digestion 1.ppt

8/14/2019 digestion 1.ppt

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Homeostasis

THE DIGESTIVE SYSTEM

The digestive systemcontributes tohomeostasis, in part, by

transferring nutrients,H2O, and electrolytesfrom the externalenvironment to theinternal environment.


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Mouth

Anus

Mouth

Anus

Geometrical shape = torus


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Mouth

Pharynx

Large Intestine:

CecumAscending colonTransverse colonDescending colonSigmoid colonRectum

Anus

Stomach

Small Intestine DuodenumJejunumIleum

Esophagus

Salivary glands

Liver

Gallbladder

Pancreas


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FUNCTIONS OF THE GI TRACT:

Protection

Non-immunological defenses Immunological defenses

Nutrition

Transfer nutrients, H2O, and electrolytes from external tointernal environment

Motility Secretion

Digestion

Absorption

Excretion Bile entering the gi tract contains organic anions and cations,

including drugs and drug metabolites that are either poorly

absorbed or not absorbed at all from the gi tract.


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Control of GI Functions Motility Secretion Digestion Absorption Excretion

Non-immunological Defenses

Immunological Defenses

Types of reflexarcs controlling GI

Functions1) Nervous

2) Endocrine

3) Paracrine

4) Immune

Reflex Arc

AfferentInput

Processingand

Integration

EfferentSecreto-Motor

OutputResponse

Presenceo f

St imulusDetected

The functions of the gi tract are controlled by a dynamic interplaybetween different cell types that interact directly, or through a large

number of signaling molecules to form reflex arcs.


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1. Mixing contractions

Mechanical disruption of ingested food

Mix luminal contents with digestivesecretions

Facilitate absorption of digestion products

2. Propulsive contractions

Propel luminal contents, usually in acaudal direction

3. Tonic contractions Maintain constant tone and intraluminal

pressure

Contraction of sphincters restrictsmovement of luminal contents

Gastrointestinal Motility

Contractions of muscles in the wall of the gastrointestinal tract mixcontents of the lumen with digestive secretions and propel contents along

the length of the gi tract.


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Surface epithelium

Mucus Exocrine glands (i.e., glands with ducts that empty into the lumen)

Salivary glands: saliva containing digestive enzymes and mucus

Gastric glands: HCl, digestive enzymes, and mucus

Intestinal glands: H2O, electrolytes, and mucus

Pancreas: pancreatic juice containing digestive enzymes,electrolytes, and mucus

Liver and gallbladder: bile containing bile salts

Gastrointestinal Secretion

1) During digestion of dietary components, a variety of substances aresecreted into the gastrointestinal lumen.

Enteroendocrine cells(i.e., secrete hormones and paracrinesubstances)

Gastric G cells: release gastrin

Gastric D cells: release somatostatin (SS)

Intestinal S cells: release secretin

Intestinal I cells: release cholecystokinin (CCK)

2) The presence of food within the gastrointestinal tract also induces thesecretion of hormones and paracrine substances.


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For example:

Proteins are broken down to single amino acids

Carbohydrates are broken down to simple one-sugar molecules

(i.e.,monosaccharides: glucose, galactose, and fructose)

Triglyceridesare broken down into free fatty acids andmonoacylglycerides

Gastrointestinal Digestion

The proteins, carbohydrates and fats contained in the diet consist oflarge macromolecules that cannot be absorbed across thegastrointestinal epithelium.

Digestive enzymes break these macromolecules down (hydrolysis) intoabsorbable subunits.


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Most of the absorption of nutrients, vitamins,

H2O, and electrolytes occurs from the smallintestine.

Gastrointestinal Absorption

The gastrointestinal epithelium separates outside(lumen) from insidethe body.

1) Absorption of digestion products (along with H2O,vitamins, and electrolytes) enter the body bybeing absorbed across the epithelial cell layer.

2) Absorbed nutrients leave the gastrointestinal tract in blood or lymph.

Hepatic Portal Vein

Monosaccharides, amino acids, water-soluble vitamins, etc.exit via hepatic portal vein en routeto the liver.

Lymphatics

Lipid digestion products and fat-soluble vitamins are packagedwithin chylomicrons(a type of lipoproteins) and cannot entercapillaries, but exit via the lymph vessels.

epithelium


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The digest ive tract wal l consists of four layers:

2) Submucosa Layer of connective

tissue, containinglarger blood andlymph vessels.

