Ch Gastrointestinal System By Wei yuanyuan. Introduction Basic processes of digestion and absorption...

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