Gastrointestinal Physiology

UNIT Ⅻ

GSATROINTESTINAL PHYSIOLOGY

Metabolism

Nutrients

Water Electrolytes Vitamine carbohydrate protein fat

Food

Alimantary tract

Chapter 62 General principles of gastrointestinal function Chapter 63 Propulsion and mixing of food in the alimentary tract Chapter 64 Secretory functions of the alimentary tract Chapter 65 Digestion and absorption in the alimentary tract

Chapter 62 General principles of gastrointestinal function

the activity of gastrointestinal smooth muscle

Control of gastrointestinal functions by nervous and hormonal systems

Activity of gastrointestinal smooth muscle Resting potentials Slow waves Spike potentials Movements of the gastrointestinal tract

Resting potentials 0 Membrane potential(mv)

-10 -20 -30 -40 -50 -60

stretch acetylcholine parasympathetics depolarization hormones norepinephrine hyperpolarization sympathetics

Slow waves 0 Membrane potential(mv)

-10

stomach 3 duodenum 12 terminal ileum 8-9

-20 -30 -40 -50 -60

Slow wave

Basic electrical rhythm

Spike potentials

Membrane potential(mv)

0

Amplitude of slow wave

-10

The resting potentials

-20 -30 -40 -50 -60

Contraction of smooth muscle

Spike potentials Spike potentials

Action potentials of nerve fibers

Mechanism of depolarization 1

Ca

2+

Na+

inflow

Na+ inflow

Movements in the gastrointestinal tract

Propulsive movement

mixing movement

Promote the food move along the alimentary tract

Keep the gastrointestinal contents mixed

Propulsive movement

Oral cavity

Peristalsis

Distention of the gut Physical irritation of the gut Chemical irritation of the gut

anus

mixing movement

Oral cavity

Peristalsis sphincter

Local intermittent constractions anus

Summary of activity 0

Resting potentials

Membrane potential(mv)

-10 -20

Slow waves

-30 -40 -50 -60

Spike potentials Contraction of smooth muscle

Control of gastrointestinal functions by nervous and hormonal systems

Nervous regulation

hormonal regulation

Hormonal regulation

cholecystokinin secretin Gastric inhibitory peptide

Break down products of food

Increase the contractility of the gallbladder

“I” cells

cholecystokinin

Increase the secretion of pancreatic juice Inhibit stomach motility

acidic gastric juice

“S” cells

Increase the secretion of pancreatic juice

secretin Inhibit the activity of most gut

Fatty acids amino acids carbohydrate

Mucosa of the upper small intestine

Gastric inhibitory peptide

Inhibit the activity of stomach

Nervous regulation Enteric nervous system(ENS)

Extrinsic nervous system

Enteric nervous system(ENS)

myenteric plexus (Auerbach’s plexus)

submucosal plexus (Meissner’s plexus)

myenteric plexus (Auerbach’s plexus) Excitatory effects Inrease the intensity of contraction Increase the rate of the rhythm of contraction Increase the velocity of conduction of contraction along the tract

Inhibitory effects

Sphincter Pyloric sphincter

Oral cavity

sphincter

anus

submucosal plexus (Meissner’s plexus)

secretion absorption contraction

Extrinsic nervous system

parasympathetic nervous sympathetic nervous

parasympathetic nervous

increase

Enteric nervous system

sympathetic nervous

norepinephrine

Smooth ENS muscle Decrease Activity of gastrointestinal tract

Irritation of the gut mucosa Distension of the gut Chemical substances

Afferent sensory nerve fibers from the gut

ENS Sympathetic ganglia Spinal cord or brain stem

Gastrointestinal reflexes stimulation

ENS

Sympathetic ganglia

Spinal cord or brain stem

Activity of gastrointestinal tract

Summary of nervous regulation Irritation of the gut mucosa Distension of the gut Chemical substances

Afferent sensory nerve fibers from the gut

ENS Sympathetic ganglia Spinal cord or brain stem

Chapter 63 Propulsion and mixing of food in the alimentary tract Oral cavity and esophagus Stomach Small intestine Large intestine

Oral cavity and esophagus

Mastication

Swallowing

Mastication 1. Mastication is important for the digestions of fruits and raw vegetables.

Cellulose membrane

2. Mastication increases the Total surface area rate of digestion by grinding exposed to the the the food into particles. digestive juices 3. Mastication prevents excoriation of GI and increase the emptying of food.

