GI physiology Flashcards
what is the digestive purpose of the mouth
chewing
moistens and lubricates food
begins digestion of polysaccharide
what is the purpose of the small intestine
digestion and absorption of most substances
what is the purpose of the large intestin
storage and concentration of undigested matter
salt and water absorption
defecation
vitamin K absorption
what is the purpose of the pancreas
secretion of enzymes and bicarbonate in order to digest carbs, fats, proteins and nucleic acids
what is the purpose of bicarbonate
to neutralise HCl entering the small intestine
what is the purpose of bile
solubilise fats
what is the purpose of the liver
bile
bicarbonate
eliminate waste products and trace metals
what is the purpose of the gall bladder
store and concentrate bile
what are the 5 main physiological processes of the GI tract
motility secretion digestion absorption excretion
what are the defence mechanisms of the gut
Gut associated lymph tissue GALT
organised aggregates of lymphoid tissue- Peyer’s patches
immune cells- lymphocytes and mast cells
gastric acid, mucin, peristalsis and epithelium
what is the lamina propria
loose connective tissue with cappilaries, neurones and immune cells
what is the muscularis mucosae
thin smooth muscle layer of the mucosa
where is striated muscle found
pharynx, upper oesophagus and external anal sphincter
how is food moved along the GI tract
rhythmic contraction and relaxation of GI wall and sphincters
what muscle types surround the myenteric plexus
circular and longitudinal
what type of smooth muscle is found in the GI tract
single unit
what does Ca2+ bind to in order to initiate contraction
calmodulin
how do neuronal and hormonal inputs affect contraction
change the force of contraction but not the speed
what links adjacent cells in single unit smooth muscle
gap junctions
what does calcium calmodulin do
activates inactive myosin light chain kinase
what does the active myosin light chain kinase do
phosphorylates myosin
what does latch state contraction do
maintains high tension
maintains the cross bridge binding with low atp consumption
how does smooth muscle relax
decrease in Ca2
leads to dissociation of Ca- calmodulin which leads to inactivation of myosin light chain kinase
which leads to dephosphorylation of myosin
cross bridges break
myosin phosphatase removes phosphate from the myosin
what causes slow waves in smooth muscle
interstitial cells of Cajal which are pacemaker cells found in the myenteric plexus
what is the intrinsic rate of the pacemaker cells
3-12 per minute
3 in the stomach
12 in the duodenum
what are the stages in slow wave generation
Depolarisation due to the cyclical opening of voltage
gated Ca2+ channels
1. Increase in the intracellular Ca2+ concentration
2. Opening of Ca2+ dependent K+ channels
3. Increased K+ permeability leads to slow
hyperpolarisation
4. Voltage gated Ca2+ channels close and intracellular
Ca2+ concentration falls
5. Ca2+ dependent K+ channels close
6. Voltage gated Ca2+ channels open
what stimulates the spikes in membrane potential
stretch
acetylcholine
parasympathetic
what stimulates hyperpolarisation
norepinephrine
sympathetics
what acts as a lubricant during chewing
mucin which is a glycoprotein
how is reflex chewing initiated
by food in mouth which is relayed from mechanoreceptors to the brain stem initiates a reflex pattern of activity in chewing muscles
what are the 3 phases of swallowing
oral phase
pharyngeal phase
oesophageal phase
where is the swallowing centre
medulla and lower pons
what innervates the lower oesophagus
vagal motor
describe the swallowing refelex
- Upper oesophageal sphincter opens to allow bolus of food to enter
oesophagus - Upper oesophageal sphincter closes
- Primary peristaltic contraction mediated by swallowing reflex involves a
series of coordinated contractions creating a region of high pressure
behind the bolus (accelerated by gravity) - Lower oesophageal sphincter opens mediated by peptidergic nerves in