4) Serosa Outer connective

tissue covering

3) Muscularis Externa Smooth muscle coat

(longitudinal and

circular musclelayers)

1) Mucosa Lines the luminal

surface (divided intothree layers).


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3) Lamina Propria

Middle layer of

connective tissue

The Mucosa:

Consists of three layers

1) Mucous membrane Inner surface

epithelial cell layer,separating outsidefrom inside thebody..

2) Muscularis Mucosae

Thin layer ofsmooth muscle

adjacent to thesubmucosa.


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The Submucosa: Area under the

mucosa betweenthe muscularismucosae and thecircular smoothmuscle layer

Contains: Connective tissue

Large blood vessels

Large lymph vessels


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2) Longitudinal musclelayer

Outermost layer

whose contractionshortens andstiffens the wall.

The Muscular is Externa:

Consists of two relatively thick layers of smooth muscle

1) Circular muscle layer Innermost layer

whose contractionnarrows the lumen.


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The Serosa:

Connective tissue covering.

Continuous with the

mesentery suspendingthe digestive organsfrom the inner wall ofthe abdominal cavity.

Mesentery


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Two connected networks of

neuronal ganglia and nervefibers.

Submucosal plexus

Myenteric plexus

The Intr in sic Nerve Plexuses:

EntericNervousSystem

(ENS)

MotilitySecretion


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The ENS is extremely large (~100 million neurons) and its ultrastructuralorganization and neuronal diversity are more like brain than peripheralnerve.

The Enteric Nervous System (ENS), the brain of the gut

Acts as a microcomputerwith itsown independent software.

Organized for programmedreflexive operations using neuralelements contained within thewall of the gi tract itself.

1. Intrinsic primary afferentneurons(IPANs) respond tochanges in luminal contentsand distention of the gutwall and convey informationto ENS interneurons.

Reflex Arc

AfferentInput

Processingand

Integration


OutputResponse

Presence o fSt imulusdetected

2. ENS interneurons integrate information and formulate programmedcommands

3. ENS efferent motor and secretomotor neurons, etc. act to changebehavior of target cells.

The ENS is a center of integrative neuronal activity that is able to controlthe behavior of the gut by neural reflexes, even in the absence of CNS.

ENSefferentneurons

ENSinterneurons

IPANs

ENS


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CNS Also organized for programmed reflexive operations that can modulate

the ENS programs.

1. Extrinsic primary afferentneurons(EPANs)respondto changes in luminalcontents and distention ofthe gut wall and conveyinformation to the CNS.

CNS interneurons andcentersintegrate informationabout events in the gi tractwith other sensory informationand formulate efferentcommands.

CNS efferent neuronsconvey commands to the gi tract

Somatic motor neuronsto areas containing skeletal muscle, and

Autonomic neurons(sympathetic and parasympathetic) to areasthat contain the ENS

Reflex ArcAfferent

Input

Processingand

Integration


OutputResponse


CNSefferentneurons

Spinal cordMedulla

Higher centers

EPANs

CNS


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An understanding of neural reflexive pathways withinthe ENS and those that involve the CNS is essential to

understanding the behavior of the gi tract.

Reflex Arc

AfferentInput

Processing

andIntegration


OutputResponse

Presence ofStimulusdetected

ENSefferentneurons

ENSinterneurons

IPANs

Reflex Arc

AfferentInput

Processingand

Integration

EfferentSecreto-MotorOutput

Response


CNSefferent

neurons

Spinal cordMedulla

Higher centers

EPANs

ENS

CNS

C t l


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InterneuronsIntegrativeCircuitry

MotorPrograms

~

SympatheticNeurons

ParasympatheticNeurons

ExtrinicPrimaryAfferent

Neurons(sensory)

Autonomic

Nervous System

InterneuronsIntegrative

Circuitry

MotorPrograms

~IntrinsicPrimaryAfferentNeurons(sensoryIPANs)

SecretomotorMotor

Vasomotor

Excitatory

Inhibitory

Neurons

Enteric Nervous System

CentralNervousSystem

IPANReceptor

EPANReceptor EFFECTORS

Smooth muscle

Epithelial cells

Vasculature

Entero-endocrinecells

Immune cells

InterstitialCells of

Cajal


Chemical

signal

Short-LoopNeuronal Reflex

PathwayUtilizes only neuralelements within the

ENS

THE BRAIN-GUT AXISShort-Loop (ENS)

andLong-Loop(CNS)

