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

G ive 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 Ca 2+ ) 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:Ca 2+ 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

sourcehormonedistribution G cellGastrinAntrum,Duodenum I cellCCKDuodenum,Jejunum K cellGIPDuodenum,Jejunum Mo cellMotilinStomach,small intestine S cellSecretinDuodenum,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.

Mastication Smell or Sight of food Conditioned reflex Fatigue dehydration dread Salivary center Parasympathetic nerve Salivary secretion ↑ （ more 、 watery ） NE Ⅶ Ⅸ + - ACh MR ΒRΒR 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 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  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 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. O meprazole,OMZ O meprazole

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

 Humoral regulation Excitatory factor ReceptorBlocker AchVagus nerveM3Ratropine HistamineECLH2Rcimetidine GastrinGcellCCK2proglumide

Regulation of gastric acidsecretion  Humoral regulation Inhibitory Somatostatin,SS Secretin

H+H+ K+K+ gastrin Histamine Ach H2H2 M3M3 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-HCO 3 barrier

Gastric Mucosal barrier  Tight junction between epithelial cell

Intrinsic factor  Secreted by the parietal cell  Essential for the absorption for vitamin B 12. 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 megaloblasticanemia

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 Intrinsic plexuses PGA (pyloric gland area) Vagus nerve 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 Intrinsic plexuses PGA (pyloric gland area) Vagus nerve 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 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

 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