GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi

8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi

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Physiology of GITChapter No=62

By

Dr. Mudassar Ali Roomi (MBBS, M.Phil.)

Assistant Professor Physiology


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GIT

Function:The GI tract provides a continuoussupply of water, nutrients and electrolytes to thebody.

To achieve this target we need:1. Movement (Motility)

2. Secretion of glands

3. Absorption of nutrients

4. Circulation of blood (splanchnic circulation)5. Control of all these functions (nervous and

hormonal)


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Two Types of Digetsive Systems

Incomplete digestivesystem

One-way, saclikedigestive cavity

Complete digestive

system A tube with an

opening at each end


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Major Components of GIT

Mouth (oral cavity)

Pharynx (throat)

Esophagus (food pipe)

Gut

Stomach Small intestine

Duodenum jejunum ileum

Large intestine Cecum appendix colon Rectum Anus


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Accessory Organs

Salivary glands (sublingual, submandibular and parotidglands)

Secrete saliva.

Pancreas: Secretes digestive enzymes.

Liver:

Secretes bile. Gallbladder:

Stores and concentrates the bile.


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Typical cross section of intestine

A typical cross section

shows:

1. Mucosa.

2. Submucosa.

3. Muscularis externa:

Circular muscle layer.

Longitudinal musclelayer.

4. Serosa.


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Mucosa: First Layer (innermost layer)

Mucosa= epithelium andlamina propria

Small amount of smoothmuscle (muscularis

mucosae) is also present

Lumen: passageway.

Epithelium may havemucus secreting glands

(also secrete digestiveenzymes)

Secretes and absorbs.


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Second Layer: Submucosa

Contains loose

Connective Tissue.

Contains Glands, blood

vessels, lymphatics andnerves.

Carries away absorbed

materials.


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Third Layer: Muscularis externa

Provides the tube

movements (peristalsis)

Has two coats of

smooth muscle tissue: Inner Circular fibers

(increase and decrease

tube diameter)

Outer Longitudinal fibers(lengthen and shorten

the tube)


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Fourth Layer: Serosa (serous layer)

Outer covering of the

tube.

Visceral peritoneum.

Secretes serous fluid tokeep the outside of the

organs lubricated and

moist, thus performs a

protective function.


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Neural Control of the GI Function

Neural Controls: Extrinsic (3)

Parasympathetic. Sympathetic. Somatic.

Intrinsic (2) Myenteric plexus. Submucosal plexus.

SANS and PANSmodulate the enteric

nervous system asopposed to directlycontrolling the smoothmuscle of the bowel.


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Extrinsic control

Autonomic Neural Pathways:Parasympathetics

Upper/Cranial via Vagus nerve:

Innervates the upper segments of GI tract to Splenic flexure

along with the pancreas.

Lower/Sacral via Pelvic Splanchnic nerves (nervi erigentes)

S2-S4to the descending colon, rectum and anus.

Post Ganglionic Neurons:

Located mainly in the Myenteric and Submucosal plexuses.

Neurotransmitter:

Acetylcholine. Function:

Stimulates GI secretion, motor activity.

Relaxes sphincters


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Extrinsic controlAutonomic Neural Pathways:

Sympathetics Fibers originate in the spinal cord between segments T-5 and L-2.

Preganglionic fibers enter the Sympathetic chains, then pass on toCoeliac and Mesenteric ganglia (PREVERTEBRAL ganglia)

Post Ganglionic Fibers:

They originate in these ganglia and pass on to all parts of the GITthrough sympathetic nerves.

Neurotransmitter:

Norepinephrine.

Some amounts of Epinephrine.

Function: Inhibition of GI secretion, motor activity.

Contraction of GI sphincters and blood vessels.


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Intrinsic/ Enteric Nervous System:

Submucosal (Meissners)plexus.

Myenteric (Auerbachs)

plexus.

Regulate segment-to-segment movement of

the gastrointestinal

tract.

May be considered a 3rd

part of the ANS.


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Enteric Nervous SystemMyenteric plexus (Auerbachsplexus):

Locatedbetween thelongitudinal and circularlayers of muscle in the wallof the GIT.

Controls tonic and

rhythmic contractions. Exerts control primarily

over digestive tractmotility.

Principal effects: Increased tone.

Increased intensity ofcontractions.

Increased rate ofcontractions.

Increased velocity ofconduction.