NeuralReflexive Pathways


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ENS or entericneurons:

1. ENS intrinsic primary afferent neurons (IPANs) Convey information about lumenal contents to ENS interneurons

2. ENS Interneurons(ascending and descending) Process and integrate sensory information Control behavior of efferent neurons

3. ENS efferent neurons

a. muscle motor neurons Excitatory and inhibitory innervation of smooth muscle

b. ENS secretomotor neurons Innervate the mucosa Control secretion

c. ENS vasomotor neurons

Control blood flow

d. ENS intestinofugal neurons Neuronal cell bodies within the ENS, but send axonal

projections to sympathetic prevertebral ganglia.


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Called IPANs, rather than sensory afferent neurons, because they do notconvey sensationfrom the intestine to the CNS.

Rather, IPANsare ENS afferent neurons that convey information aboutlumenal contents to ENS interneurons to initiate short-loop neuralreflexes.

No IPAN nerve endings reach the lumen of the gut.

How do IPANs detect the presence and composition of lumenalcontents?

1. Presence of contents within the lumen

IPANs within the wall of the gut respond directly to beingstretched when the wall is distended due to contents withinthe lumen.

2. Composition of the lumenal contents IPANs may respond indirectlyto intralumenal stimuli

IPAN nerve endings possess receptors for chemicalmessengers (hormones, paracrine substances) releasedfrom entero-endocrine cells in the mucosal epithelium

into the lamina propria.

Intrinsic Primary Afferent Neurons (IPANs)


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Entero-endocrine cellsin the mucosal epithelium

1. When stimulated, release chemical messengers (hormones,

paracrine substances)into the lamina propria

a) These messengers bind to receptors on IPAN (and EPAN (seelater)) nerve endings.

2. Tasteand feelthe presence of luminal contents

a) Tastethe chemical constituents of the luminal contents.

Receptors (taste buds like on the tongue) on the apicalmembrane sensitive to:

Changes in pH

Protein digestion products

Fat digestion products

D-glucose

Chemical irritants

Changes in solute concentration

b) Feelthe mechanical distortion of the mucosa by shear force,pressure, volume, etc. caused by the presence of luminalcontents.


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Intraluminal

chemical ormechanicalstimulus

Intrinsic Primary AfferentNeuron (IPAN)

ENS neuronal network

Altered motility

Altered secretion

Short loop neural reflexAll neural elements involved in thereflex are contained within the wallof the GI tract.

Entero-

endocrinecell

Distentionand stretch ofthe gut wall


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The ENS and Entero-Endocrine Cells

Specific intraluminal stimuli lumen first activate entero-

endocrine cel lsare strategically positioned in the mucosa totaste and feel luminal contents and release mediators thatactivate IPANs.

Entero-endocrine cells in the stomach

G-cells secrete gastrin

D-cells secrete somatostatin

etc.

Entero-endocrine cells in the small intestine

S-cells secrete secretin

I-cells secrete cholecystokinin

Enterochromaffin cells (EC) secrete serotonin (5HT)

etc.


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Enterochromaffin cell (EC)

Synthesize and store serotonin (5-hydroxytryptamine or 5HT) Taste luminal contents

Nutients

Hyperosmolality

Change in pH

Luminal irritants

Invading enteropathogenic microorganisms

Feel luminal contents

Mechanical forces on the exerted on the mucosal surface

In response to the detection of luminal stimuli, EC release 5HT into thelamina propria.

5HT binds to receptors on IPANs in the lamina propria to initiate ashort-loop neural reflex within the ENS that result in changes insecretion and motility.

An example of an entero-endocrine cell:


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Chemical or Mechanical Stimulation

Circular Muscle

Serosa

Submucous plexus

Myenteric plexus

Mucosa

Lumen

Lamina propria IPANs

SecretomotorNeurons

Interneurons

ECcellHT

ACh

Cl-

Intraluminal stimuli may initiate short-loop reflexes that altersecretion by epithelial cells.

Longitudinal Muscle


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Circular Muscle

Longitudinal Muscle

Submucous plexus

Myenteric plexus

Mucosa

Lumen

Lamina propria

IPANs

Excitatory motor neuron

Inhibitory motor neuron

ECcell

Interneurons

HT


Intraluminal stimuli may initiate short-loop reflexes that altercontractions in the muscularis externa.

