Chapter 23: Digestive System 1

Stomach

• Nervesupply

• Sympathe0candparasympathe0cfibersoftheautonomicnervoussystem

• Bloodsupply

• Celiactrunk,andcorrespondingveins(partofthehepa0cportalsystem)

Stomach

Figure 23.14a

Chapter 23: Digestive System 2

MicroscopicAnatomyoftheStomach

• Muscularisexterna

• Hasanaddi0onalobliquelayerthat:

• Allowsthestomachtochurn,mix,andpummelfoodphysically

• Breaksdownfoodintosmallerfragments

• Epithelialliningiscomposedof:

• Gobletcellsthatproduceacoatofalkalinemucus

• Themucoussurfacelayertrapsabicarbonate‐richfluidbeneathit

• Gastricpitscontaingastricglandsthatsecretegastricjuice,mucus,andgastrin

MicroscopicAnatomyoftheStomach

Figure 23.15

Chapter 23: Digestive System 3

HistologyoftheStomach• Mucosa

• Simplecolumnarepithelium

• Transi0onfromstra0fiedsquamousatgastric‐esophagealjunc0on

• Mostlymucouscells

• Tokeepfromdiges0ngitself,thestomachhasamucosalbarrierwith:

• Athickcoatofbicarbonate‐richmucusonthestomachwall

• Epithelialcellsthatarejoinedby0ghtjunc0ons

• GastricglandsthathavecellsimpermeabletoHCl

• Damagedepithelialcellsarequicklyreplaced

• 3‐6days

HistologyoftheStomach

• MuscularisExterna

• Threelayersinsteadoftwo

• Addi0onalinternalobliquelayer

• Middlecircularlayer

• Outerlongitudinallayer

Mucosa

Submucosa

Muscularisexterna

Serosa

Chapter 23: Digestive System 4

GastricpitsandGlands

• Gastricpits

• Apicalsurfacetolaminapropria

• Mucousneckcells

• Secreteacidmucus

• Parietalcells

• SecreteHClandintrinsicfactor

• pH1.5‐3.5

GastricGlands• Chiefcells

• Producepepsinogen

• Ac0vatedtopepsinby:

• HClinthestomach

• Pepsinitselfviaaposi0vefeedbackmechanism

• Enteroendocrinecells

• Secretegastrin,histamine,endorphins,serotonin,cholecystokinin(CCK),andsomatosta0nintothelaminapropria

Chapter 23: Digestive System 5

Diges9onintheStomach

• Thestomach:

• Holdsingestedfood

• Degradesthisfoodbothphysicallyandchemically

• Enzyma0callydigestsproteinswithpepsin

• Deliverschymetothesmallintes0ne

• Secretesintrinsicfactor

• Requiredforabsorp0onofvitaminB12

Regula9onofGastricSecre9on

• Neuralandhormonalmechanismsregulatethereleaseofgastricjuice

• Parasympathe0cnervouscontrol

• Vagusnerve(X)

• S0mulatesdras0cincreaseinallgastricglandsecre0ons

• HormonalControl

• Gastrin

• Sameresultasparasympathe0cs0mulus

Chapter 23: Digestive System 6

Regula9onofGastricSecre9on

• S0mulatoryandinhibitoryeventsoccurinthreephases

• Cephalic,Gastric,Intes0nal

• Cephalic(reflex)phase

• Priortofoodentry,ini0atedbysight,smell,tasteoffood

• Reflexarc‐sensoryreceptor⇒cerebralcortex/hypothalamus⇒Vagalnucleiinmedulla⇒stomachglandsviavagalmotorfibers

• Inhibitedbyfearorsa0a0on

Regula9onofGastricSecre9on• Gastricphase:oncefoodentersthestomach

• Hormonalandneuralcontrol

• Vagovagalreflex

• Stomachdistension

• Ac0va0onofstretchreceptors(neuralac0va0on)

• Impulsesenttomedullaandbacktomyentericplexustriggeringacetylcholinerelease,causinggastrinrelease

• Ac0va0onofchemoreceptorsbyprotein,caffeine,andrisingpH,alcohol

• S0mulatereleaseofgastrinandincreasedsecre0onandmo0lity

• Inhibitoryeventsinclude:

• ApHlowerthan2

• Emo0onalupsetthatoverridestheparasympathe0cdivision

Chapter 23: Digestive System 7

Intes9nalPhase

• TwoPhases‐excitatoryandinhibitory

• Excitatoryphase

• LowpH;par0allydigestedproteinenterstheduodenum

• S0mulatesduodenalmucosatosecreteenteric(intes0nal)gastrin

• Causesgastricglandac0vityandmaintainsmo0lity

Intes9nalPhase• Inhibitoryphase

• Distensionofduodenum,presenceoffa`yacids,lowpH,orhypertonicchyme,and/orirritantsintheduodenum

• Triggersenterogastricreflex

• Inhibi0onoflocal(gastric)reflexesandvagal(parasympathe0c)nucleiinmedulla

• Sympathe0callyclosesthepyloricsphincter

• Increasedvasoconstric0on

• Releasesenterogastronesthatinhibitgastricsecre0on

• Secre0n:lowersgastricsecre0ons

• Cholecystokinin(CCK):decreasesgastricmo0lity

• Gastricinhibitorypep0de(GIP):lowersgastricsecre0onsandmo0lity

Chapter 23: Digestive System 8

ReleaseofGastricJuice

Figure 23.16

Regula9onandMechanismofHClSecre9on

• HClsecre0on

• S0mulatedbyACh,histamine,andgastrin

• Throughsecond‐messengersystems

• Releaseofhydrochloricacid:

• Islowifonlyoneligandbindstoparietalcells

• Ishighifallthreeligandsbindtoparietalcells

• An0histaminesblockreceptorsanddecreaseHClrelease

Chapter 23: Digestive System 9

Regula9onandMechanismofHClSecre9on

Figure 23.17

ResponseoftheStomachtoFilling

• Stomachpressure

• Remainsconstantun0labout1Loffoodisingested

• Rela0veunchangingpressureresultsfromreflex‐mediatedrelaxa0onandplas0city

• Reflex‐mediatedeventsinclude:

• Recep0verelaxa0on–asfoodtravelsintheesophagus,stomachmusclesrelax

• Adap0verelaxa0on–thestomachdilatesinresponsetogastricfilling

• Plas0city–intrinsicabilityofsmoothmuscletoexhibitthestress‐relaxa0onresponse

Chapter 23: Digestive System 10

GastricContrac9leAc9vity

• Peristal0cwavesmovetowardthepylorusattherateof3perminute

• Thisbasicelectricalrhythm(BER)isini0atedbypacemakercells(cellsofCajal)

• Mostvigorousperistalsisandmixingoccursnearthepylorus

• Chymeiseither:

• Deliveredinsmallamountstotheduodenumor

• Forcedbackwardintothestomachforfurthermixing