the vagus releasing VIP (vasoactive intestinal peptide) - Receptive relaxation of the orad region of the stomach
- Lower oesophageal sphincter closes
what may result from achalasia (absence of relaxation)
dysphagia
failure of lower oesophageal sphincter to relax
impaired peristalsis in distal regions
could be caused by Selective loss of neurons which regulate the lower oesophageal sphincter by releasing vasoactive intestinal
peptide (VIP)
what does the parasympathetic NS do
stimulate gastric smooth muscle motility and secretions
what does the sympathetic NS do
inhibit motility and secretions
what myenteric nerves innervate the stomach
parasympathetic innervation via the vagus
sympathetic via the coeliac ganglion
how does the stomach receive sensory innervation
sensory receptors and the ENS
and centrally via the vagal and splanchnic nerves
what can the stomach sense
(pressure, distension,
pH, pain
what layers make up the muscularis externa
oblique, circular and longitudinal muscle
what is the orad
upper proximal part of the stomach which distends in order to make room for food
the stomach can be increased to 1.5 l
what afferent information is sent via the vagus
Mechanoreceptors associated with chewing, oesophageal
and stomach distension relay information to CNS via
sensory neurons
what efferent information is sent via the vagus
efferent information from the CNS causes orad relaxation
what neuropeptide is released from postganglionic peptidergic vagal neurons in order to relax the orad
vasoactive intestinal peptide
what part of the stomach is responsible for mixing
thick muscular wall of the caudad
– contraction waves begin in the middle of the body, move
distally with increasing strength towards the pylorus
describe the contraction patterns of the stomach
- propulsion - bolus is pushed towards the closed pylorus
- grinding - the antrum churns the trapped material
- retropulsion -bolus is pushed back towards the proximal stomach
some food may be allowed to exit via the pylorus
what is the slow wave frequency of the stomach
3-5 per min
what factors affect the force of contraction
para. stimulation, gastrin and motilin increase AP frequency and force of contraction
what are Migrating myoelectric complexes
• mediated by motilin – released from endocrine cells in the
upper GI tract
occur every 90 minutes
Clear stomach of residue remaining from previous
meal
how long does it take to empty the gastric contents after a meal
3 hours
what Physical factors affect
emptying
- Liquids empty more rapidly than solids
- Isotonic fluids empty more rapidly than
hypo- or (especially) hypertonic fluids - Solids must be reduced to particles
what is the effect of fat on the GI system
cholecystokinin is secreted
what is the function 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
how many waves are in the duodenum
12 per minute
how many waves are in the ileum
9 per minute
what coordinates contraction
ENS
what causes orad contraction
aCh and substance P
what causes caudad relaxation
VIP and nitric oxide
what is the taeniae coli
longitudinal muscle concentrated in 3 bands
what parts of the colon are innervated by the vagus
caecum, ascending colon and transverse colon
what parts of the colon are innervated by the pelvic nerves
• Descending and sigmoid colon, rectum and anal canal
Stimulation causes expulsive contractions of the distal colon
what does the sympathetic ns do to the colon
stops colonic movements
why might the faeces be retained
Na+ and water retention
what are the haustra
the sac like segments of the colon
why would you have hard faeces in constipation
more water is absorbed because more time in GI tract
what reflexes cause propulsion of faeces into the rectum
gastrocolic and duodenocolic reflexes
what is the gastro colic reflex
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.
The efferent limb of the reflex increasing colon
motility is mediated by CCK and gastrin.