Serosa


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Entero-Endocrine Cells

Example: enterochromaffin cells (EC) Removal of 5HT from the lamina propria

5HT is a base At physiological pH, 5HT is positively charged and cannot

freely enter cells to be metabolized by intracellular enzymes(e.g.,monoamine oxidase or MAO)

Inactivation of 5HT occurs mainly by transporter-mediateduptake into enterocytes.

The serotonin reuptake transporter(SERT or HTT) is theprimary molecule responsible for inactivating 5HT in the gut.

NOTE:

Transcription of SERT is decreased in patients with inflammatory

bowel disease (IBD) or irritable bowel syndrome (IBS). This contributes to

Increased water in stools

Increased colonic motility

Alternating patterns of diarrhea and constipation

InterneuronsCentral


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MotorPrograms

~

SympatheticNeurons


ExtrinicPrimaryAfferentNeurons(sensory)

AutonomicNervous System


Circuitry

Motor

Programs


SecretomotorMotor

Vasomotor

Excitatory

Inhibitory

Neurons



IPANReceptor


Smooth muscle

Epithelial cells

Vasculature


Immune cells


Cajal


Chemical

signal

Short-LoopNeuronal Reflex

PathwayUtilizes only neuralelements within the

ENS

BRAIN OF THE GUT

InterneuronsCentral


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Long-LoopNeuronal Reflex

PathwayUtilizes neural elementswithin both the CNS and

the ENS


Chemical

signal


MotorPrograms

~

SympatheticNeurons


ExtrinicPrimaryAfferentNeurons(sensory)



Circuitry

Motor

Programs


SecretomotorMotor

Vasomotor

Excitatory

Inhibitory

Neurons



IPANReceptor


Smooth muscle

Epithelial cells

Vasculature


Immune cells


Cajal


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Circular Muscle

Longitudinal Muscle

Submucous plexus

Myenteric plexus

Mucosa

Lumen

Lamina propria

Extrinsic primary afferent neuron

CNS

HT


Pain and Nausea


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1. CNS sensory extrinsic

primary afferentneurons (EPANs)carryinformation from the GItract to the CNS.

2. CNS efferent autonomicnervesfrom the CNSinnervate the ENS.

Sympathetic nerves

Parasympatheticnerves

Nerves Extr in sic to the Gut Wall:Connect ions to the CNS

Autonomic nervous systeminput from the CNS modifiesthe ongoing activity of the

ENS.


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Altered motility

Altered secretion

Long loop neural reflex

ENS neuronal network

Autonomic Nervous Output

ParasympatheticNervous System

SympatheticNervous System

CNS neural elements in the brainand spinal cord are involved inlong-loop neural reflexes and alterparasympathetic and sympatheticneural input to the ENS.

CNS neuronal network(brain and spinal cord)

Intraluminal

chemical ormechanicalstimulus

CNS Extrinsic PrimaryAfferent Neuron (EPAN)

Entero-

endocrinecell

Distentionand stretch ofthe gut wall


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EPANs that convey information from the gastrointestinaltract to the CNS consist of the following three types:

1) Vagal afferent neurons

EPANs within the vagus nerves (X) that convey information tothe medulla

Neuronal cell bodies within vagal ganglia outside of themedulla.

2) Spinal visceral afferent neurons EPANs within the splanchnic nerves (i.e., spinal nerves levels

that convey information from the viscera to the spinal cordsegments T1 to L2 .

Neuronal cell bodies within dorsal root ganglia outside ofthe spinal cord at these levels

3) Pelvic afferent neurons EPANs within the pelvic nerves (i.e., spinal nerves at spinal cord

levels S2 to S4) that convey information to spinal cord levels

Neuronal cell bodies contained in dorsal root gangliaoutside of the spinal cord at these levels


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Circular muscle

Longitudinal muscle

Submucous plexus

Myenteric plexus

Medulla

Vagalganglion

Vagal Primary AfferentNeurons (EPANs)

Parasympatheticpreganglionic

neuron

Vagus Nerve


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Circular muscle

Longitudinal muscle

Submucous plexus

Myenteric plexus

Spinal cord(S2-S4)

Dorsal rootganglion

Parasympatheticpreganglionic

neuron

Pelvic Nerve

Pelvic Primary AfferentNeurons (EPANs)

