Gastric Motility, Secretion and Digestion Flashcards
capacity of the stomach
orad region relaxes receptively to accomadate food from oesophagus
what nerve relaxes the stomach
vagus (CN X)
functions of the stomach
digestion;
proteins
carbohydrates
mixes food with gastric secretions, producing chyme
limited amount of absorption
stores food before passing to small intestine
enzymes for protein digestion
pepsin
HCl
enzymes for carbohydrate digestion
salivary amylase
secretion of gastric juice from stomach
secretes gastric juice from gastric gland in the gastric mucosa
describe fundus of the stomach
next to oesophagus
thin smooth muscle layer
function of fundus
receives food, little mixing
little food stored here - usually a pocket of gas
describe body of the stomach
middle section
thin smooth muscle layer
function of body
little mixing
food stored here
describe antrum of stomach
next to duodenum
thick smooth muscle layer
function of antrum
highly contractile
much mixing at time of gastric secretions
mechanical activity of the stomach
2 types;
orad stomach
caudad stomach
describe orad stomach
fundus and proximal body
tonic - maintained
describe caudad stomach
distal body and antrum
phasic - intermittent
electrical and mechanical activity of the orad region - relaxation
occurs during swallow, permitting storage of ingested material
no slow activity
tonic contractions - weak due to relatively thin musculature
contents propelled intermittently to caudad region by low amplitude tonic contractions – decrease stomach size as it empties
minimal mixing of contents for long periods allows for carbohydrate partial digestion
hormone that decreases contractions (and stomach emptying) in the orad region
gastrin
electrical and mechanical activity of caudad region
slow waves occur simultaneously, only those that reach threshold contract
phasic peristaltic contractions driven by suprathreshold slow waves progress from midstomach to gastroduodenal junction (the antral wave, or pump) propelling contents towards pylorus through which 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 – this is retropulsion
retropulsion mixes gastric contents reducing chyme (‘grinding’ function) to small particles that pass through the pylorus
what controls the emptying of the stomach
strength of antral wave, or pump, determines the escape of chyme through pyloric sphincter
what governs the strength of the antral wave
gastric factors
duodenal factors
gastric factors
rate of emptying proportional to volume of chyme in the stomach - distension increases motility
consistency of chyme - emptying facilitated by thin liquid chyme
gastric factors - how does distension increase motility
stretch of smooth muscle
stimulation of intrinsic nerve plexus
increased vagus nerve activity and gastrin release
duodenal factors
duodenum must be ready to receive chyme, it can delay entry
duodenal factors - how does the duodenum delay emptying
neuronal response - entergastric reflex
hormonal response - release of enteragastrones
enterogastric reflex
decreases antral activity by signals from intrinsic nerve plexus and the ANS
enterogastrones
e.g. cholecystokinin CCK
inhibits stomach contraction
stimuli within duedenom driving neuronal and hormones response
fat (potent)
acid
hypertonicity
distension
how does fat within duedenom drive neuronal and hormones response
delay in gastric emptying required for digestion and absorption in small intestine
how does acid within duedenom drive neuronal and hormones response
time is required for neutralisation of gastric acid by bicarbonate secreted from the pancreas - important for optimal function of pancreatic digestive enzymes
how does hypertonicity within duedenom drive neuronal and hormones response
products of carbohydrate and protein digestion are osmotically active and draw water into the small intestine - danger of reduced plasma volume and circulatory disturbances
e.g. dumping syndrome
how is the mucosa of the stomach class in consideration of secretion
the oxyntic gland area (proximal stomach including the fundus and body)
the pylorlic gland area (distal stomach, designated the antrum)
what is gastric mucosa composed of
surface lining the stomach
pits, invaginations of the surface
gland, at the base of the pits responsible for several secretions
functions of oxyntic mucosa
secretes; HCl pepsinogen intrinsic factor and Gastroferrin histamine mucus
HCl function in secretion
activates pepsinogen to pepsin
denatures protein
kills most micro-oragnisms ingested with food
pepsinogen function in secretion
inactive precursor of the peptidase, pepsin
intrinsic factor and gastroferrin function in secretion
bind vitamin B12 and iron respectively, facilitating subsequent absorption
histmaine function in secretion
stimulates HCl secretion
somatostatin function in secretion
inhibits HCl secretion
what regulates/induces HCl secretion from gastric parietal cell
ACh
gastrin (parietal cell)
histamine
functions of pyloric gland area secretion
gastrin
somatostatin
mucus
gastrin function in secretion
stimulates HCl secretion
receptor and signal-transduciton pathways for secretion of HCl from gastrin
gastrin, ACh - PLC-IP3
histamine - cAMP PKA
receptor and signal-transduciton pathways for inhibiting HCl from gastrin
somatostatin, prostaglandins - cAMP - PKA
3 phases of gastric acid secretion
cephalic phase (in the head) gastric phase intestinal phase
cephalic phase
before food reaches the stomach preparing it stomach to receive food
nerve control of cephalic phase - neurotransmitter action
vagus nerve (CN X); release ACh - directly activating parietal cells
gastric phase
when food is in stomach - involving both physical and chemical mechanisms
intestinal phase
after food has left stomach;
chyme entering upper small intestine causes weak stimulation of gastric secretion via neuronal and hormonal mechanisms
nerve control of cephalic phase - endocrine action
vagus nerve;
via release of GRP, causing release of gastrin from G cells into systemic circulation - activating parietal cells
nerve control of cephalic phase - paracrine action
vagus nerve;
via release of histmaine from ECL cells that locally activate parietal cells
nerve control of cephalic phase - D cells
vagus nerve;
via inhibition of D cells, decreasing the inhibitory effect of ss on G-cells
physical and chemical mechanisms of the gastric phase
distension of stomach activates reflexes that cause acid secretion
food buffers pH, D cel inhibition via ss of gastrin release is decreased
amino acids stimulate G cells
other stimulates - calcium, caffeine, alcohol
phases of inhibition of gastric acid secretion
cephalic phase
gastric phase
intestinal phase
phases of inhibition of gastric acid secretion - cephalic phase
vagal nerve activity decreases upon cessation of eating and following stomach emptying
pain, nausea and negative emotions also decrease vagal activity and increase sympathetic activity - reducing gastric secretion
phases of inhibition of gastric acid secretion - gastric phase
antral pH falls when food exits stomach (due to decreased buffering of gastric HCl) – release of somatostatin from D cells recommences, decreasing gastrin secretion prostaglandin E2 (PGE2) continually secreted by the gastric mucosa acts locally to reduce histamine- and gastrin-mediated HCl secretion
phases of inhibition of gastric acid secretion - intestinal phase
The factors that reduce gastric motility also reduce gastric secretion (e.g. neuronal reflexes, enterogastrones)
