physiology of gastric motility and secretion Flashcards
characteristics of the stomach
> orad region of the stomach relaxes receptively to accommodate food from oesophagus
starting point of digestion of proteins ie pepsin and continues carbohydrate digestion ie amylase
mixes food with gastric secretions to produce semi-liquid chyme
stores food before passing it into small intestine as chyme for further digestion and absorption
secretes gastric juice from gastric glands in the gastric mucosa
what are the two types of mechanical activity of the stomach
orad stomach - fundus and proximal body - tonic
caudad stomach - distal boy and antrum - phasic
what does the orad region of the stomach do
> relaxation driven by vagus occurs during a swallow permitting storage of ingested material
no slow wave activity
tonic contractions, when they occur are weak due to relatively thin musculature
the contents are propelled intermittently to caudad region by low amplitude tonic contractions of about 1 minute duration - decrease stomach size as it empties
minimal mixing of contents for long period s - allows for carbohydrate partial digestion
what does the caudad region of the stomach do
> slow waves occur continuously but only those reaching threshold illicit a contraction
phasic peristaltic contractions driven by suprathreshold slow waves progress from mid stomach to gasproduodenal junction ( the antra wave or pump ) propelling content towards the pylorus through which is a very small volume of chyme flows into the duodenum
velocity of contraction increases towards the junction overtaking the movement of chyme that rebounds against constricted distal antrum back into the relaxed body of the stomach
what is retropulsion
basically when (peristaltic) contraction speed is faster than the chyme moving so it hits the wall (the antrum) and falls back into the body of the stomach
what is the purpose of retropulsion
it mixes gastric contents reducing chyme to small particles that pass through the pylorus
what is the delivery of chyme to the duodenum governed by
gastric and duodenal factors
gastric factors
> consistency of chyme
rate of emptying proportional to volume of chyme in stomach
*distension increases motility due to:
stretch of smooth muscle
stimulation of intrinsic nerve plexuses
increased vagus nerve activity and gastrin release
duodenal factors
> duodenum must be ready to receive chyme - delays emptying by:
neurohormonal response ie enterogastric reflex
hormonal response
stimuli within the duodenum that drive the neuronal and hormone responses include :
fat
acid
hypertonicity
distension
secretion - the mucosa of the stomach are classed as ..
> oxyntic gland area
>pyloric gland area
what is gastric mucosa composed of
> a surface lining the stomach
pits, invaginations of the surface
glands at the base of the pits responsible for several secretions
gastric secretions
OM = HCl pepsinogen intrinsic factor and gastroferrin histamine mucus PGA = gastrin somatostatin mucus
HCl
activates pepsinogen to pepsin
denatures protein
kills most micro-organisms ingested with food
>secreted by gastric parietal cell
pepsinogen
inactive precursor of the peptidase - pepsin
intrinsic factor and gastroferrin
binds with B12 and iron respectively facilitating subsequent absorption
histamine
stimulates HCl secretion
mucus
protective
gastrin
stimulates HCl secretion and motility
somatostatin
inhibits HCl secretion
what are the three important secretagogues that induce acid secretion from the parietal cells
ACh
gastrin
histamine
**acting by direct and indirect pathways
direct pathway
the secretagogues stimulate the parietal cell triggering the secretion of hydrogen ions into the lumen
indirect pathway
ACh and gastrin aslo stimulate the ECL cell resulting in secretion of histamine
this histamine then acts on the parietal cell
what stimuli is needed for the secretion of hydrogen ions
PLC-IP3 (gastrin ACh) and cAMP-PKA (histamine) signalling pathways
what stimuli is needed for the inhibitions of hydrogen secretion
cAMP-PKA (somatostatin, prostaglandins) signalling pathways
**histamine = stimulation
somatostatin and prostaglandins = inhibition
of adenylate cyclase
relationship between secretagogues and H/K ATPase
can cause trafficking of the pump
> in the resting state the pump is largely within the cytoplasmic tubulovesicles
but in the stimulated state the pump traffics to the apical membrane taking residence in extended microvilli
what are the three phases of gastric acid secretion
CGI !
cephalic
gastric
intestinal
what is gastric secretion rate controlled by
stimulatory and inhibitory mechanisms
CGI
cephalic phase
before food reaches the stomach
>driven directly and indirectly by the CNS and vagus nerves
Vagus stimulates enteric neurones that:
release ACh directly activating parietal cells (neurotransmitter action)
via release of GRP causes release of gastrin from G cells in to systemic circulation that activates parietal cells (endocrine action)
via release of histamine from ECL cells that locally activates parietal cells (paracrine action)
via inhibition of D cells decreases the inhibitory effect of ss on G-cells
gastric phase
when food is in stomach
>involves both physical and chemical mechanisms
distension of stomach activates reflexes that cause acid secretion
food buffers pH, D cell inhibition via ss of gastrin release is decreased
amino acids (e.g. tryptophan, phenylalanine) stimulate G cells. Other stimulants include: Ca2+, caffeine and alcohol
intestinal phase
after food has left the stomach
>chyme entering the upper small intestine cause weak stimulation of gastric secretion via neuronal and hormonal mechanisms
how is gastric acid secretion inhibited
> cephalic = vagal nerve activity decreases upon cessation of eating and following an empty stomach
gastric phase = antral pH falls when food exits stomach - D cells start releasing somatostatin again
intestinal phase = the factors that reduce gastric motility also reduce gastric secretion
**pain, nausea and negative emotions also decrease vagal nerve activity and increase sympathetic activity that combined reduce gastric acid secretion
