Transcript Regulation of Gastric juice secretion

Regulation of Gastric juice
secretion
M.I.Glad Mohesh,
Assistant Professor,
Department of Physiology,
Shri Sathya Sai Medical College & RI,
Ammapettai, Kancheepuram District,
Tamilnadu state, India.
[email protected]
Specific learning objectives,
At the end of this class you will be able to,
1.Describe how gastric juice secretion is regulated
during digestive phase in stomach.
2.Describe the factors that stimulate secretion of
gastric juice in stomach.
3.Describe the factors that inhibit secretion of gastric
juice in stomach.
4.Explain the physiological basis of the action of
Cimetidine.
Ivan Petrovich Pavlov
(1849-1936)
Russian Physiologist
Nobel Prize for Physiology
or Medicine in 1904
Canis lupus
familiaris
Secretions in the stomach
Gastric juice secretion is,
*Accurately synchronized with the need for it.
*Secretion persists since, food is about to
enter the stomach and continues till food is
present in the stomach.
Digestive phase x Inter-Digestive phase.
Secretion is regulated by,
Neurocrine /endocrine/ paracrine methods.
Agents that stimulate parietal cells.
1.Acetylcholine.
Post ganglionic parasympathetic fibres (vagus)
Acetylcholine
Parietal cells
G cells(gastrin)
(HCL)
Atropine-blocker.
ECL cells
(histamine)
2.Gastrin : G cells in Antral mucosa
Expt : gastric antral removal -- gastrin secretion.
Vagal stimulation – Ach - gastrin
Inhibited by – somatostatin
Gastrin + Ach – more powerful ( potentiation)
(A+B+C) > (A)+(B)+(C)
3.Histamine:
*very potent stimulator of gastric acid secretion
(mast cells - ECL cells (enterochromaffin cells)
Histamine
H2 receptors
parietal cells
Eg) Cimetidine –H2 receptor antagonist – gastric
secretions.
Agents that inhibit parietal cells
1.Acid
Secreted acid--inhibits release of gastrin
from G cells
Negative feedback mechanism
(prevents damage of gastric mucosa)
Acidic chyme –enters duodenum-gastric secretion is
further inhibited.
(intramural neural reflex operating via pH sensitive
duodenal receptors).
2.Somatostatin
D cells – inhibits {G cells + ECL cells}
Stimulus-gastric acidity
Negative feedback mechanism
Others involved- gastrin, CCK-PZ, secretin and
vasoactive intestinal peptide{VIP},prostaglandins
Stimulus-entry of acid & food into the duodenum.
3.Food
*Carbohydrates, lipids & high osmolar contents of the
chyme entering the duodenum.
GIP & CCK-PZ Inhibits gastric juice secretion
Substances that alter HCl secretion
Digestive phase regulation can be explained in
terms of 3 arbitrary phases,
1.Cephalic phase;
Sight/Smell or even thought of food—stomach
starts secreting the gastric juice.
Stimulus 1.Unconditioned reflex : food in mouth
2.Conditoned reflex : smell/ sight/
thought or awareness of food in the mouth.
(psychic or appetite juice)
30% of total gastric juice secretion secreted.
Experimental evidence:
1.Sham feeding : Animal feels that it is eating
whereas nothing really reaches its stomach.
Oesophageal fistula
Mediated by : Vagus nerve
Proof: Vagotomised animal does not show any
secretion.
Vagus – Ach + GRP released on G cell – gastrin
secretion- gastric juice secretion.
Pavlov’s esophagostomy and sham-feeding experiments
http://physiologyonline.physiology.org/content/19/6/326
2.Gastric phase:
Food is in the stomach
*Secretion rate is increased.
*60% of the gastric juice secretion during the
digestive phase of stomach happens in this phase.
Stimuli:
1.distension of the stomach
2.chemicals released due to distension
3.chemicals present in the food.
Expt: Infusing saline directly into the stomach.
Proof: 20-50% of the peak acid output(PAO).
Via Vagus nerve
afferent
convey the information to CNS
Vagus
efferent
Motor nucleus of vagus
gastric secretion
Expt :Role of vagus nerve
Sectioning of one of the two vagi leaving the
other intact + stimulation of the central end of
vagus - gastric secretion
Stretching of the gastric mucosa by the presence
of food in stomach leads to secretion of acid and
enzyme.
1.Long vagovagal reflex action –Ach secretion by
vagus.
2.Local reflexes in the intrinsic neural plexus.
stretch of mucosa causes activation of Meissner’s
plexus which stimulates secretions.
local reflex
Stretch
vagovagal reflex.
2.Chemical stimuli
Expt : 1) Watery meal infusion
60-70% of peak acid output
2) equivalent amount of saline
Most important stimuli,
a) Proteins b) partially digested proteins and
aminoacids.
Humoural mechanism
Gastrin + Acetylcholine + Histamine
a) Gastrin
G cells in the antrum
stimuli – a) distension of the gastric mucosa by
food in the stomach.
b) chemicals released on digestion,
1) peptones + aminoacids
Released gastrin increases gastric juice secretion
By some hypothetical pathways.
1) incr.HCL + pepsin secretion
2) Incr. Histamine secretion from ECL cells.
b) Acetylcholine
stimuli: distension of gastric mucosa
Ach release from vagus—acts via M1 receptors –
can be blocked by atropine.
Ach – release of Histamine from ECL cells.
c) Histamine
ECL (mast cells) in gastric mucosa.
distension of gastric mucosa.
Ach & gastrin stimulates its secretion
acts via H2 receptors – Cimetidine and Ranitidine.
Intestinal Phase
Presence of food in intestine-mechanical & chemical
stimuli.
Mechanism is unclear-how it regulates gastric
secretion.
presence of chyme in duodenum- Enterooxyntin
contribution is very minimal
Actually this phase has a major role in inhibition of
gastric juice secretion.
a) Acid / fat and hyper osmolar substances enters
duodenum.
Duodenum and jejunum secretes,
a) secretin, b) bulbogastrone, c)CCK PZ, d)GIP
Inhibits gastrin release –gastric juice secretion.
b) Enterogastric reflex.
food distends duodenum- local plexus is activatedinhibits through sympathetic fibres-secretion &
motility of the stomach is inhibited.
Stimuli : Fat
Cerebral cortex, hypothalamus and limbic system
influences gastric secretion.
INTER DIGESTIVE Phase.(Resting Phase)
No food in stomach – minimal secretion of
Gastric juice
Represents basal gastric secretion.
References
1.Textbook of Medical Physiology,11th edition,
Guyton & Hall, Arthur C Guyton, John E Hall.
2.Understanding Medical Physiology,4th edition,
Bijlani, Manjunatha.S.
3.Fundamentals of Medical Physiology,4th edition,
L Prakasam Reddy.