Swallowing The process of food coming into the stomach from oral cavity . 1.Voluntary stage

Oral cavity

2.Pharyngeal stage

Pharynx

3.Esophageal stage

Esophagus

Pharynx Esophagus Stomach

1.Voluntary stage

Oral cavity

The pressure of the tongue upward and backward against the palate.

Pharynx

2.Pharyngeal stage

Pharynx

Esophagus

Receptors(pharynx) Trigeminal nerves

Glossopharyngeal nerves

Brain stem 5th ,9th ,10th ,12th cranial nerves Series of automatic pharyngeal muscle contractions

2.Pharyngeal stage

Pharynx

1.the trachea is closed 2.the passage between the pharynx and nasal cavities is closed 3.the esophagus is opened 4.a fast peristaltic wave originates in the pharynx and forces the bolus of food into the upper esophagus.

Esophagus

3.Esophageal stage

Esophagus

Stomach

peristalsis primary The continuation of the peristaltic wave that begins in the pharynx and spread into the esophagus

secondary Distension of the esophagus Myenteric nervous system

Vagal afferent

medulla Vagal efferent Glossopharyngeal nervous

esophagus

Lower esophageal sphincter (gastroesophageal sphincter)

Oral cavity

Prevents reflux of stomach contents into the esophagus sphincter

stomach

Recpetive relaxation of the stomach Oral cavity

sphincter

stomach stomach

Stomach Orad portion First two thirds of the body body

Caudad portion The remainder of the body plus the antrum

antrum

Stomach 1.The storage of large quantities of food 2. Mixing this food with gastric secretions to from chyme 3. Slow emptying of chyme from the stomach into the small intestine

Chyme

The mixture of food and gastric secretions

The storage of large quantities of food Food enter the stomach

Brain stem

stomach

afferent efferent

Vagal nerve

Vago-vagal reflex

Mixing and propulsion of food into the stomach

Basic electrical rhythm

Constrictor waves Mixed waves

Mixing and propulsion of food into the stomach Constrictor waves

Propulsive movements mixing movements

polyrus

Pyloric pump

Mixing and propulsion of food into the stomach Constrictor waves

mixing movements

The opening of pylorus is small Pyloric muscle contracts

pylorus

Pyloric sphincter Water and fluids polyrus

Hunger contractions Constrictor waves occur when the food is present in the stomach Hunger contractions occur when the stomach has been empty for several hours or more Hunger contractions cause mild pain in the stomach which is called hunger pang after much longer starvation

Regulation of stomach emptying Pyloric pump Pyloric sohincter polyrus

Emptying of the stomach Nervous factor Gastric factors Hormonal factor Nervous factor Duodenal factors Hormonal factor

Gastric factors

Stretching of the stomach Local myenteric reflexes

Slightly inhibit the pylorus

Accentuate the ativity of the pyloric pump

Gastric factors

foods (especially digestive products of meat) Antral mucosa releases gastrin

Increase the movement of the gastric body

Accentuate the ativity of the pyloric pump

Nervous factor Duodenal factors Hormonal factor

Duodenal factors

Nervous factor

Chyme in the duodenum stimulate the duodenum in different ways

Nervous reflex Slightly increase the contraction of pyloric sphincter

Decrease the activity of the pyloric pump

The stimulation of chyme in the duodenum 1.The distention of the duodenum 2.The irritation of the duodenal mucosa 3.The acidity of chyme 4.The osmolality of chyme 5.The breakdown products in the chyme

Nervous reflex

Enterogastric reflexes

1. through enteric nervous system 2. Stimulation of duodenum Extrinsic nervous Sympathetic ganglia Sympathetic nervous fibers stomach 3. Through the vagus nerves to the brain stem, then inhibit the excitatory signals transmitted to the stomach through vagi

Hormonal factor foods Duodenal mucosa Release hormones increase the contraction of pyloric sphincter

Decrease the activity of the pyloric pump

Hormonal factor

cholecystokinin secretin

Decrease the stomach motility

Gastric inhibitory peptide

summary Pyloric pump

The force of emptying

Pyloric sphincter

Control the emptying

polyrus

summary excitatory Gastric factors

Duodenal factors inhibitory

Nervous factor Hormonal factor

Nervous factor Hormonal factor

Small intestine

Mixing contractions propulsive contractions

Mixing contractions

segmentation contractions

Propulsive movements segmentation contractions peristalsis Peristaltic rush

peristalsis 1.Occur in any part of the small intestine 2. Move analward at a slow velocity 3. Are weak and die out after traveling only 3 to 5 centimeters 4. The net movement of chyme in the small intestine is very slow

peristalsis

the entry of chyme into the duodenum

Distention of the stomach elicit peristalsis of small intestine through myenteric plexus, which is called gastroenteric reflex