S i l d


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Circular muscle

Longitudinal muscle

Submucous plexus

Myenteric plexus

Spinal cord(T1-L2) Dorsal root ganglion

Sympathetic prevertebral ganglion

Sympatheticparavertebral ganglion

Sympatheticpost-ganglionic neuron

Sympatheticpre-ganglionic neuron

ENSintestinofugalneuron

Splanchnic Nerves

Spinal Visceral Afferent Neurons (EPANs)


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CNS Efferent Secreto-Motor Output to the GI Tract:

1. Somatic efferents (motor neurons) to striated muscle

Cranial nerves from the brainstem (medulla and pons)

Spinal nerves from sacral spinal cord (pudendal nerves)

2. Visceral efferents (autonomic nervous system (ANS))

Cranio-Sacral Division: Parasympathetic preganglionicneurons

Cranial nerves

Spinal nerves from segments S2- S4 forming the pelvic nerves

Thoraco-Lumbar Division: Sympathetic preganglionic neuronsfrom spinal cord segments T1 to L2

Splanchnic nerves

CNS Eff t S t M t O t t t th GI T t


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Somatic motor neurons innervating striated muscle within the

gastrointestinal tract are found in:1. Cranial nerves (VII, IX, X, and XII), originating from the

brainstem, innervating striated muscle of the jaws, tongue, oralcavity, pharynx, and upper esophagus.

2. Pudendal nerves, originating from the sacral spinal cord,

innervating the striated muscle of the external anal sphincter.

CNS Efferent Secreto-Motor Output to the GI Tract:1. Somatic efferents (motor neurons) to striated muscle


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CNS Efferent Secreto-Motor Output to the GI Tract:2. Visceral efferents (autonomic nervous system (ANS))



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a. Cranio-Sacral Division: Parasympathetic preganglionicneurons

1) Cranial nerves from the brainstem

Axons from parasympathetic preganglionic neurons within thebrainstem exit and synapse on postganglionicparasympathetic neurons within parasympathetic gangliaclose to, or within, the target organ (long preganglionic fiber;

short postganglionic fiber). For example, parasympathetic postganglionic neurons

that innervate the salivary glands are located within thesubmandibular and otic ganglia close to the salivaryglands.

Where there is ENS, the postganglionic

parasympathetic neurons are located within the ENS(except for the distal colon).

2) Pelvic nerves from the sacral spinal cord

Synapse on postganglionic parasympathetic neuronspostganglionic neurons within the ENS in the distal colon.



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b) Thoraco-Lumbar Division: Sympathetic preganglionic neuronsfrom spinal cord segments T1 to L2

1) Axons from sympathetic preganglionic neurons exit the spinalcord and enter the sympathetic paravertebral ganglia at thesame level.

2) The synapse with the sympathetic postganglionic neuron mayoccur at different locations

(a) Within the sympathetic paravertebral ganglion (at the samelevel or at another level)

In this case, the sympathetic postganglionic fiber exits theparavertebral ganglion chain to innervate the distant target(short preganglionic fiber; long postganglionic fiber).

(b) Within a sympathetic prevertebral ganglion, close to thewall of the gi tract, after passing through the paravertebralganglion (long preganglionic fiber; short postganglionicfiber).

In this case, the sympathetic postganglionic fiber exits theprevertebral ganglion to innervate the target (the ENS).

Splanchnicnerves

Parotid


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SpinalNerves(pelvic nerve)

Sympathetic

Liver

Gall

bladder

Pancreas

Small

intestine

Colon

Rectum

Cranialnerves

Salivary

glands

Parasympathetic

Parotidgland

Stomach

Spleen

The thoraco-lumbar

efferent autonomicneuronscomprisethe sympatheticdivisionof theautonomic nervoussystem.

Splanchnicnerves

AutonomicNervous System:

The cranio-sacralefferent autonomicneuronscomprisethe parasympatheticdivisionof theautonomic nervoussystem.

Parasympathetic preganglionic fiber


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Salivary Glands

Salivary glands

Preganglionic parasympathetic neurons

Neuronal cell bodies in superiorand inferior salivary nucleiof the medulla

Preganglionic nerve fibers in cranial nerves (VII, and IX) project tosubmandibular and otic ganglia near salivary glands.

