15 GI tract motility: motility of stomach, small and large intestine Flashcards
What is are the major functions of gastric motility? (3)
- Allows stomach to act as a reservoir for the large volume of food ingested at a single meal (receptive relaxation)
- Breaks food into smaller particles and mixes with gastric secretions
- Empties gastric contents into duodenum at controlled rate
Smooth muscle of the stomach:
• 3 layers?
• Change in thickness?
3 layers:
• Outer= Longitudinal
• Middle= Circular
• Inner= Oblique
Muscle wall thickness increases from proximal to distal
What does contraction in the caudad region do?
Mix food and propel it into the small intestine
List the innervation of the stomach (3)
- Extrinsic nerves
• Parasympathetic
• Sympathetic - Enteric nervous system
• Myenteric plexus
(a) parasympathetic via vagus
(b) sympathetic via coeliac ganglion - Sensory nerve fibres
What are the extrinsic nerves that innervate the stomach?
- Parasympathetic - stimulate gastric smooth muscle motility and secretions
- Sympathetic - inhibit motility and secretions
What is the enteric nervous system that innervates the stomach?
Myenteric plexus:
- Parasympathetic innervation via vagus
- Sympathetic innervation via coeliac ganglion
What are the sensory afferent fibres that innervate the stomach?
- Between sensory receptors and the ENS (pressure, distension, pH, pain)
- Centrally via the vagal and splanchnic nerves
Discuss the receptive relaxation of the stomach and its functions
Orad region has a thin muscular wall:
- Distension of the lower oesophagus induces relaxation of the lower oesophageal sphincter and the road region of stomach
- Reduces pressure and increases volume of the stomach (1.5 L)
What nerve fibres are involved in vagovagal reflex?
Afferent and efferent nerve fibres in the vagus
Discuss the afferent information in vagovagal reflex
Mechanoreceptors associated with chewing, oesophageal and stomach distension relay information to CNS via sensory neurons
Discuss the efferent information in vagovagal reflex
Efferent information from CNS causes oral relaxation:
• Neurotransmitter - VIP (vasoactive intestinal peptide): released from postganglionic peptidergic vagal neurons is responsible for orad relaxation
What substance is responsible for orad relaxation?
Neurotransmitter: VIP (vasoactive intestinal peptide)
Describe the muscle layers and type of contractions in the fundus and body of the stomach
- Muscle layers are thin
* Weak contractions - contents settle into layers based on density
Which region of the stomach is responsible for mixing?
- Thick muscular wall of caudad region is responsible for mixing
- Contraction waves begin in the middle of the body, move distally with increasing strength towards the pylorus
Describe the movement patterns in the stomach
- Propulsion - bolus is pushed toward the closed pylorus
- Grinding - antrum churns the trapped material
- Retropulsion - bolus is pushed back into the proximal stomach
What occurs in retropulsion?
Propels the gastric contents back for further mixing in the stomach
What is the slow wave frequency in the stomach?
3 - 5 per minute
Control of slow wave frequency in the stomach?
- Neural hormonal input
- Hormonal input
- Parasympathetic stimulation
- Gastrin
- Motilin
- Sympathetic stimulation
- Secretin
What does not affect slow wave frequency in the stomach but does affect action potential frequency?
- Neural input
* Hormonal input
What increases action potential frequency (and force of contraction) in slow wave frequency in the stomach
- Parasympathetic stimulation
- Gastrin
- Motilin
What decreases action potential frequency in slow wave frequency in the stomach?
- Sympathetic stimulation
* Secretin
What is the activity of the stomach during fasting?
Periodic gastric contractions (MMCs)
• Migrating myoelectric complexes:
– Mediated by motilin: released from endocrine cells in the upper GI tract
– 90 minute intervals
• Clear stomach of residue remaining from previous meal
After a meal, how much solids, liquids and gastric secretions are contained in the stomach?
1.5 L
How long does it take to empty gastric contents?
~ 3 hours
Why is the rate of gastric emptying regulated?
To ensure that gastric H+ is neutralised in the duodenum and there is adequate time for digestion and absorption of nutrients
What type of meal has the quickest gastric emptying time?
- Saline meal (fastest emptying time)
- Acid meal
- Oleate meal
What physical factors affect gastric emptying? (3)
- Liquids empty more rapidly than solids
- Isotonic fluids empty more rapidly than hypo - or (especially) hypertonic fluids
- Solids must be reduced to particles < 1mm^3 or less: retropulsion continues until this is achieved
What chemical factors inhibit gastric emptying?
Presence of fat and H+ ions in the duodenum
Presence of fat inhibits gastric emptying. What is fat mediated by?
Cholecystokinin - secreted when fat reaches the duodenum
Presence of H+ ions inhibits gastric emptying. What is H+ mediated by?