Peristaltic rush Powerful and rapid peristalsis Is produced by intense irritation of the intestinal mucosa, infectious diarrhea Travel long distances to sweep the contents of the intestine into the colon and thereby relieve the small intestine irritative chyme and excessive distention

Ileocecal valve and ileocecal sphincter Sphincter remains constricted and slows the emptying of ideal contents into the cecum

cecum ileum

The lips of the ileocecal valve protrude into the lumen of the cecum to prevent the backflow of fecal contents from the colon into the small intestine

Large intestine 1. Absorption of water and electrolytes from the chyme to form solid feces Proximal half of the colon 2. Storage of fecal matter until it can be expelled distal half of the colon

Mixing movements

haustrations

Circular constrictions is longer Circular constrictions is larger so the constriction of the lumen could produce occlusion

propulsive movements

Mass movements

Defecation

Is initiated by defecation reflexes

Enteric nervous system reflex Parasympathetic reflex

Cerebral cortex

Pudendal nerve

Descending colon Sigmoid

contract

rectum Internal anal sphincter

relax

external anal sphincter

Enteric nervous system reflex When feces enter the rectum Distend the rectal wall

Myenteric plexus

external anal sphincter Internal anal sphincter Descending colon Sigmoid rectum

Parasympathetic reflex When feces enter the rectum Nerve endings in the rectum are stimulated

Pelvic nerves, spinal cord

external anal sphincter Internal anal sphincter Descending colon Sigmoid rectum

Chapter 64 Secretory functions of the alimentary tract Subserve two primary functions 1.digestive enzymes are secreted in most areas from the mouth to the distal end of the ileum 2. Mucous glands, from the mouth to the anus provide mucus for lubrication and protection of all parts of the alimentary tract

1. General principles of alimentary tract secretion 2. Secretion of saliva 3. Gastric secretion 4. Pancreatic secretion 5. Secretion of bile 6. Secretion of the small intestine and the large intestine

General principles of alimentary tract secretion

mucus

Act as a lubricant that protects the surfaces from excoriation and digestion

Organic substances

Digestive enzyme

Water and electrolyte

Chloride, sodium

General principles of alimentary tract secretion

Basic mechanisms of stimulation of the alimentary tract glands Basic mechanism of secretion by glandular cells

Basic mechanisms of stimulation of the alimentary tract glands The presence of food in the gastrointestinal tract Tactile stimulation chemical irritation distension Extrinsic nervous system

Enteric nervous system hormones

Secretion of glands

Extrinsic nervous system Secretion of glands

Parasympathetic nerves

Increase the secretion

Sympathetic nerves Constriction of the blood vessels decrease the secretion

Extrinsic nervous system Sympathetic stimulation can have s dual effect : 1. Sympathetic stimulation alone usually increase secretion 2. If other factors have caused copious secretion, superimposed sympathetic stimulation reduces the secretion because of vasoconstrictive reduction of the blood supply

hormones In the stomach and intestine, several different gastrointestinal hormones help regulate the secretion of glands. These hormones are liberated from the gastrointestinal mucosa in response to the presence of food in the lumen of food These hormones are absorbed into the blood and carried to the glands and stimulate the secretion

Gastric secretion Oxyntic glands (gastric glands) Tubular glands

Hydrochloric acid, pepsinogen, intrinsic factor, mucus

Pyloric glands pepsinogen, gastrin, mucus

Gastric secretion Mucous neck cells: mucus and pepsinogen Oxyntic glands (gastric glands)

Peptic(chief) cells: pepsinogen Parietal(oxyntic) cells: Hydrochloric acid, intrinsic factor

Basic mechanism of hydrochloric acid secretion Parietal cells secrete an acid solution, pH is about 0.8, which is about 3 million times that of the arterial blood canaliculi

apical

canaliculi

Na+ K+ Cl-

basal

Extracellular fluid

K+ HCl

Cl-

H2O

OH-

H+

H+-K+ATPase

H+ K+

Cl-

Osmotic pressure H2O OH- + CO2

HCO3-

Cl-

K+ HCl H2O H+ OH- + CO2

HCO3-

Cl-

Secretion and activation of pepsinogen Peptic(chief) cells and Mucous neck cells secrete deferent types of pepsinogen which have the same functions pepsinogen