Postganglionic parasympathetic neurons

Neuronal cell bodies in submandibular and otic ganglia

Postganglionic nerve fibers project to and innervate the salivary

glands

Submandibularand

Otic Ganglia

Parasympathetic postganglionicfiber

y p p g g

ParasympatheticNervous System:

Salivary Glands

Parasympathetic Nervous System:


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Esophagus through Transverse Colon, Liver,Gallbladder, and Pancreas


Preganglionic neuronal cell bodies in dorsal motornucleusand nucleus ambiguusof the medulla

Preganglionic nerve fibers in cranial nerve X (vagusnerves) project to postganglionic neurons within theganglia within target organs

Postganglionic parasympathetic

neuronsParasympathetic postganglionic

neuronal cell bodies in gangliawithin target organs (e.g.,intrapancreatic ganglia, ENS,etc.)

Parasympathetic postganglionicnerve fibers project to targettissues (e.g., pancreatic acinarcells, ENS interneurons, etc.)

Alter secretion, motility,

absorption, etc.


X



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Descending Colon to Anus


Neuronal cell bodies in intermediolateral cell column ofspinal cord segments S2 to S4

Preganglionic nerve fibers contained in spinal nervesform the pelvic nerveswhich project to postganglionicneurons in the ENS of the distal colon

Postganglionicparasympathetic neurons

Postganglionic nerve fibersproject to ENS interneurons

Alter activity within the

ENSAlter secretion, motility,

and absorption

Pelvicnerve


Spinal

nerves

Parotid


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Sympathetic

Spinal nerves

Splancnicnerves

Liver

Gall bladder

Pancreas

Small

intestine

Colon

Rectum

Salivary

glands

Parotidgland

Stomach

Spleen

Sympathetic Nervous

System:the thoraco-lumbar divisionof the autonomic nervoussystem.

Sympathetic preganglionic fiber

Sympathetic postganglionic fiber

Prevertebralsympathetic ganglia

Paravertebralsympathetic ganglia

(chain ganglia)

(e.g.,superior mesenteric

ganglia, inferior mesentericganglia, celiac ganglia, etc.close to the wall of the gi tract)

Sympathetic preganglionic fibers leave the spinal cord through spinalnerves (T1 to L2)


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nerves (T1 to L2) These axons enter the nearest sympathetic

paravertebralganglion and have one of three fates:

1

2

23

The postganglionic neuron exits to innervatean ENS neuron.

1. Synapse on a

postganglionic neuron withinthe paravertebral ganglionat the same level

2. Travel up or down a fewsegments to synapse on apostganglionic neuron within

a paravertebral ganglion In the above cases, the

postganglionicsympathetic neuron exitsthe paravertebral chainganglia to innervate a

target tissue.3. Pass thru the paravertebral ganglion

to synapse on a postganglionicneuron within a prevertebralganglion;

Salivary glands

P li i h i


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Preganglionic sympathetic neurons

Preganglionic neuronal cell bodies in spinalcord intermediolateral cell column (T1-L2)

Preganglionic nerve fibers enter

paravertebral (i.e., beside the vertebra)sympatheticchain gangliato synapse onpostganglionic neurons

Postganglionic fibers ascendthe paravertebral sympathetic

chain ganglia to the superiorcervical gangliaand exit toinnervate the salivary glands

Alter salivary gland secretionand blood flow

Postganglionicsympathetic neurons

Postganglionic neuronalcell bodies withinparavertebral sympatheticchain ganglia

Paravertebralganglia

Superiorcervicalganglion

2

1

Esophagus through Entire Colon, Liver, Gallbladder, andPancreas


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Pancreas

Preganglionic sympathetic neurons

Neuronal cell bodies in spinal cord intermediolateral cellcolumn (T1-L2)

Preganglionic nerve fiberspass through paravertebral sympathetic chain

ganglia without synapsing and

project toprevertebral sympathetic ganglianear theGI tract where they synapse on postganglionicneuronal cell bodies.

Postganglionic sympatheticneurons

Neuronal cell bodies in outlyingprevertebral sympathetic ganglia(e.g.,superior mesenteric, inferior

mesenteric, and celiac ganglia)Postganglionic sympathetic fibers

project to ENS interneurons

Alter neuronal activity withinthe ENS

Alter secretion, motility,and absorption, etc.

