Motility in the GI tract Flashcards
What motor activity does the GI tract perform and its functions?
Segmental contractions (mixing) Peristaltic contractions Tonic contraction (sphincters)
What is the function of the small intestine?
Location for almost all digestion and absorption
Secretion of Chyme into the colon
What types of motility are in the small intestine during the fed state?
Segmentation
Peristalsis
(these types are rhythmic phasic contractions (RPCs))
How does segmentation work?
Rings of contractile muscle at intervals contract then relax pushing bolus around.
Used for mixing mainly
How does peristalsis work?
Sequential contractions of rings of circular muscle followed by sequential relaxation.
Combination of activity between muscular layers, with sequential activity in each of them.
Propels chyme along
What regulates peristalsis during the fasting state?
Migratory motor complex (MMC).
What is the MMC?
Large sweeps of electrical activity causing contraction down the small intestine.
What is the purpose of the MMC?
Propel particles from the stomach into the duodenum
Clear small intestine of its residual content (undigested food)
Stops colonic bacteria from entering into the terminal ileum
What types of reflex are present in the ileum?
Ileogastric reflex
Gastroileal reflex
Mediated via the ENS
What is the Ileocaecal Sphincter?
Separates the ileum from the colon.
Operate based on sensory neurones detecting pressure changes via the ENS.
What is the Colon?
Last 4ft of the GIT
Surface area only 1/30th of the small intestine.
No villi
What are the parts of the colon?
Ascending Transverse Descending Sigmoid Rectum
What are Taeniae Coli?
Longitudinal muscle layers thickened to form 3 muscular bands which run the length of the colon
What is the function of the Colon?
Absorb large quantities of fluid and electrolytes thus converting liquid content to solid.
Metabolises fibres into short chain fatty acid (glutarate) to be absorbed. (caused by microbiota fermenting the fibres into the SCFA)
Storage and reservoir function
Regulate release of faecal material
Provides environment for comensal bacteria which synthesise vitamins
Secretes mucous and ions.
What are the functions of the proximal colon?
site of absorption and bacterial fermentation
What are the functions of the distal colon?
reservoir/storage function
What do Colonic rhythmic phasic contractions do?
cause slow net distal propulsion with extensive mixing/turning over
Short duration - contractions have no propagation and varying amplitudes
Long duration - contractions over short distances which turn over and propel the semi-solid to solid contents more effectively
What is Colonic giant migrating contractions?
Colonic motility:
GMCs are large amplitude lumen occluding contractions over entire colon that precede defecation.
How is motility stimulated in the colon?
When something is taken (food) resulting in distention and therefore stimulation of motility.
What are some western diseases and what causes them?
Constipation Diverticular disease Haemorrhoids Polyps Cancer of colon Irritable colon Ulcerative colitis Caused by populations which have a low fibre diet
What is a laxative?
Something which stimulates motility
What are the classifications of laxatives?
Osmotic laxatives
Bulk laxatives
Stimulant laxatives
Faecal laxatives
What part of the colon is innervated by which nerve?
Vagus - innervates ascending and transverse
Pelvic - innervates descending and rectum
What does stimulation of the pelvic nerve bring about?
Causes expulsive movement (GMCs) of the distal colon and sustained contraction of some segments.
What is the process of the defaecation reflex?
- distention of the rectum.
- stimulation of stretch receptors
- stimulation of myenteric plexus.
- stimulates motility and peristalsis.
- reinforcement in the form of a parasympathetic defaecation reflex
- mediated by via the sacral segments of the spinal cord
- results in amplification of peristaltic waves and relaxation of the internal anal sphincter.
- defaecation can be inhibited by conscious control over the external anal sphincter
What are the functions that motility has in the small intestine?
Mixing - chyme from stomach with secretions of pancreas, liver and intestines
Propulsion - Moving food to the correct sites for digestion, absorption and make way for further material to be emptied from stomach
Release of Chyme into the colon
What regulates ileal flow into the colon through the Ileocaecal Sphincter
Luminal contents and pressure both proximal and distal to the sphincter.
Distention of the ileum = relaxation of the sphincter
Distention of the proximal colon = contraction of the sphincter
What are the phases of MMC movement?
- A prolonged quiescent period
- Period of increasing action-potential frequency and contractility
- Period of peak electrical and mechanical activity that lasts a few minutes,
- Period of declining activity that merges into the next quiescent period.
What are the types of motility in the Colon?
- Colonic Rhythmic Phasic Contractions
2. Giant Migrating Contractions
What are bulk laxatives?
- Indigestible material passes quickly activating pressure receptors which initiate the Myenteric plexus
What are osmotic (saline) laxatives?
- create a hyperosmotic environment in the colon drawing water into the colon (eg. lactulose)
What are Faecal laxatives?
- stimulate defecation (eg. docusate)
What are stimulant laxatives?
-stimulate the ENS directly to cause motility (eg. senna danthron)
What is the ileogastric reflex?
- ileal distention leads to decreased gastric motility (inhibitory reflex)
What is the Gastroileal reflex?
- increased gastric activity causing increased ileal motility and increased movement of chyme through the ileoceacal valve (excitatory reflex)
