Gut Motility Flashcards
what is the GI tract innervated by
The gastrointestinal (GI) tract is innervated by intrinsic enteric neurons and by extrinsic efferent and afferent nerves
describe the extrinsic innervation parts
o Parasympathetic by the vagus nerve - this responds to cholinergic receptors and is excitatory
o Sympathetic by the greater splanchnic, lumbar colonic and hypogastric nerves SLH, this responds to noradrenergic and is inhibitory
what makes up the intrinsic enteric nervous system
- myenteric plexus
- submucosal plexus
- interneurones
- motor neurones
- sensory neurones
- Interstitial cells of Cajal
describe what the parts of the intrinsic enteric nervous system do
o Myenteric plexus - primary motility controller
o Submucosal plexus – primary fluid exchange controller
o Interneurons – move information up and down the gut
o Motor neurones – releases neurotransmitter onto effectors (ACh (cholinergic) causes contraction, NO causes relaxation)
o Sensory neurones – carries information from receptors to the motor neurones
o Interstitial cells of Cajal
where are the interstitial cells of Cajal (ICCs)
Around the myenteric and submucosal plexus
what do the ICCs do
- they are myogenic therefore they act as a pacemaker for gut contraction by spontaneously depolarising
Create rhythm of electrical slow waves causing phasic muscle contractions
Influence ability of hormones/ neurotransmitters to induce propulsive and other movements + facilitate motor nerve transmission to muscle
Higher frequency near greater curvature
Slow waves reaching mid-lower corpus form into complete ring wave-fronts
what does the ENS control
Gut motility
Local blood flow
Transmucosal movement of fluids
Modulates immune and hormonal function
what nervous system is in the oesophagus
CNS control - vagus
what nervous system is in the in the stomach
myogenic, ENS, and CNS vagal control
what nervous system is in the small intestine
ENS
what nervous system is in the large intestine
ENS and myogenic control
what nervous system is in the rectum
ENS and CNS control
how many phases of the migrating motor complex are there
• 3 phases every 90-120 minutes
what is the function of the migrating motor complex
o Clear undigested material
o Prevent bacteria overgrowth
o Hunger sensations
where does the migrating motor complex start
o Can originate in the stomach (vagus dependent) or small intestine (vagus-independent)
what are the three phases of food intake
cephalic
gastric
intestinal
describe the cephalic phase
o Triggered by the thought, sight, smell, taste of food and chewing and swallowing
o Prepares GI tract salivation, gastric acid release, pancreatic secretion, inhibition of MMCs, ghrelin secretion
o Stimulated by vagus nerve to parietal cells and G cells
describe the gastric phase
o Satiation, early digestion
o Triggered by mechanical effect stomach distension
o Stimulated by local reflexes and vagovagal reflexes to parietal cells and G cells
describe the intestinal phase
o Feedback and satiation
o Triggered by chemoreceptor activation in small intestine from the products of protein digestion in the duodenum as well as intestine distension
o Stimulate by amino acids and intestinal endocrine cells (entero-oxyntin)
How does peristalsis take place
- the primary peristaltic wave occurs on swallowing when bolus enters the oesophagus, this is controlled by skeletal muscle
- then stretch receptors cause a secondary peristaltic wave in the smooth muscle at the base of the oesophagus this causes the bolus to be forced into the stomach
- there is contraction proximal to the bolus and relaxation distal to the bolus
- this is controlled by the myenteric plexus of the intrinsic enteric nervous system
- there is then the ascending wave of peristalsis
- then a descending wave of peristalsis
what controls and causes the ascending wave of peristalsis
o Excitatory neuro-transmission to muscle (mostly by release of acetylcholine)
what controls and causes the descending wave of peristalsis
o Inhibitory neuro-transmission to muscle (mostly by release of the gas nitric oxide)
how does the enteric plexus control peristalsis
o detect intraluminal stimuli
o initiate peristalsis, plus increased secretion & vascular flow
what is the function of the proximal stomach
- receptive relaxation
- adaptive relaxation
- burp reflex
how is receptive relaxation caused
– vagus reflex – release of CCK - cholecystokinin
how is adaptive relaxation caused
enteric reflex – release of NO
how does the burp reflex work
- it allows excess air in the stomach to escape
- it is caused by the brief relaxation of the lower oesophageal sphincter this allows the gas to escape