Has no digestive activity

HCl pepsin

Has digestive activity

Secretion and activation of pepsinogen Pepsin is an active proteolytic enzyme in a highly acid medium(optimum pH 1.8to 3.5). but above a pH of 5 pepsin become inactiveted in a short time. HCl functions: 11111111111 1. Activate the pepsinogen 2222 2. Provide the acid circumstances for the activity of pepsin

Pyloric glands - secretion of mucus and gastrin Pyloric glands pepsinogen mucus gastrin

Lubricate food movement

Surface mucous cells Surface mucous cells lie on the surface of gastric mucosa between the glands. Surface mucous cells secrete large quantities of mucus Mucus is insoluble and coats the stomach mucosa with a gel layer, providing a mayor shell of protection for the stomach wall and contributing to lubrication of food transport Mucus is alkaline, therefore the normal underlying stomach wall is not directly exposed to the highly acidic

Surface mucous cells

mucus

Regulation of gastric secretion The basic neurotransmitters and hormones that directly stimulate secretion by the gastric glands: Peptic cells acetylcholine mucus cells parietal cells gastrin parietal cells histamine

Regulation of gastric secretion Located in the pyloric glands Stimulate the release of histamine

gastrin G-34, G-17

Gastrin cells (G cells)

Meats reach the antral stomach

Regulation of gastric secretion

Stimulate the secretion of HCl

Located in the body of stomach and is adjacent to the gastric glands histamine

gastrin

enterochromaffin cells

acetylcholine

Regulation of gastric secretion acetylcholine Pepsinogen secretion HCl The rate of secretion of pepsinogen is strongly influenced by the amount of acid in the stomach

Feedback inhibition of gastric secretion of both acid and pepsinogen by excess acidity When the acidity of the gastric juices increases to the pH below 3.0, the gastric mechanism for stimulating gastric secretion becomes blocked. Block the secretion of gastrin excess acidity

Cause an inhibitory nervous reflex that inhibits gastric secretion

Feedback inhibition plays an important roles in protecting the stomach against excessive acidity and excessive pepsin concentration and also in maintaining optimal pH for function of the peptic enzymes

Summary of gastric secretion Oxyntic glands (gastric glands) Tubular glands

Hydrochloric acid, pepsinogen, intrinsic factor, mucus

Pyloric glands pepsinogen, gastrin, mucus Surface mucous cells

Summary of Regulation of gastric secretion gastrin acetylcholine histamine secretion of HCl inhibit excess acidity

Pepsinogen secretion inhibit

Pancreatic secretion acini duct

acinus

Pancreatic enzymes

+ ducts

H2O, Na+, HCO3-, other ions Pancreatic secrfetion

Pancreatic digestive enzymes Pancreatic secretion contains enzymes for digesting all three major types of food: protein,

carbohydrates,

fats

Pancreatic amylase

Pancreatic lipase

trypsin chymotrypsin Carboxypolypeptidase Elastases, nucleases

Cholesterol esterase phospholipase

inactive

enterokinase

trypsinogen chymotrypsinogen

trypsin chymotrypsin

procarboxypolypeptidase Carboxypolypeptidase

protein trypsin chymotrypsin peptides Carboxypolypeptidase Amino acids

starches

glycogen

Pancreatic amylase

Disaccharides and trisaccharides

Pancreatic lipase

Neutral fat

Fatty acids and monoglycerides

Cholesterol esterase

Cholesterol esters

phospholipase

phospholipid

Acini also secrete trypsin inhibitor trypsin inhibitor prevents the activation of trypsin, therefore inhibit the other proteolytic enzymes trypsin inhibitor prevents the digestion of pancreas by trypsin and other enzymes When the pancreas is severely damaged, large quantities of pancreatic secretion become pooled in the damaged areas of the pancreas. Under these conditions, the effect of trypsin inhibitor is sometimes overwhelmed, so pancreatic secretions become activated and literally digest the pancreas, which produce a kind of disease called acute pancreatitis

Secretion of bicarbonate ions

ducts

HCO3-

There are large numbers of HCO3- in the pancreatic juice, which could neutralize the HCl emptied into the duodenum from the stomach

Luminal border Osmotic pressure increase

Na+

HCO3-

H+ Carbonic anhydrase

H2O H2CO3 H2O+CO2 CO2

Na+ blood

Blood border

Regulation of pancreatic secretion three basic stimuli are important in causing pancreatic secretion

Nervous endings acetylcholine Intestinal mucosa cholystokinin Intestinal mucosa secretin

