chain kinase enzyme when phosphorylates myosin, allowing it to interact with actin and therefore mediates contraction.
How does relaxation occur in smooth muscle?
MLCP phosphatase dephosphorylates myosin so it no longer interacts with actin molecules and hence contraction is halted.
How is Ca controlled to control contraction?
Ca ions are exchanged by a Na-Ca exchanger of Ca pump, but this eventually would deplete the cell of Ca ions.
Instead Ca ions re-uptake into the sarcoplasmic reticulum
How can relaxation of smooth muscle be maintained?
MLCP can itself be phosphorylated meaning that it cannot interact with calmodulin and as such no muscle contraction will be cause
What does activation of the myenteric plexus do?
Increases tonic contraction
increases intensity of rhythmic contractions
increases the rate of rhythmic contractions
increases velocity of conduction
What does activation of the submucosal plexus do?
Involved primarily with the absorptive function of the GIT
What are the different types of movements in the GIT?
A.Propulsive movements: PERISTALSIS
B.Non-propulsive movements: SEGMENTATION
C.Interdigestive: MIGRATING MOTOR COMPLEX (MMC)
What sensory receptors are involved in the GIT?
Enteric sensory neurones
What are the roles of the para/symp NS in motility of the GIT?
Sympathetic inhibits motility
Parasympathetic initiates motility
What type of reflexes are involved in the gut wall?
Ileogastric
gastrocolic
enterogastric
What is the Gastro colic reflex?
sensory distension initiating a reflex of contractions in the large intestine allowing more room for new food in the colon.
What is the intestine intestinal reflex?
(inhibitory reflex)
prevents excess chyme being secreted into the ileum when distention hasn’t been fully completed
What is Hirschsprung disease?
Congenital lack of neuronal ganglionic cells in the ENS leading to a lack of motility and therefore can cause MEGAcolon
What is Chagas disease?
infectious disease of a parasitic nature, resulting in the significant reduction in the number of ganglionic cells in the ENS
What is Achalasia?
a condition in which the muscles of the lower part of the oesophagus fail to relax, preventing food from passing into the stomach caused by a dramatic reduction in the number of neuronal cells in the lower oesophageal segment.
What are some gastric functions of the stomach?
gastric emptying/release
Food storage
gastric motility and mixing
How is the stomach adapted for storage function?
Can be highly folded and upon filling the folds flatten out allowing an increase in volume.
Once the stomach folds extend fully (via the vago-vagal reflex) then the intraluminal pressure increases and it becomes harder to eat.
what are the phases of gastric motility in the stomach?
Propulsion - rapid flow of liquids and suspended small particles are pushed down towards the pylorus.
Grinding - the emptying of liquids and small particles but larger particles are retained in the bolus and are subject to grinding.
Retropulsion - this is where large particles are cleared in the terminal antrum
What is the function of the pylorus?
allows carful regulated emptying of gastric contents
prevents regurgitation of duodenal contents
What are the phases of gastric emptying?
– Cephalic - approach of food or food in the mouth
– Gastric - food in the stomach
– Intestinal - food in duodenum, ileum or colon
What occurs during the cephalic phase of gastric emptying?
An inhibitory phase - Inhibitory nerve fibres in the vagus nerve
Results in relaxation of stomach enabling it to store large volumes
What occurs during the gastric phase of gastric emptying?
Is an excitatory phase in which the stomach empties at rate proportional to the volume in it.
What occurs during the intestinal phase of gastric emptying?
It is (mainly) an inhibitory phase, through which the duodenum adapts the “work-load” as a function of the state of the digestive process.
what is the predominant excitatory neurotransmitter for smooth muscle control?
ACh
What is the predominant inhibitory neurotransmitter for smooth muscle control?
VIP and NO
Which fibre inhibits and stimulates motility by controlling the resting membrane potential?
Stimulation of membrane potential by Sympathetic fibres inhibit motility
Stimulation of membrane potential by parasympathetic fibres increases motility.
What are the symptoms of Hirschsprung disease?
Vomiting
constipation
distension of abdomen
Intestinal obstruction
What patient evaluation takes place to diagnose Hirschsprung’s disease?
plain abdominal radiography
Rectal manometry
Biopsy
What is the treatment for Hirschsprung’s Disease?
Removal of Aganglionic segment and subsequent anastomosis
What is the clinical presentation of Achalasia?
Solid Dysphagia (difficulty swallowing) Chest pain Regurgitation Pyrosis Weight loss Nocturnal cough and recurrent aspiration.
What is the diagnosis of Achalasia?
Plain film
Barium esophagogram
Endoscopy
Oesophagal manometry
What is the treatment for Achalasia?
Nitrates and Calcium Channel Blockers
Botulinum toxic preventing ACh release at NMJ
Pneumatic dilation
Surgical myotomy.
What causes the emptying of the stomach during the gastric phase of
Myogenic reflex: stretching smooth muscle causes reflex contraction
Activation of pressure receptors which send impulses in local nerve plexi and in the vagus nerve
Gastrin released in response to peptides etc
What controls the intestinal phase of gastric emptying?
This phase is mainly controlled through hormonal and paracrine mechanisms, activated by duodenal chemoreception.
There are also some motor reflexes such as ileogastric reflex delaying gastric emptying
What are the intestinal mechanisms controlled by hormonal and paracrine mechanisms?
Low pH - activates secretion of Secretin
High fats/lipids - increase secretion of CCK
High levels of amino acids - increase secretion of Gastrin
High levels of carbohydrates - increased secretion of GIP
What is the significance of The intestinal phase of gastric emptying being inhibitory?
delays gastric emptying increasing the time taken for the GI tract to empty and thus allow more time for digestion.
