physiology of gastric motility and secretion

Question 1

characteristics of the stomach

Answer 1

> 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

Question 2

what are the two types of mechanical activity of the stomach

Answer 2

orad stomach - fundus and proximal body - tonic

caudad stomach - distal boy and antrum - phasic

Question 3

what does the orad region of the stomach do

Answer 3

> 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

Question 4

what does the caudad region of the stomach do

Answer 4

> 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

Question 5

what is retropulsion

Answer 5

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

Question 6

what is the purpose of retropulsion

Answer 6

it mixes gastric contents reducing chyme to small particles that pass through the pylorus

Question 7

what is the delivery of chyme to the duodenum governed by

Answer 7

gastric and duodenal factors

Question 8

gastric factors

Answer 8

> 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

Question 9

duodenal factors

Answer 9

> 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

Question 10

secretion - the mucosa of the stomach are classed as ..

Answer 10

> oxyntic gland area

>pyloric gland area

Question 11

what is gastric mucosa composed of

Answer 11

> a surface lining the stomach
pits, invaginations of the surface
glands at the base of the pits responsible for several secretions

Question 12

gastric secretions

Answer 12

OM =
HCl 
pepsinogen 
intrinsic factor and gastroferrin 
histamine
mucus 
PGA = 
gastrin 
somatostatin 
mucus

Question 13

HCl

Answer 13

activates pepsinogen to pepsin
denatures protein
kills most micro-organisms ingested with food
>secreted by gastric parietal cell

Question 14

pepsinogen

Answer 14

inactive precursor of the peptidase - pepsin

Question 15

intrinsic factor and gastroferrin

Answer 15

binds with B12 and iron respectively facilitating subsequent absorption

Question 16

histamine

Answer 16

stimulates HCl secretion

Question 17

mucus

Answer 17

protective

Question 18

gastrin

Answer 18

stimulates HCl secretion and motility

Question 19

somatostatin

Answer 19

inhibits HCl secretion

Question 20

what are the three important secretagogues that induce acid secretion from the parietal cells

Answer 20

ACh
gastrin
histamine

**acting by direct and indirect pathways

Question 21

direct pathway

Answer 21

the secretagogues stimulate the parietal cell triggering the secretion of hydrogen ions into the lumen

Question 22

indirect pathway

Answer 22

ACh and gastrin aslo stimulate the ECL cell resulting in secretion of histamine
this histamine then acts on the parietal cell

Question 23

what stimuli is needed for the secretion of hydrogen ions

Answer 23

PLC-IP3 (gastrin ACh) and cAMP-PKA (histamine) signalling pathways

Question 24

what stimuli is needed for the inhibitions of hydrogen secretion

Answer 24

cAMP-PKA (somatostatin, prostaglandins) signalling pathways

**histamine = stimulation
somatostatin and prostaglandins = inhibition
of adenylate cyclase

Question 25

relationship between secretagogues and H/K ATPase

Answer 25

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

Question 26

what are the three phases of gastric acid secretion

Answer 26

CGI !
cephalic
gastric
intestinal

Question 27

what is gastric secretion rate controlled by

Answer 27

stimulatory and inhibitory mechanisms

CGI

Question 28

cephalic phase

Answer 28

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

Question 29

gastric phase

Answer 29

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

Question 30

intestinal phase

Answer 30

after food has left the stomach
>chyme entering the upper small intestine cause weak stimulation of gastric secretion via neuronal and hormonal mechanisms

Question 31

how is gastric acid secretion inhibited

Answer 31

> 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