- Reflexes in the enteric nervous system
- H+ receptors in the duodenum detect low pH and relay information to the gastric smooth muscle via interneurons in the myenteric plexus
3 functions of the motility of the small intestine
- Mixes chyme with digestive enzymes and pancreatic secretions
- Exposes nutrients to the intestinal mucosa for absorption
- Propels unabsorbed chyme into large intestine
Parasympathetic innervation of small intestine:
• Nerve?
• Action?
• Neurotransmitter?
- Nerve - vagus
- Action - increases contraction
- Neurotransmitter -
- ACh
- Motilin
Sympathetic innervation of small intestine:
• Nerve?
• Action?
• Neurotransmitter?
- Nerve - coeliac and superior mesenteric ganglia
- Action - decreases contraction
- Neurotransmitter - NA
What is the slow wave frequency in the small intestine?
- Duodenum - 12 per minute
* Ileum - 9 per minute
How often does migrating myoelectric complexes occur in the small intestine? What is its function?
- Every 90 mins
* Clear small intestine of residual chyme
In the small intestine, what are the 2 types of contractions coordinated by ENS and their functions?
- Segmental - responsible for mixing
2. Peristalsis - create forward movement
What substances cause orad contraction?
- ACh
* Substance P
What substances cause caudad relaxation?
- VIP
* Nitric oxide
What is the structure of the colon?
Longitudinal muscle concentrated in 3 bands:
• Taeniae (tenia) coli
What is the innervation of the colon?
- Parasympathetic:
(a) Vagus
• Caecum, ascending colon and transverse colon
• Stimulation causes segmental contractions of the proximal colon
(b) Pelvic nerves
• Descending and sigmoid colon, rectum and anal canal
• Stimulation causes expulsive contractions of the distal colon
- Sympathetic:
• Simulation stops colonic movements
Segmental contractions in caecum and proximal colon?
- Contractions mix contents
* Reverse peristalsis and segmental propulsion towards the caecum can occur (retention favours Na+ and water absorption)
Large intestinal motility?
- Material not absorbed in the small intestine enters the large intestine - faeces destined for excretion
- After contents of of the small intestine enter the caecum and proximal colon, the ileoceacal sphincter contracts. Faecal material moves from the caecum, through the colon to the rectum and on to the anal canal
- Colon receives 500 - 1500 ml of chyme per day but most of the salt and water are absorbed (100 ml per day lost in faeces)
- Contractions associated with sac-like segments called haustra (haustrations)
How much chyme does the colon receive per day?
500 - 1500 ml
• Most of salt and water are absorbed (100 ml lost)
What are haustra?
Sac-like segment in colon associated with contraction
What does poor motility in colon result in?
- Causes greater absorption
* Hard faeces in transverse colon causes constipation
What does excess motility in colon result in?
- Less absorption
* Diarrhoea or loos faeces
Describe mass movements in the colon
- Caused by gastrocolic and duoedenocolic reflexes
- Move contents of large intestine over long distances (20 cm)
- Occur 1-3 times per day
- After water is absorbed in distal colon, the faeces is semi solid
- Final mass movement propels faecal content into rectum
What is gastrocolic reflex? What are the afferent and efferent limbs mediated by?
- Distension of the stomach by food increases the motility of the colon and the frequency of mass movements in the large intestine
- Afferent limb in the stomach mediated by the parasympathetic nervous system
- Efferent limb of reflex increasing colon motility is mediated by CCK and gastrin
What is rectosphincteric reflex?
As rectum fills with faeces, the smooth muscle of the rectum contracts and the internal snap sphincter relaxes
Muscle relaxation and contraction process of defecation?
- Rectosphincteric reflex
- External anal sphincter (composed of striated muscle and under voluntary control) remains tonically contracted
- External anal sphincter is relaxed voluntarily, the smooth muscle of the rectum contracts and the pressure forces faeces through the anal canal
Vomiting:
• Centre where?
• Afferent information?
• Efferent response?
- Vomiting centre in the medulla
2. Afferent information: • Vestibular system • Back of throat • GI tract • Chemoreceptor trigger zone in the 4th ventricle
- Efferent information:
• Reverse peristalsis in small intestine
• Relaxation of the stomach and pylorus
• Forced inspiration to increase abdominal pressure
• Relaxation of the lower oesophageal sphincter
• Forceful expulsion of gastric and duodenal contents
What may newborns vomit in the 1st few hours after birth? Why?
- May vomit mucus, occasionally blood-streaked in the 1st few hours after birth
- Rarely persists after the 1st few feedings (irritation of the gastric mucosa by material swallowed during delivery)
Why do newborns in neonatal period vomit?
• Relatively frequents symptom
- Regurgitation from overfeeding
- Failure to permit infant to educate swelled air
When voimiting occurs shortly after birth and is persistent, what must be considered?