what is the rectosphincteric reflex
As rectum fills with faeces, the smooth muscle of the rectum
contracts and the internal anal sphincter relaxes but the external anal spincter remains closed
where is the vomiting centre
in the medulla
how does vomiting work
- 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 content
what afferent information stimulates vomiting
- Vestibular system
- Back of throat
- GI tact
- Chemoreceptor trigger zone in the 4th ventricle
what are the main breakdown products of carbohydrate absorption
glucose
galactose
fructose
how are glucose and galactose absorbed
Na dependant cotransport
how is fructose absorbed
facilitated diffusion
what is SGLT1
sodium dependent glucose transporter 1 located on the apical membrane transports glucose and galactose
what is GLUT5
glucose transporter 5
transports fructose across the
apical membrane
how does protein digestion happen
in the stomach with pepsin
then in the small intestine with pancreatic and brush border proteases
where is alpha amylase found
mouth
what is endopeptidase
hydrolyses the interior peptide bonds of proteins
what is exopeptidase
hydrolyses one amino acid at a time from the exterior
how are pancreatic proteases secreted in the small intestine
as inactive precursors
how is trypsinogen activated
brush border enzyme enterokinase to the active form trypsin
what does trypsin do
catalyses the conversion of other inactive precursors to active enzymes
how are amino acids absorbed from the small intestine
Na dependant co transport
how are dipeptides and tripeptides absorbed from the small intestine
H+ dipeptide cotransport
where does fat digestion begin
stomach with the action of lingual and gastric lipases
how is fat digestion completed
in the small intestine with the action of pancreatic enzymes
what is fat digested to
glycerol and free fatty acids
what is the function of gastric lipase
digest fats
slow the rate of gastric emptying so that pancreatic enzymes are able to digest lipid
where is cholecystokinin secreted from
I cells of the duodenal and jejunal mucosa in response to the presence of monoglycerides and fatty acids and peptides
what substances emulsify dietary lipids
Bile salts, lysolecithin and products of lipid digestion
what does emulsification produce
small droplets of lipids
dispersed in an aqueous solution creating a large
surface area for pancreatic enzyme digestion
what pancreatic enzymes complete lipid digestion
pancreatic lipase, cholesterol
ester hydrolase and phospholipaseA2 and the protein, colipase
what are chylomicrons made from
fats, cholesterol (inside) and proteins (outside)
how do chylomicrons enter the blood
lacteals to thoracic duct which empties into the blood stream
how are fatty acids absorbed
bile salts which form micelles
what are Crypts of Lieberkuhn in the small intestine for
secretion
what are colonic glands for
secretion
how often are villous cells replaced
approx. every 2-3 days
where is the majority of water reabsorbed
small intestine
where is sodium reabsorbed
jejunum, ileum and colon
what cells in the pancreas produce digestive enzymes
basophillic cells
stimulated by CCK
what cells in the pancreas produce bicarbonate ions
centroacinar cells stimulated by secretin
where is the submucosal plexus
between the submucosa and the circular muscle layer
what is found in the sub mucosa
– Loose connective tissue – Larger blood vessels – Lymphatics – Secretory glands – Enteric neurones in the submucosa -submucosal plexus
what is found in the muscularis externa
– Inner layer of circular muscle
– Outer layer of longitudinal muscle
– Enteric neurones between the muscle layers – myenteric plexus
what is found in the serosa
outer layer of connective tissue and squamous epithelia
what is the extrinsic autonomic NS
sympathetic and parasympathetic
what is the intrinsic component of the ANS
enteric NS
Meissner’s plexus (submucosal)
and Auerbach’s plexus- myenteric
describe the organisation of the somatic NS
single motorneurone
no ganglia
ACh
describe the sympathetic organisation
have ganglia close to the spinal cord which secrete ACh
have long
postganglionic neurones
thoracolumbar (ps is craniosacral)
norepinephrine is secreted postganglion
what are the 3 plexuses in the sympathetic GI supply
coeliac plexus
sup mesenteric
inferior mesenteric
what reflexes are controlled by the ENS
secretion, peristalsis, mixing, inhibition
what reflexes travel from the GI tract to the prevertebral sympathetic ganglia and then back to the GI tract
reflexes that transmit signals to other areas of the stomach
gastrocolic, enterogastric, colonoilial
what reflexes travel from the GI tract to the brain stem or spinal cord
reflexes controlling motility and secretion
pain reflexes which inhibits the GI tract
defecation which produces reflex contractions
what is catarrh
a build up of mucus in a body cavity
results from inflammation of the mucus membrane