what is the function of the distal stomach
propulsion with grinding and mixing (acid and peptidases)
what are the three phases that take place in the distal stomach
- propulsion
- emptying
- retropulsion
what happens if food is too large
o If food too large it will push it back from the sphincter into the distal stomach for further grinding and mixing
describe propulsion
Rapid flow of liquids with suspended small particles
Delayed flow of large particles towards pylorus
describe emptying
Emptying of liquids with small particles into duodenum
Large particles retained in the bulge of the terminal antrum
describe retropulsion
Retropulsion of large particles back into the body of the stomach for further mixing
Clearing of terminal antrum
what does contraction into the mid corpus of the stomach do
o Contraction of mid corpus mixes & breaks down food, together with gastric acid + peptidases
what does movement into the antrum of the stomach do
o Movement into the antrum enables powerful contractions (against a closed pylorus) to further break food into particles small enough to be propelled into the duodenum
describe the rates of gastric emptying
Liquids = exponential
Large solid particles = lag phase at the start
Viscous chyme = linear fashion
what does feedback in gastric emptying do
- allows more time for digestion and absorption
describe the feedback regulation of gastric emptying
- within duodenum - nutrients, osmalitiy, HCL
- CCK release
- Activation of vagal afferents
- reduces opening of pyloric sphincter
- reduces contraction in corpus-antrum
- enhances relaxation of fundus
- Activation of vagal afferents
what does the illegal brake do
– promotes satiety
Peptide YY
Glucagon-like-peptide 1
Oxyntomodulin
what are enteroendocrine cells
The primary source of feed-back signalling for physiological regulation
what are the movements of the upper gastrointestinal tract
• Primary & Secondary Peristalsis in Oesophagus
• Role of Lower Oesophageal Sphincter
o Prevents reflux
o TLESRs
• Gastric fundus relaxation
o Initial vagal reflex, maintained by the ENS
• Gastric ‘mixing’, ‘grinding’ and emptying
o Particle size
• Feedback regulation
o Nutrient, pH, Osmolarity
o Hormonal and neuronal
• Control exerted mostly by ENS, vagal, hormonal systems
what does the ascending colon do
o Mixing o Absorption o Fermentation o Rich in living bacteria o Slow transit
what does the transverse colon do
o Absorption
o Rapid transit
what does the descending colon do
o Storage
o Slow
o Partly voluntary transit
what are the movements of the ascending proximal colon
- propulsion
- retropulsion
- segmentation
how is the movements of the ascending colon controlled
Interstitial cells of Cajal - generate slow waves of electrical activity propagating short distances in oral or anal directions
Release of local mediators – 5-HT3 receptor antagonists – cause constipation
how does mass movement occur
o Substance enters ascending colon – haustra visible
o As more enters a constrictive ring occurs (giant migrating contraction) and the haustra disappear from a portion of the ascending colon
o Area of the colon distal to the constrictive ring loose their haustrations and contract as a unit which propels faecal material into the transverse colon
o Haustra then return
how does defecation occur
o As more faecal matter enters the rectum, the pressure increases
o Internal sphincter relaxes to prevent excessive pressure build up
o External sphincter contracts to prevent release of faecal matter
what prevents defecation
tone of internal anal sphincter and puborectalis
Mechanical effects of acute anorectal angle
what imitates defecation
Relaxation of puborectalis and external anal sphincter
Increase in intrabdominal pressure – facilitated by squatting
Sphincter relaxation and rectal propulsive contractions
what are the two types of enteric nerves
• Intrinsic Primary Afferent Neurones (IPAN’s)
o AH-neurones; Slow, Long-Lasting ability to become Hyperexcitable
o Short axon ~30 dendrites; Projects circumferentially from mucosa to motor/ interneurones + IPAN’s
• Motor-/ Interneurones
o S-neurones; Long axon projecting along the gut, short dendrites; Fast depolarisation
what do the enteric neurones do
- detect intraluminal stimuli (eg., histamine from mast cells, 5-HT from enterochromaffin cells, motilin from endocrine cells)
innervate specialised cells of the muscle (ICC’s)
communicate with the brainstem, spinal cord and paracrine/ endocrine systems
initiate complex behaviours involving immune, vascular, muscular and epithelial transport systems