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Myenteric plexus (Auerbachsplexus):

It is not totally excitatory in nature.

Some of the neurons of the myenteric nervous

system secrete VIP. This is an inhibitory peptide.

These inhibitory signals are useful for controlling

the intestinal sphincters e.g. pyloric sphinter

Partially controlled by autonomic nervous

system.


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Enteric Nervous System

Submucosal plexus (Meissnersplexus)

Buried in the Submucosa.

Senses the environmentwithin the lumen.

Regulates GI blood flow.

Controls epithelial cellfunction (local intestinalsecretion and absorption).

May be sparse or missingin some parts of GI tract.

Partially controlled byautonomic nervoussystem.


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Enteric Nervous System

Types of Neurons in the Enteric System:

1. Sensory neurons: Chemoreceptors sensitive to acid, glucose and amino acids have

been demonstrated which, in essence, allow "tasting" of luminalcontents. Sensory receptors in muscle respond to stretch andtension.

2. Motor neurons:

Control GI motility and secretion, and possiblyabsorption.

3. Interneurons: Largely responsible for integrating information from

sensory neurons and providing it to motor neurons.


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Types of Enteric Neurotransmitters in enteric

neurons

Acetylcholine: Excitatory

Stimulates smooth muscle contraction.

Increases intestinal secretions.

Releases enteric hormones.

Dilates the blood vessels.

Norepinephrine: Derived from extrinsic sympathetic neurons.

Inhibits the GI activity.

Causes vasoconstriction

Others (some are excitatory some are inhibitory) Adenosine tri phosphate (ATP), Serotonin, Dopamine,

Cholecystokinin, VIP, Somatostatin, Substance P,

Leu-enkephalin, Met-encephalin and Bombesin.


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Electrical activity in the Smooth Muscle of GIT

Smooth muscle in the GIT is excitable.

Slow, intrinsic electrical activity takes place

along the membranes of this smooth muscle.

The GI smooth muscle acts as a functional syncytium.

Due to presence of Gap junctions.

Therefore, whenever an action potential is generated

within the muscle mass, it travels in all directions.


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Electrical activity in GIT

The RMP is -50 to -55

mV. This can be

changed to different

levels to control themotor activity of the

gut.

There are two types of

electrical waves: Slow waves-

Spike potentials.


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Slow Waves/ Basic Electrical Rhythm of the gut.

These are spontaneous,rhythmic fluctuations inthe RMP between -55 to -40 mV.

These are not true actionpotentials, but are

localized/graded potential They do not cause muscle

contraction in the GIT. Frequency of slow waves

in different parts of gut: 3/ min in the body of the

stomach. 12/ min in the duodenum. 8-9/ min in the terminal

ileum.


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Cause of Slow Waves

Pacemakercells/Interstitial cells ofCajal:

These pacemaker cells

undergo cyclic changesin the membranepotential due to uniqueNa ion channels which

periodically open andproduce inward,pacemaker currantswhich in turn generateslow waves.


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Slow Waves

Function: They lead the RMP to

threshold value so that thespike potential can take place.

Stimulated by(which causeshypopolarization):

Stretch. Acetylcholine. Parasympathetic nervous

system (vagus) Parasympathomimetic drugs

e.g. Pilocarpine Slow wave activity is greatly

reduced by Vagotomy,Norepinephrine andsympathetics (which causeshyperpolarization)


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Spike Potentials

These are true action potentials.

They occur whenever the RMP of the smooth musclebecomes more positive than -40 mV (threshold)

Higher the slow wave potential, greater is the frequency ofthe spike potentials

Each spike lasts for up to 10- 20 m sec which is 10- 40 timeslonger than the action potential in the nerves fibers.

Ionic basis of action potentials/spike potential: In GIT, the ion channels involved are slow Calcium- Sodium

channels. The slowness accounts for the long duration of the action

potential in the GI smooth muscle.


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Three types of Gastrointestinal Reflexes

1. Reflexes that are integrated entirely within the enteric nervoussystem. These are the reflexes which control: Secretion.

Peristalsis.

Contractions etc.

2. Reflexes from the gut to the prevertebral sympathetic ganglia andthen back to the GI tract. Gastrocolic reflex.

Enterogastric reflex.

Colonoileal reflex.

3. Reflexes from the gut to the spinal cord or brain stem and thenback to the GI tract. Reflexes from stomach and duodenum to the brain stem and back via

the vagus nerve.

