Transcript Gastric secretion

Ch Gastrointestinal System
By Wei yuanyuan
Introduction
Basic processes
of digestion and absorption
1.Propulsion and mixing of food in the
alimentary tract
2.Secretory functions of the alimentary tract
3.Digestion and absorption in the gastrointestinal
tract
Give a accurate explanation for
Digestion
Absorption
Before ingested food can be absorbed,it must be
dissolved and broken down into small
molecules.This process is called digestion.
The molecules produced by digestion then move
from the lumen of the alimentary tract across a
layer of epithelial cells and enter the blood or
lymph.This process is called absorption.
Functions of the digestive system
Motility:mix the luminal contents with the various
secretions and propels food through the digestive
system.
Secretion:release of digestive juices in response to a
specific stimulus.
Digestion:breakdown of food into molecular components
small enough to cross the plasma membrane.
Absorption: the molecules produced by digestion then
move from the lumen of the gastrointestinal tract
across a layer of epithellial cells and enter the blood
Anatomy:
Components of the
digestive system
Structure of the alimentary canal
Structure of the alimentary canal


Mucosa
Submucosa
circular muscle

Muscularis
longitudinal muscle


Serosa
Smooth muscle
General properties of gastrointestinal smooth muscle

Low excitability

High distensibility stomach 50ml-1000ml

Tonic contraction
maintain shape and location of GI tract
it is the basis of other movements as well.


High sensitivity to temperature,stretch and chemical
stimulation
Autorhythmicity
Mechanism of autorhymicity
Electrophysiological properties of gastrointestinal
smooth muscle

Resting membrane potential
-50～-60mv
Ionic basis
Em (selective membrane
permeability to K+, Na+ ,Cl-and Ca2+)
Electrogenic Na+ -K+ pump
Slow wave (basic electrical rhythm)




It is a kind of spontaneous slow and recurring
depolarization wave based on resting membrane
potential.
Mechanism:
Initiated in the interstitial cells of
Cajal(ICC)(pacemaker cell)
Intensity:5～15mv
Frequency:3 ～12cpm
Santiago Ramon Y Cajal

He and Camillo Golgi
received the Nobel Prize
in 1906 for introduction
of the silver-chromate
stain
Normal BER frequencies in the gastrointestinal
system
Spike potential (Action potential)


Duration:10～20ms
Ionic mechanism:
Depolarization:Ca2+ influx
Repolarization:K+ efflux
Innervation of the Gut


Autonomic nervous system(extrinsic)
Enteric nervous system (intrinsic)
Neural control of gastrointestinal function


Autonomic nervous system
sympathetic nerve
parasympathetic nerve vagus nerve
pelvic nerve
Enteric nervous system(ENS)
myenteric plexus
submucosal plexus
Autonomic nervous system
Sympathetic nerve
NE
Inhibitory(-)
Parasympathetic nerve
Mainly ACh
Stimulatory(+)
GI reflexes

Short reflexes
from receptors through the nerve plexuses
to effector cells

Long reflexes
from receptors in the tract to the CNS by
way of afferent nerves and back to the nerve
plexuses and effector cells by way of
autonomic nerve fibers
GI hormone


The hormones synthesized by a large number of
endocrine cells within the gastrointestinal tract
Physiological functions
Control of the digestive function
Control of the release of other hormones
Trophic action
GI hormone

Four main types
Gastrin
Secretin
Cholecystokinin (CCK)
Gastric inhibitory peptide(GIP)
GI peptides
GI peptides
source
hormone
distribution
G cell
Gastrin
Antrum,Duodenum
I cell
CCK
Duodenum,Jejunum
K cell
GIP
Duodenum,Jejunum
Mo cell
Motilin
Stomach,small intestine
S cell
Secretin
Duodenum,Jejunum
Mouth and esophagus

Saliva
secreted by the three pairs of glands:
submaxillary gland
sublingual gland
parotid gland
Salivary gland
saliva