Possibilities of:
- Intestinal obstruction
- Increased intracranial pressure
What does a history of maternal hydramnios suggest?
Upper gastrointestinal (oesophageal, duodenal, ileal) atresia
What does bile stained emesis suggest?
Intestinal obstruction beyond the duodenum and requires investigation
How to diagnose duodenal atresia in an upright abdominal film?
Characteristic “double-bubble” sign
- Dilated stomach
- Dilated proximal duodenum
What is normal gut rotation?
- In normal embryo, physiologic herniation of the gut through umbilicus at 6 weeks is accompanied by a 270° counterclockwise rotation of the developing intestine around the superior mesenteric artery
- By 10 - 12 weeks, the intestine returns to the abdomen and assumes its normal adult anatomic position
- Normal small bowel mesentery has a broad attachment stretching diagonally from the duodenojejunal junction (LUQ) to caecum (RLQ)
The physiologic herniation of the gut through the umbilicus, accompanied by a 270° counterclockwise rotation of the developing intestine around SMA occurs when in an normal embryo?
6 weeks
When does the intestine return to the abdomen in an normal embryo?
10 - 12 weeks
What are the broad attachments of normal small bowel mesentery?
From duodenojejunal junction (in LUQ) to caecum (in RLQ)
Malrotation intestinal obstruction may result in what 3 things?
- Obstruction of the duodenum may result from congenital peritoneal bands (Lads bands) which run over the duodenum from the caecum in the RUQ
- Midgut volvulus
- Internal hernia in
the mesentry
What causes malrotation of the intestine?
- Results when the normal embryologic sequence of bowel development and fixation is interrupted
- Malrotated bowel is prone to torsion, resulting in midgut volvulus
Vomiting may occur from what disturbances that don’t obstruct the digestive tract?
- Milk allergy
- Adrenal hyperplasia of the salt - losing variety
- Galactosaemia
- Hyperammonaemias
- Organic acidaemias
- Increased intracrhail pressure
- Septicaemia
- Meningitis
- Urinary tract infections
What are valvulae conniventes/ Kerckring folds/ plicae circulares/ small bowel folds?
Where do they start and end? Size?
• Mucosal folds of small intestine
- Starts from 2nd part of duodenum
- Large and thick at jejunum and considerably decrease in size distally in the ileum to disappear entirely in the distal ideal bowel loops
What is the % of mechanical intestinal obstruction that is caused by small or large bowel obstruction?
• 80% - small bowel obstruction
20% - large bowel obstruction
Small bowel obstruction can be divided into what types?
- Congenital
- Acquired:
• Extrinsic causing compression
• Intrinsic
• Luminal
What are the most common cause of small bowel obstruction in developed and developing countries?
• Developed countries:
~ 75% - adhesions
• Developing countries:
- 80% - incarcerated hernias
What is the structure of large intestine?
- 1.5 m muscular tube
- Extends from caecum to rectum
- Has 3 outer longitudinal muscular layers called taenia coli - 30 cm shorter than the length of the large bowel (causes characteristic sacculations interrupted by incomplete rings called haustra)
What are the causes of large bowel obstruction?
- Underlying aetiology is age-dependent
- In adulthood, most common cause:
- Colonic cancer (50-60%) typically in sigmoid
- Acute diverticulitis (involving sigmoid colon)
• Adhesions don’t tend to cause large bowel obstruction unlike small bowel obstruction
What is Hirschsprung disease?
- Most common cause of neonatal colonic obstruction
- Commonly characterised by a short segment of colonic aganglionosis affecting term neonates (esp. boys)
- Affects ~ 1:5000 - 8000 live births
- Never seen in premature infants
How is Hirschsprung disease diagnosed and treated?
- Condition typically presents in term neonates with failure to pass meconium in the first 1-2 days after birth
- Definitive diagnosis requires a full thickness rectal biopsy
- Surgical treatment is by removal of affected portion of the colon
Why can Hirschsprung disease have high mortality rates?
- Mortality rates can be as high as 30% due to enterocolitis
* In surgical removal m 3-4% of cases colonic perforation complicates the presentation
What is paralytic ileus?
Functional intestinal obstruction without an actual physical obstruction
What causes paralytic ileus?
- Multifactorial malfunction in the nerves in the intestine and subsequent impairment in intestinal peristalsis
- Mneumonic - 5 Ps:
- Postoperative
- Peritonitis
- Potassium - low
- Pelvic and spinal fractures
- Parturition
How do patients with paralytic ileus present?
- With symptoms similar to a mechanical bowel obstruction e.g. nausea/ vomiting, distension, and reduced or absent bowel movements
- Bowel sounds will be decreased/ absent whereas they are increased in mechanical obstruction
- On radiographs: generalised, uniform, gaseous distension of the large and small bowel