Pain reflexes.

Defecation reflexes.


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1. GASTRIN:

Secretion: By G cells in gastric pits of the mucosa.

Stimulus: Stomach distention , the products ofproteins and Gastrin releasing peptide, which isreleased by the nerves of the gastric mucosa as aresult of Vagal stimulation.

Actions:

1. Increases HCl production in stomach.

2. Increases gastric motility. 3. Stimulates growth of gastric mucosa.

4. Contracts lower esophageal sphincter.

5. Relaxes pyloric sphincter.


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2. CHOLECYSTOKININ (CCK):

Secretion: By I cells in the mucosa of the duodenumand jejunum.

Stimulus: Chyme rich in digestive products oftriglycerides and fatty acids and monoglycerides and

amino acids.Actions: Contracts the gallbladder.

Opens the Sphincter of Oddi.

Increases secretion of pancreatic juice rich in digestiveenzymes.

Inhibits gastric secretion and motility.

May reduce hunger.


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3. SECRETIN

Secretion: By S cells in the mucosa of theduodenum.

Stimulus: Acid chyme (H+), fatty acids in the smallintestine causes secretion of Secretin.

Actions: Stimulates the watery secretion of pancreatic juice and

bile that is rich in bicarbonate ions. MCQ

Inhibit production of HCl in stomach.

Promotes growth and maintenance of the pancreas. Enhances the effects of Cholecystokinin (CCK)


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5. Motilin

Secretion: By the cells of the upper

duodenum.

Stimulus: It is released during fasting.

Actions:

Increases the gastrointestinal motility.

Causes formation of Interdigestive myoelectric

complexes/Migratory motor complexes (MMC) in afasting person.

House keeping function


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Functional Types of Movements in the GIT

1. Propulsive movements/peristalsis

2. Mixing movements.

Both are brought about by the Enteric nervous system, but

are influenced by the Extrinsic nervous system, especially theParasympathetic nervous system.


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Propulsive movements/peristalsis

It is an inherent property of many syncitial smoothmuscle tubes.

Occurs in the gut, bile ducts, glandular ducts andureters.

Stimulus: Distension of the gut, physical or chemical irritation of the

surface epithelium and stimulation by the Parasympatheticnervous system.

Controlled by: Actual peristalsis is brought about by the Myenteric

nervous system.

But it is influenced by the Parasympathetic nervoussystem.


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Propulsive movements/peristalsis

Mechanism of myenteric

reflex or the peristalticreflex: Distension of the gut-

Stimulation of the entericnervous system.

The gut wall contracts 2-3 cmbehind the point of stimulus-formation of a contractilering.

Initiation of a wave ofcontraction- travels in the

anal direction. Relaxation of the gut wall

distal to the point ofstimulation- receptiverelaxation.


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Function of the Myenteric plexus inperistalsis:

Weak or no peristalsis in areas of the gut where

the Myenteric plexus is absent ( e.g. congenitalmegacolon/Hirschprungsdisease)

If the Cholinergic nerve endings of the Myentericplexus are blocked by Atropine- No peristalsis.

Therefore, effectual peristalsis requires an intactfunctional Myenteric plexus.


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Properties of peristalsis:

1. Directional movement: Theoretically, it can occur in both the orad and anal directions.

But the wave of peristalsis dies out if it travels in the orad direction.

Exact reason is not known.

Probably because of the fact that the Myenteric plexus is itself polarized.

2. Receptive Relaxation: Part of the gut immediately after the peristaltic ring relaxes to allow the

easy movement of food.

Is a function of the Myenteric plexus.

This relaxation takes place only the anal direction, ahead of the contractilering.

Therefore, peristalsis moves towards the anus.

3. The anal ward moving complex in association with the Myentericplexus is called the Myenteric/ peristaltic reflex. The peristaltic reflex Plus the anal direction of the movement is called the

Law of the Gut.


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Mixing Movements

Differ in most parts of the gut. Segmentations- in smallintestine

Haustrations- in large intestine In many cases, most of the

mixing is provided byperistalsis itself, especially

when it pushes the foodagainst a closed sphincter. Local , intermittent,

constrictive contractionsoccur at some places in thegut. They are present afterevery few cms.

They last for 5 to 10 secs.

Then new contractionsappear at new places.

This pattern is calledchopping and shearing offood.

GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi

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