Functions of saliva
1 Moistening and lubricating the food
particles before swallowing.
2 It also contains the enzyme amylase,which
partially digests polysaccharides.
3 Dissolving some of food molecules.
Control of salivary secretion
Mastication
Smell or
Sight of food
Conditioned
reflex
+
Salivary center
-
Fatigue
dehydration
dread
Parasympathetic
nerve
Ⅶ Ⅸ
ACh
Sympathetic
nerve
MR
NE
Β
R
Salivary gland
Salivary secretion↑
（more、watery）
Salivary secretion↑
（less、viscous ）
Note:


1.There is no hormonal regulation of salivary secretion.
2.Unlike their antagonistic activity in most organs,both
sympathetic and parasympathetic systems stimulate salivary
secretion.
Chewing and swallowing
bolus
(一)Chewing
•Saliva begins carbohydrate
digestion---amylase
•Hygiene---lysozyme
•Facilitating speech
swallowing
(二)swallowing
three stages：
oropharyngeal stage：oral cavity
→pharynx voluntary movement
pharyngeal stage：pharynx→upper
end of esophagus
esophageal stage：
esophagus→stomach (Peristalsis)
swallowing
pharyngoesophageal sphincter
1. Subatmospheric intrapleural pressure.
2.Preventing air from entering the esophagus
and stomach during breathing.

Peristalsis：active process
pharynx→upper end of esophagus
Primary peristaltic wave:controlled
by the swallowing center
Secondary peristaltic wave:intrinsic
nerve plexuses
swallowing
esophageal stage：
 Lower esophageal sphincter（gastroesophageal
sphincter）
1. a higher-pressure zone of 5-10mmHg above
intragastric pressure.
2.Preventing reflux of the stomach’s contents into the
esophagus.
3.heartburn

Digestion in the stomach
stomach
Function:
1 Store ingested food until it can be emptied into the
small intestion
2 Secrete HCl and enzymes that begin protein
digestion.
3 Through the stomach’s mixing movements,the
ingested food is pulverized and mixed with gastric
secretions to produce a thick liquid mixture known as

chyme.


Oxyntic mucosa:lines the body and fundus
Gastric secretion
Pyloric gland area(PGA):lines the antrum
Gastric secretion
oxyntic mucosa:
Mucous neck cell:
line the entrance or neck of the gastric pit
secrete a thin,watery mucus
 Chief cells
deeper portions of the pit
secrete the enzyme precursor pepsinogen
 Parietal cells:
outer wall
HCl and intrinsic factor

surface
epithelial
cells:
Gastric
secretion


Between the gastric pits,line the gastric mucosa
Secrete a thick,viscous,alkaline mucus
pyloric
glandsecretion
area:
Gastric



Primarily secrete mucus + small amount of
pepsiogen
Gastrin:telecrine
No acid
Gastrin:




major stimulus :protein
Stimulate the parietal and chief cells
Trophic to the mucosa of the stomach and
small intestine
The most potent stumulant of the parietal cells
Gastric juice


Properties
pH 0.9-1.5
1-2.5 L/day
Major components
Hydrochloric acid
Pepsinogen
Mucus
Intrinsic factor
Hydrochloric acid


Secreted by the parietal cells
Output
Basal:0-5 mmol/h
Maximal:20-25 mmol/h
How does HCl produced?
Mechanism of HCl
secretion
Active transport
2. Huge H+ gradient
(3 million)
3. Omeprazole,OMZ
1.
Functions of HCl





Convert pepsinogen into pepsin,and provide low pH
evironment for pepsin’s action.
Dissolve the particulate matter in food
To stimulate pancreatic and bile secretions when acid
enters in the duodenum
Assisted absorption of iron and calcium
Kill most of the bacteria that enter along with food.
Regulation of gastric acid secretion
Regulation of gastric acidsecretion

Humoral regulation
Excitatory factor
Receptor
Blocker
Ach
Vagus nerve
M3R
atropine
Histamine
ECL
H2R
cimetidine
Gastrin
Gcell
CCK2
proglumide
Regulation of gastric acidsecretion

