GIT physiology 1 Flashcards
mechanical digestion
mixing waves - gentle, rippling peristaltic movements - creates chyme
chemical digestion
different enzymes require different environments
many molecules require stepwise digestion
all require specific conditions
3 types of chemical digestion
secreted enzymes
brush border enzymes
cytosolic enzymes
carbohydrases
chemically digest carbohydrates
- pancreatic amylase
- a-dextrinase, sucrase, lactase, maltase in brush border
- ends with monosaccharides which can be absorbed
proteases
chemically digest proteins
- trypsin, chymotryp[sin, carboxypeptidase, and elastase from pancreas
lipases
chemically digest lipids
- pancreatic lipase most important in triglyceride digestion
- emulsification by bile salts increases surface area
nucleases
chemically digest nucleic acids
- ribonuclease and deoxyribonuclease in pancreatic juice
- nucleosides and phosphates in brush border
3 types of movement in the GI tract
- interdigestive clearing
- squishing
- peristalsis
interdigestive clearing
driven by a pacemaker
rhythm that sweeps the gut clean
squishing
stretch driven rhythmic contraction
for mechanically breaking up food
peristalsis
local stretch driven propulsion
mechanical aspects of digestion are modulated by
ANS/hormones/paracrines
basal electrical rhythm
cells opening and closing channels constantly
enteric smooth muscle
tonic/phasic
bundles electrically connected via gap junctions contraction stimulated by Ca
tonic/phasic
on/off or oscillating
bundles of enteric smooth muscle is connected via
gap junctions
contraction of enteric smooth muscle is stimulated by
calcium
Ca2+ channels in enteric smooth muscle are
slow to open/close
long AP duration/contraction
excitation of smooth muscle
lots of things can open smooth muscle Ca channels
- ANS activity - acetyl choline
- enteric nervous system activity
- local chemistry
- autorhythmicity - some GIT smooth muscle cells are pacemakers
- hormones, lumocrines
- stretch
how does stretch excite enteric smooth muscle
stretching smooth muscle opens mechanically gated channels;s and causes it tp contract
stretch triggering enteric smooth muscle is called
myogenic stretch response
how does ANS open Ca channel;s
acetylcholine stimulates Gi tract smooth muscle
autorhymicity
some GIT smooth muscle cells are pacemakers
when there is no food to digest
phasic pacemaker dominates - continual slow intrinsic electrical activity - basal electrical rhythm
2 types of phasic pacemaker pattern
- slow waves
- spikes
when a slow pave passes the threshold
an AP is generated
this is a spike
migrating motor complex
rhythmic contraction event passing from cell to cell via gap junctions
lasts 90-120 minutes
migrating motor complex stops when
feeding/eating occurs
migrating motor complex restarts when
resumes automatically when there is no food
segmentation and grinding
stretch initiates local rhythmical circular muscle contraction and relaxation
when there is food in the gut there is two stimuli
stretch and osmotic stimuli (hyperosmolar)
frequency and strength of segmentation and grinding and peristalsis is affected by
PNS - increased
SNS - decreased
peristalsis
retrograde circular contraction and anterograde circular relaxation
moves
2 signals regulating peristalsis
retrograde signal and anterograde signal
retrograde signal results in
results in contraction
anterograde signal results in
results in relaxation
retrograde signal molecule
acetyl choline
anterograde signal molecule
NO and ATP
during absorption
segmentation and mixing
after completion of absorption
peristalsis to move food onto next section of the gut
colon motility
a reservoir for the undigested
slow to absorb sodium
ileocaecal valve
restricts colon bugs from leaving the colon
prevents back flow
increase in ileum pressure
ileocaecal valve opens
increase in colon pressure
ileocaecal valve shuts
ileocaecal calve opens when
increase in ileum pressure
opens briefly with each peristaltic wave
involuntary sphincters are controlled by
myogenic tone and relative pressures
three states of pressure control
resting state - closed, inhibits movement between segments
relaxation - open, pressure infront of sphincter>after
constriction - very closed, pressure after sphincter»in front, prevents retrograde flow
neural control of sphincters
indirect mechanism
- excites interneurons
- uses NO/ATP, facilitates flow
neural control of defamation
parasympathetic stimulation causes internal anal sphincter relaxation
external anal sphincter is under voluntary control
2 gut nerve plexuses
- submucosal plexus - meissner’s
- myenteric plexus - aurbach’s
submucosal plexus
between the mucosa and circular muscle layer
myenteric plexus
between circular and longitudinal muscle layer
myenteric plexus controls
GI tract motility - mechanical digestion
submucosal plexus controls
coordinates local secretions and local blood flow
chemical digestion
intrinsic reflexes
enteric nervous system only
control secretion, peristalsis, local effects
extrinsic reflexes
prevertebral ganglia
transmission of long distance signals
entero-gastric reflex
acidic duodenum contents promote colonic contractions
gastrocolic reflex
stretched stomach increases colon motility causing evacuation - so stomach can empty into duodenum
gastro-ileal reflex
stretch stomach causes ileocaecal valve to open
long reflexes
uses the CNS
vago-vagal reflex
pain reflex defecation reflex
3 things regulating hormonal control of motility
secretin
cholecystokinin
motilin
motilin
starts migrating motor complex
cholecystokinin (CCK) and secretin
produced by duodenum in response to food
once contents have left the stomach - they slow down the stomach to prevent overload of the duodenum
secretin produced in response to
acid (food)
cholecystokinin is produced in response to
FAs/peptides (food)
motilin initiates
interdigestive clearing - promotes migrating motor complex
motilin produced by
small intestine M cells
motilin is surpassed by
feeding
phases of digestion
cephalic phase
gastric phase
intestinal phase
cephalic phase
largely neural control
smell, sight and thought of food
gastric phase
neural and hormonal control
food enters stomach
intestinal phase
largely hormonal control
begins when food enters small intestine
inter digestive phase
when there is no food
what does cephalic phase do
reflex from taste buds stimulates vagal centre in the CNS
cranial nerves from medulla stimulate secretion from salivary glands, stomach, pancreas
vagus also increases motility of stomach, intestine
what does gastric phase do
food in the stomach stimulates vago-vagal reflexes and the release of hormones
stimulates gastric and pancreatic secretion, priming for digestion
hormones further stimulate gastric secretion
what does intestinal phase do
food releases stimulatory hormones from the gut mucosa
- hormones stimulate secretion of exocrine pancreases, contraction of gallbladder
- inhibitory hormones acting on stomach
what is excreted during the intestinal phase
HCO3- to nuetralise the acid, enzymes to further digest food,
gallbladder delivers bile salts into intestines
why is the stomach inhibited during the intestinal phase
to prevent overloading the intestines
