GI Motility Flashcards
Which parts of the GI tract have voluntary skeletal muscle rather than involuntary smooth muscle?
Proximal third of the stomach, upper oesophageal sphincter and external anal sphincter
Describe what is meant by the slow wave potential of the GI tract
These are not action potentials but oscillating depolarisation and repolarisation of the membrane potential of smooth muscle cells. The depolarisation stage brings the smooth muscle closer to the threshold and if an action potential occurs at the top of these slow waves the cyclical contraction of the GI tract will occur
What is the intrinsic rate of the slow waves in the stomach?
3 waves per minute
What is the intrinsic rate of the slow waves in the duodenum?
12 waves per minute
Which cells are considered the pacemaker cells of the GI tract due to their role in controlling the slow wave mechanism?
Interstitial cells of Cajal
The intrinsic rate of the contractions of the GI tract doesn’t change but the strength of contraction can be increased or decreased. True or false?
True
Describe the mechanism of swallowing?
The upper oesophageal sphincter open which is mediated by the swallowing reflex. The sphincter closes when the food bolus enters the oesophagus. The swallowing reflex results in a peristaltic wave which pushes the bolus towards the stomach. The vagus nerve mediates opening of the lower oesophageal sphincter and at the same time to topmost part of the stomach relaxes to allow the bolus to flow into the stomach
What factors will increase the frequency and force of contractions in the stomach?
Parasympathetic innervation
Motillin
Gastrin
What factors will decrease the frequency and force of contractions in the stomach?
Sympathetic stimulation
Secretin
GIP
What is the migrating myoelectric complex?
This is a contraction which sweeps the length of the stoma h every 90mins in the interprandial state in order to clear the stomach of any residue. This is mediated by motilin
Where is motilin synthesised?
M cells in the small bowel
What is the affect of erthromycin on motilin?
It is a motilin antagonist
What is the emptying rate from the stomach for inert liquids?
20minutes
How long can it take solids to empty from the stomach?
3-4 hours
Fat and hydrogen ions in the duodenal lumen will inhibit gastric emptying. How is this achieved?
Fat stimulates cholecystokinin which increases contraction of the pyloric sphincter
The effect if hydrogen ions is mediated by the enteric nervous system
If the small intestine transports solids and liquids at the same rate why are liquids the first to reach the caecum?
Because liquids are released earlier from the stomach and into the duodenum than solids
Describe the segmentation contractions of the small intestine
A section of the small intestine contraction and splits the chyme sending some forward and some backwards. It then relaxes which allows the bolus of chyme to merge back together. This action severs to mix the chyme
What is the name of the contractions of the large intestine which move food towards the caecum?
Mass movements
Explain the control of continence and defecation.
As the rectum fills with farces the smooth muscle wall of the rectum and internal anal sphincter relaxes in the recrosphincteric reflex. However, defecation does not occur because the external anal sphincter is still tonically contracted. When this sphincter is voluntarily relaxed, the smooth muscle of the rectum contracts to create pressure that forces faeces through the anal canal
Describe what a normal high resolution mannometry trace of a person swallowing would look like
Here areas of red shiw high pressure and blue shows low pressure. First, the upper oesophageal sphincter is relaxed (as seen by a break in the solid green line to blue which is the to ically contracted sphincter), a wave kf red and yellow peristaltic wave follows this and the bottom of this wave, contraction of the lower sphincter ceases to allow food to enter the stomach
Describe how achalasia leads to difficulties swallowing
Here there is no peristaltic contraction due to nerve damage and the lower oesophageal sphincter fails to relax properly. This makes it difficult to eat and patients often have to take large amounts of water to eat a meal and will regurgitate
How can achalsia be treated?
A balloon can be used to decrease the pressure in the lower oesophageal sphincter
Laprascopic surgery can also be used ti lower oressure in the lower oesophageal sphincter
How does scleroderma lead to severe oesophagitis and excessive reflux?
Scleroderma is a condition which effects connective tissues and this results in very weak peristalsis and a very weak lower oesophageal sphincter which results in excessive reflux and oesophagitis
What is nut racker oesophagus?
Hypertensive peristalsis of the oesophagus which has no evident therapies and is associated with pain on swallowing
What are the main general causes of delayed gastric emptying?
Idiopathic Longstanding diabetes Macro vascular disease Drug use particularly opiates Post viral
How can delayed gastric emptying treated?
Small meals eaten frequently - liquid food
Treat underlying cause
Erythromycin acts as a motilin antagonist
Metoclopramide and cisalpride are 5HT4 agonists
Dopamine antagonists such as dimperidone can be used
Botox can be injected into the pyloric sphincter
Gastric electrical stimulation via an enterra device
Acute post operative ileus leads to constipation in the absence if mechanical obstruction. How long does this last?
0-24 hours if it occurs in the small intestine
24-48 hours if it occurs in the stomach
48-72 hours if it occurs in the large intestine
List the risk factors for acute post operative ileus
Open surgery
Prolonged abdominal or pelvic surgery
Delayed enteral nutrition
Peri-operative complications or opiate analgesia
Caecal perforations can occur due to acute colonic pseudo obstruction. How is this caused?
Large bowel parasympathetic dysfunction commonly after cardiothoracic or spinal surgery
What are some of the causes of chronic intestinal pseudoobstruction?
Myopathic - Scleroderma or amyloidosis
Neuropathic - parkinsons
Endocrine - diabetes mellitus, severe hypothyroidism
Drugs such as phenothiazines
How can colonic transit be investigated?
Serial x rays with radioopaque markers
How does the drug loperamide work to treat diarrhoea?
It stimulates an opiate receptor to decrease tone and activity of the myenteric nerve plexus
How does prucalopride work as a laxative?
It stimulates the 5HT4 receptor to increase colonic motility
How does linaclotide work as a treatment for IBS constipation?
Increases the secretion of chloride and bicarbonate ions into the lumen ti increase intestinal fluid and thus speed up colonic transit
What are the main general causes of incontinence.?
Damage to pudendal nerve
Muscle damage of external anal sphincter
Excessive rectal distension
What is hirschrungs disease?
A failure of migration of neural crest cells to the distal colon and results in the affected segment if the colon being aganglionic
How is hirschrungs disease managed?
Surgical resection of the affected colon
Approximately how often to haustral contractions occur?
Every thirty minutes
Explain how the location of hasutra changes with contraction of the large intestine?
The relaxed segment which has formed a sac slowly contracts whilst a previously contracted segment simultaneously relaxes to form a new sac - thus the location of the haustra changes
What is the purpose of haustral contractions?
To mix the colonic contents and expose them to the colonic mucosa to aid absorption
How often to mass movements of the large intestine occur?
3-4 times a day
Which reflex triggers mass movements in 5e large intestine?
Gastrocolic reflex
Describe the defecation reflex
The movement of faces into the rectum distends the rectum causes stretch receptors to initiate the defecation reflex. This reflex causes relaxation of the smooth muscle internal anal sphincter and contraction of the rectum and sigmoid colon walls. Defecation will not occur however until the voluntarily controlled skeletal muscle of the external anal sphincter is relaxed.
What happens if, following the initiation of the defecation reflex, the urge to defecate is not followed by relaxation of the external anal sphincter?
The distended rectal wall will gradually relax and the urge to defecate will subside. This lasts until the next mass movements pushes more faces into the rectum, distends the rectum further and again initiates the defecation reflex
