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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
Mucosa Submucosa circular muscle Muscularis longitudinal muscle Serosa
Structure of the alimentary canal
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 systemSympathetic nerve NE
Inhibitory(-)
Parasympathetic nerve
Mainly AChStimulatory(+)
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.
MasticationSmell orSight of
foodConditioned
reflex
Fatiguedehydration
dreadSalivary center
Parasympathetic nerve
Salivary secretion↑（ more 、 watery ）
NE
Ⅶ Ⅸ
+ -
ACh
MR
ΒR
Sympathetic nerve
Control of salivary secretion
Salivary secretion↑ （ less 、 viscous
）
Salivary gland
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 sphincter1. Subatmospheric intrapleural
pressure.2.Preventing air from entering the
esophagus and stomach during breathing.
Peristalsis ： active processpharynx→upper end of esophagusPrimary peristaltic wave:controlled by the swallowing centerSecondary 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 intestion2 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 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
Gastric secretion
surface epithelial cells: Between the gastric pits,line the gastric
mucosa Secrete a thick,viscous,alkaline mucus
Gastric secretion
pyloric gland area: Primarily secrete mucus + small amount of
pepsiogen Gastrin:telecrine No acid
Gastric secretion
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
1. Active transport
2. Huge H+ gradient
(3 million)
3. Omeprazole,OMZ
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
InhibitorySomatostatin,SSSecretin
H+K+
gastrin
Histamine AchH2 M3
Gastrin receptor
Parietal cell
图：胃溃疡的主要发病机制及药物治疗。
proglumide
cimetidine
atropine
Omeprazole
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 phasefeed-forward fashion
Smelling,chewing,
swllowing food
Intrinsicplexuses
PGA(pyloric
gland area)
Vagusnerve
Gastrin
HCl pepsinogen
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
Intrinsicplexuses
PGA(pyloric
gland area)
Vagusnerve
Gastrin
HCl pepsinogen
+
+
+
+
+
Phases of gastric secretion Intestinal phase 10% only humoral mechanisms involved in this
phase.
intestinal phase
Sti.in the duodenum:
digested protein products
Intestinalgastrin
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
