Intestinal motility Flashcards
gastrointestinal motility
GI motility is the movements of the digestive system and how contents transit within it.
GI motility involves coordinated contractions and relaxations of the GIT that moves content from the mouth to the anus.
There are 3 main patterns of motility: peristalsis, segmentation and tonic contraction.
Intestinal motility
The duodenum and jejunum are the sites of most digestion and absorption
Tonic and phasic muscle is essential for these properties
Mixing - chyme from stomach with secretions of pancreas, liver and intestines
Propulsion - moving food to correct sites for digestion, absorption and make way for further material emptied from stomach
segmentation
- non propulsive movement
- mainly for mixing
-enhances the digestion and absorption of dietary nutrients
peristalsis
- propulsion
- movement of food and its digestive products in a caudal direction
- result in elimination of nondigested , nonabsorbed material
reserboir
- holds content
- made possible by sphincters
intestinal motility- fed state
Segmentation: Rings of circular muscle at intervals contract and then relax. Then adjacent rings contract and relax
Overall result: mixing mainly.
intestinal motility - fed state
Peristalsis: Sequential contractions of rings of circular muscle followed by sequential relaxation.
Combination of activity between muscular layers, with sequential activity in each of them.
Overall result: Propels chyme along
intestinal motility - fasted state
In the fasting state the small bowel is relatively dormant
But it does exhibit synchronised, rhythmic changes in both electrical and motor activity which is termed the migrating motor complex (MMC)
function - both nutritional and cleansing
Migrating motor complex
MMCs in humans occur at intervals of 90 to 120 minutes and consist of four distinct phases:
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.
The role of the MMC is to propel particles greater than 2 mm in diameter from the stomach into the duodenum
Migrating motor complex ..
In addition, the MMCs clear the small intestine of its residual content, including undigested food, desquamated cells, and intestinal and pancreatic biliary secretions.
Stops colonic bacteria from migrating into the terminal ileum.
MMCs usually originate in the stomach and often travel to the distal end of the ileum, but approximately 25% are initiated in the duodenum and proximal part of the jejunum.
Feeding terminates MMCs and initiates the appearance of the “fed motor pattern-segmentation and peristalsis
The ENS, humoral factors and extrinsic innervations all impact to regulate both MMCs and transition to the fed state.
A major determinant of the MMC pattern is the hormone motilin, a 22-amino-acid peptide that is synthesized in the duodenal mucosa and released just before the initiation of phase 3 of the MMC cycle.
The colon
This is made up of the last 4 ft of the GIT. It has no villi
Functions
To absorb large quantities of fluid and electrolytes
Absorb short chain fatty acids (product of carbohydrate fermentation)
Regulate release of faecal material
Store/reservoir
Provides environment for synthesis of vitamins B complex and K- beneficial bacteria
Secretes mucus and ions (K+, Cl-, HCO3-)
Proximal colon- site of absorption and bacterial fermentation
Distal colon - reservoir/storage function
Proximal colon: Transverse colon, ascending colon and cecum
Distal colon: Descending colon and sigmoid colon
Colon motility
- rhythmic phasic contractions - segmentation ( ascending and transverse colon )
- giant migrating contractions - propulsion ( transverse and descending colon )
rhythmic phasic contractions
This is also known as Haustral contractions
Increases contact of faeces to mucosal wall = increased absorption of water, electrolytes and vitamins
The contractions vary widely in amplitude and duration.
The contraction causes ‘pouches’ known as Haustra.
giant migrating contractions
This is also known as mass movement peristalsis.
Alternating waves of contraction and relaxation gives large amplitude over long distances= propulsion.
Occurs during both fasting and postprandial states.
Occurs randomly about 2 to 10 times a day
Hirschsprung disease
The aganglionic, aperistaltic bowel segment effectively prevents the propulsion of the fecal stream, resulting in in megacolon) above the point where the nerves are missing and hypertrophy of the normal proximal colon.
Hirschsprung Disease – congenital lack of neuronal ganglionic cells in the ENS plexi.
regulation of colon motility
faeces in caecum —> distention of colon (stress reflex) —> activation of stress receptors —> signal to myenteric nerve plexus —> triggers Haustral contractions —> distention in stomach —> G cells secrete gastrin —> Gastrin trigger mass movement in colon —> distention of pylorus —> acetylcholine is secreted —> Ach signal system parasympathetic nervous —> increased motility in colon
Autonomic nervous control: Sympathetic inhibit; Parasympathetic increase motility
Gastrocolic reflex: increased motility of the colon due to stretch of the stomach
Long range intestinal reflexes are important in the regulation of colon motility
Defecation reflex
Faeces in sigmoid colon and rectum initiate stretch reflexes
Stretch receptors are activated- parasympathetic nervous system-motor fibres
Acetylcholine is released –> contraction of sigmoid colon and rectum
Internal anal sphincter (made up of an involuntary smooth muscle) are activated by PNS —> relaxation of the muscle
External anal sphincter (made up of voluntary smooth muscle) are innervated by somatic nervous system
CNS send signal to external anal sphincter to relax —> faeces out