Humoral regulation
Inhibitory
Somatostatin,SS
Secretin
K+ H+
Omeprazole
H2
Histamine
cimetidine
Parietal
cell
M3
Ach
atropine
Gastrin receptor
gastrin proglumide
图：胃溃疡的主要发病机制及药物治疗。
Pepsinogen


Secreted by the chief cells as an inactive
precursor of pepsin
Activated in the stomach,initially by H+ ions
and then by active pepsin,is referred to as
autocatalytic(self-activating) process.
Pepsinogen
Function:
It hydrolyzes peptides bonds of ingested protein
molecules into peptones,and polypeptides,with little
amino acids
Note:
If it absence,protein can be completely digested by
enzymes in the small intestine.

How stomach protect itself?
Mucus-HCO3 barrier
Gastric Mucosal barrier

Tight junction between epithelial cell
Intrinsic factor




Secreted by the parietal cell
Essential for the absorption for vitamin B12. endocytosis
Receptor:cubulin , which is located only in terminal ileum
Note:
surgical removal of fundus and body of stomach results
in B12 deficiency and pernicious anemia
megaloblastic anemia
Regulation of gastric secretion
Regulation of gastric secretion
Nervous regulation
 Short reflex pathways
 Long autonomic pathways
long excitatory reflexes:
parasympathetic
long inhibitory pathways:
sympathetic
Phases of gastric secretion

Cephalic phase
30% of total gastric juice with higher acidity
and pepsinogen is secreted during this phase.
Sham feeding
Cephalic phase
feed-forward fashion
Smelling,chewing,
swllowing food
Vagus
nerve
Intrinsic
plexuses
PGA
(pyloric gland
area)
HCl
pepsinogen
Gastrin
Phases of gastric secretion
Gastric phase
60% of total gastric juice with higher acidity is
secreted during this phase.
the pepsinogen concentration secreted is less
than in the cephalic phase.

gastric phase
+
Sti.in stomach
Protein,distention
caffine,alcohol
+ Vagus
+
Intrinsic
plexuses
HCl
pepsinogen
nerve
+
PGA
(pyloric gland
area)
+
Gastrin
Phases of gastric secretion

Intestinal phase 10%
only humoral mechanisms involved in this phase.
intestinal phase
Sti.in the duodenum:
digested protein
products
+
Intestinal
gastrin
HCl
pepsinogen

How is the flow of gastric juices shut off as
chyme begins to be emptied from the
stomach into the small intestine?
Inhibition of gastric secretion
Inhibition of gastric secretion


Protein is withdrawn
HCl itself ( pH ＜2)-inhibits the PGA from releasing
gastrin
negative feedback
2.High acidity ( pH ＜2.5) in the duodenum triggers
enterogastric reflexes and enterogastrone that
inhibit gastric acid secretion.
Inhibition of gastric secretion


3 Distention,hypertonic solution,solutions containing
amino acids and fatty acids in the small intestine.
Enterogastrones:the hormones released by the
intestinal tract that reflexly inhibit gastric activity
are collectively called
Gastric motility
Motor function of the stomach


Proximal stomach
cardia
fundus
body
Distal stomach
antrum
pylorus
pyloric sphincter
Motor function of the stomach


Receptive relaxation-proximal portion
storage function(1-1.5L)
vago-vagal reflex
Peristalsis-distal portion
BER in the stomach
Contractions in the empty stomach


Migrating motor complex (MMC)
periodic waves of contraction,which move along the
gastrointestinal tract from stomach to colon
purpose of this activity:to “sweep”debris out of the
digestive tract during the interdigestive period
MMCs can lead to hunger contractions ,which are
associated with discomfort,referred to as “hunger pains”
Emptying of the stomach

Emptying rate
fluid ＞viscous
small particle ＞large particle
isotonic ＞hyper-＆hypo-osmotic
carbohydrates ＞protein ＞fat
Regular meal 4-6hrs
Regulation of stomach emptying


Gastric factors that promote emptying
Gastric food volume
Gastrin
Duodenal factors that inhibit stomach emptying
Fat
hypertonicity
distention
Enterogastric nervous reflexes