Prevertebralganglia

Paravertebral

ganglia

333

Spinal cord


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ENSintestinofugalneuron

Sympatheticpost-ganglionic neuron

Sympatheticpre-ganglionic neuron

Spinal Visceral Afferent Neurons (EPANs)

Neural input to the sympathetic postganglionicneuron within the prevertebral sympatheticganglia:

a) Sympathetic preganglionic fiber from thespinal cord

b) Collateral fiber from spinal visceralafferent neuron

c) Collateral fiber from ENS intestinofugalneuron

Spinal cord(T1-L2) Dorsal root ganglion

Sympathetic prevertebralganglion

Sympathetic paravertebralganglion


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1) Short-loop (Intrinsic) reflex

All elements within the ENS.

2) Long-loop (Extrinsic) reflex

Afferent: spinal visceral, vagal, and pelvic nerves

Convergence of sensory input, processing, integration:spinal cord, brainstem, midbrain, hypothalamus

Efferent: sympathetic or parasympathetic

3) Intermediate loop (prevertebral ganglionic) reflex

Collateral sensory fibers from sensory ENS intestinofugalneurons and spinal visceral afferents to prevertebral ganglia

modify efferent sympathetic neural traffic.

*There is redundancy!!

Neural reflexes possible at three levels:


CentralNervous THE BRAIN-GUT AXIS


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IntegrativeCircuitry

MotorPrograms

~

SympatheticNeurons


ExtrinicPrimaryAfferentNeurons

(sensory)



Circuitry

Motor

Programs


SecretomotorMotor

Vasomotor

Excitatory

Inhibitory

Neurons


NervousSystem

IPANReceptor


Smooth muscle

Epithelial cells

Vasculature


Immune cells


Cajal


Chemical

signal

Short-Loop (ENS)and

Long-Loop(CNS)Neural

Reflexive Pathways


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When stimulatedappropriately, entero-endocrine cells in themucosa release paracrinesubstances into thelamina propria andhormones into the blood.

GI paracrinesubstances andhormones influence

Secretion

Motility

Gastrointest inal Paracr ine

Substances and Hormones:

Endocrine/Paracrine Regulation


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Endocrine/Paracrine Regulationof Gastrointestinal Function

GI tract is the largest endocrine organMucosal endocrine cells -- peptide hormones

Mucosal paracrine cells -- paracrine peptides

ENS neurons -- neurocrine peptides

GI tract is the largest immune organ Immune cells -- paracrine peptides

-- histamine

-- prostaglandins

-- etc.

Endocrine/paracrine regulators of gi functionoperate in concert with neural regulation.



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GI tract as a paracrine organ

GI paracrine cellsare similar to endocrine cells Paracrine substances are secreted into interstitial space in

response to appropriate stimuli.

Paracrine substances diffuse to local target cells

IPANs and EPANs

Absorptive cells

Other endocrine cells

ENS cells

Secretory cells

Paracrine cells

Immune cells

Etc.

Example:Gastric D cells release somatostatin(SS), a paracrine peptide, that inhibits acidsecretion by nearby gastric parietal cells.



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GI tract as an endocrine organ

GI endocrine cells GI hormones are secreted into blood in response to an

appropriate stimulus.

GI hormones act at distant target cells

Absorptive cells

Other endocrine cells IPANs and EPANs

ENS cells

Secretory cells

Paracrine cells

Immune cells

Etc.

Example: Secretin, a hormone released into theblood by the small intestine, stimulates gastric Dcells to release SS that inhibits acid secretion bygastric parietal cells.



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gof Gastrointestinal Function

GI tract as an immune organ GI immune cells

Release paracrine substances into interstitial space inresponse to appropriate stimuli.

bacterial antigenic proteins

cytokines

histamine

peptides prostaglandins

Paracrine substances released from immune cells diffuseto local target cells

Absorptive cells

Endocrine cells IPANs and EPANs

ENS cells

Secretory cells

Paracrine cells

Other immune cells

Etc.

Control of GI Functions


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Control of GI Functions Digestion Secretion

Absorption Motility Excretion

Non-immunological Defenses Immunological Defenses

Types of reflexarcs controllingGI Functions

1) Nervous

2) Endocrine

3) Paracrine

Reflex Arc

AfferentInput

Processingand

Integration

EfferentSecreto-MotorResponse

Presenceo f

s t imu lus

The functions of the gi tract are controlled by a dynamic interplaybetween different cell types that interact directly, or through a large

number of signaling molecules to form reflex arcs.

Documents

digestion 1.ppt