Stimulate the acinar cells of the pancreas much more than the ductal cells Cause production of large quantities of pancreatic enzymes but relatively small quantities of fluid and ions Cause production of large quantities of fluid and ions but relatively small quantities of pancreatic enzymes

Phases of pancreatic secretion

Cephalic phase

Account for 20% of the total secretion of pancreatic enzymes after a meal

gastric phase

5 to 10% Secretin

intestinal phase

cholecystokinin

Secretin Acid chyme “S” cells secretin Secretin is absorbed into the blood

Secrete large quantities of water and NaHCO3

pancreas

NaHCO3 Neutralize HCl coming from the stomach HCl+ NaHCO3 =NaCl+H2CO3

CO2

H2O

Protect the stomach mucosa

Provides an appropriate pH for action of the pancreatic enzymes, which function optimally in a slightly alkaline or neutral

cholecystokinin Break down products of food “I” cells

Secrete large quantities of pancreatic enzymes

cholecystokinin cholecystokinin is absorbed into the blood

pancreas

Secretion of bile Large quantities of bile salts Emulsifying function Detergent action on the fat particles in the food, so decrease the surface tension of the particles Break the fat globules into small size to increase the area of the fats exposed to lipase

Promote the digestion of fats

Promote the absorption of fats

Promote the absorption of fats Some breakdown products of fats are insoluble and can not be absorbed by intestine, Bile salts form minute complexes with these lipids, which are called micelles Micelles are soluble, so could ferry these lipids to the mucosa

Secretion of the small intestine and large intestine

mucus Small intestinal secretions

bicarbonate Water and other ions

Secretion of the small intestine and large intestine

Large intestinal secretions

mucus bicarbonate

link Spike potentials

Ca

Action potentials of nerve fibers

2+

Na+ inflow

Na+ inflow

ENS myenteric plexus

submucosal plexus

Inrease the intensity of contraction Increase the rate of the rhythm of contraction Increase the velocity of conduction of contraction Inhibit the Sphincter

secretion absorption contraction

Sympathetic ganglia Spinal cord or brain stem sympathetic nervous parasympathetic nervous

ENS

ENS

Movement of alimentary tract Secretion of digestive juices and digestion of the food Absorption of nutrients Circulation of blood through the gastrointestinal organs Control of these functions by nervous and hormonal systems

myenteric plexus (Auerbach’s plexus)

submucosal plexus (Meissner’s plexus)

neurotransmitters acetylcholine

Excitatory effects

norepinephine

Inhibitory effects

Secretion of saliva parotid Salivary glands

Serous secretion

submandibular

Serous secretion

sublingual

mucous secretion

buccal

mucous secretion

Saliva contains two types of protein secretion: 1.Serous secretion: contains ptyalin, which is an enzyme for digesting starches 2.mucous secretion: contains mucin, which is for lubricating and surface protective purpose

Secretion of ions in the saliva Resting conditions Large quantities 7 times as great as of K+ that in plasam

saliva

Large quantities of HCO3A few of Na+ A few of Cl-

2 or 3 times One seventh or one tenth their concentrations inplasam

Secretion of ions in the saliva Submandibular salivary glands: acini and salivary ducts ducts

acini

Salivary secretion is a two-stage operation:

Acini secrete primary secretion

Ptyalin, mucin, ions

As the primary secretion flows through the ducts, two major active transport pocesses take place to modify the ionic

Secretion of ions in the saliva ducts ClHCO3Na+ K+ HCO3acini

1. Na+ are actively reabsorbed and K+ are actively secreted in exchange for the Na+ 2. There is excess of Na+ reabsorption over K+ secretion, so creates electrical negativity in the ducts, and in turn cause Clto be reabsorbed passively. 3. HCO3- are secreted into the lumen actively or by exchange of Cl-

Secretion of ions in the saliva 1. During maximal salivary, the salivary ionic concentrations change considerably ducts

acini

2. The rate of formation of primary secretion by the acini increase greatly 3. The acinar secretion then flows through the ducts so rapidly that the ductal reconditioning of the secretion is considerably reduced. 4. Compared with the saliva under resting conditions, the [Cl-] increase and the [K+] decrease.

Nervous regulation of salivary secretion Parasympethetic nervous salivary secretion

Taste and tactile stimuli from the oral cavity, sour taste, presence of smooth objects, rough objects Sympethetic nervous

Basic mechanism of secretion by glandular cells Water and electrolyte Ductal cell Extracellular fluid

Glandular duct