Cranial nerves

Cranial Nerve Anatomy

2. Cranial Nerves
Twelvepairsofcranialnervesareassociatedwith
thebrain
Thefirsttwopairsattachtotheforebrain,andtherest
areassociatedwiththebrainstem.
Otherthanthevagusnerves,whichextendintothe
abdomen,cranialnervesserveonlyheadandneck
structures.

5. I. Olfactory
Thesearethetinysensorynerves(filaments)ofsmell,whichrunfrom
thenasalmucosatosynapsewiththeolfactorybulbs.
Originandcourse:Olfactorynervefibersarisefromolfactory
sensoryneuronslocatedinolfactoryepitheliumofnasalcavityandpass
throughcribriformplateofethmoidbonetosynapseinolfactorybulb.
Fibersofolfactorybulbneuronsextendposteriorlyasolfactorytract,
whichrunsbeneathfrontallobetoentercerebralhemispheresand
terminatesinprimaryolfactorycortex.
Function:Purelysensory;carryafferentimpulsesforsenseofsmell.
Clinicaltesting:Asksubjecttosniffandidentifyaromatic
substances,suchasoilofclovesandvanilla.
AppliedAnatomy:-Fractureofethmoidboneorlesionsofolfactory
fibersmayresultinpartialortotallossofsmell,aconditionknownas
anosmia

7. II. Optic.
Originandcourse:Fibersarisefromretinaofeyetoformoptic
nerve,whichpassesthroughopticcanaloforbit.Theoptic
nervesconvergetoformtheopticchiasmawherefiberspartially
crossover,continueonasoptictracts,enterthalamus,and
synapsethere.
Thalamicfibersrun(astheopticradiation)tooccipital(visual)
cortex,wherevisualinterpretationoccurs.
Function:Purelysensory;carryafferentimpulsesforvision.
Clinicaltesting:Assessvisionandvisualfieldwitheye
AppliedAnatomy:-Damagetoopticnerveresultsinblindness
ineyeservedbynerve.Damagetovisualpathwaybeyondthe
opticchiasmaresultsinpartialvisuallosses.Visualdefectsare
calledanopsias

9. III OculomotorNerves
Originandcourse:Fibersextendfromventralmidbrainandpass
throughbonyorbit,viasuperiororbitalfissure,toeye.
Function:Eachnerveincludesthefollowing:
Somaticmotorfiberstofourofthesixextrinsiceyemuscles(inferior
obliqueandsuperior,inferior,andmedialrectusmuscles)thathelp
directeyeball,andtolevatorpalpebraesuperiorismuscle,which
raisesuppereyelid.
Parasympathetic(autonomic)motorfiberstosphincterpupillae
(circularmusclesofiris),whichcausepupiltoconstrict,andto
ciliarymuscle,controllinglensshapeforvisualfocusing.Some
parasympatheticcellbodiesareintheciliaryganglia.
Sensory(proprioceptor)afferents,whichrunfromsamefour
extrinsiceyemusclestomidbrain.

10. Clinicaltesting:Examinepupilsforsize,shape,and
equality.Testpupillaryreflexwithpenlight.Test
convergencefornearvisionandsubject'sabilityto
followobjectswiththeeyes.
Appliedanatomy:-Inoculomotornerveparalysis,
eyecannotbemovedup,down,orinward.Atrest,eye
rotateslaterally[externalstrabismus]becausethe
actionsofthetwoextrinsiceyemusclesnotservedby
cranialnervesIIIareunopposed.Uppereyeliddroops
(ptosis),andthepersonhasdoublevisionandtrouble
focusingoncloseobjects.

12. IV TrochlearNerves
Originandcourse:Fibersemergefromdorsalmidbrain
andcourseventrallyaroundmidbraintoenterorbit
throughsuperiororbitalfissurealongwithoculomotor
nerves.
Function:Primarilymotornerves;supplysomaticmotor
fibersto(carryproprioceptorfibersfrom)oneofthe
extrinsiceyemuscles,thesuperiorobliquemuscle,which
passesthroughthepulley-shapedtrochlea.
Clinicaltesting:TestwithcranialnerveIII(oculomotor).
AppliedAnatomy:Damagetoatrochlearnerveresultsin
doublevisionandimpairsabilitytorotateeye
inferolaterally.

14. V Trigeminal Nerves
Largest cranial nerves; fibers extend from ponsto face,
and form three divisions (trigemina5 threefold):
Ophthalmic
maxillary,
mandibular
As main general sensory nerves of face, transmit
afferent impulses from touch, temperature, and pain
receptors. Cell bodies of sensory neurons of all three
divisions are located in large trigeminal ganglion.
The mandibulardivision also contains motor fibers
that innervate chewing muscles.

16. Ophthalmic division (V1)
Origin and course:-Fibers run from face to pons
via superior orbital fissure.
Function Conveys:-sensory impulses from skin of
anterior scalp, upper eyelid, and nose, and from nasal
cavity mucosa, cornea, and lacrimalgland.
Clinical testing Corneal reflex test:-Touching cornea
with wisp of cotton should elicit blinking.

17. Maxillary division (V2)
Originandcourse:-Fibersrunfromfacetoponsvia
foramenrotundum.
FunctionConveys:-Conveyssensoryimpulsesfrom
nasalcavitymucosa,palate,upperteeth,skinofcheek,
upperlip,lowereyelid.
ClinicaltestingCornealreflextest:-Testsensations
ofpain,touch,andtemperaturewithsafetypinandhot
andcoldobjects.

18. Mandibulardivision (V3)
Originandcourse:-Fiberspassthroughskullvia
foramenovale.
FunctionConveys:-Conveyssensoryimpulsesfrom
anteriortongue(excepttastebuds),lowerteeth,skin
ofchin,temporalregionofscalp.Suppliesmotorfibers
to,andcarriesproprioceptorfibersfrom,musclesof
mastication.
ClinicaltestingCornealreflextest:-Assessmotor
branchbyaskingpersontoclenchhisteeth,open
mouthagainstresistance,andmovejawsidetoside.

19. Applied Anatomy
Trigeminalneuralgia:-causedbyinflammationoftrigeminal
nerve,iswidelyconsideredtoproducemostexcruciatingpain
known.Thestabbingpainlastsforafewsecondstoaminute,but
itcanberelentless,occurringahundredtimesaday.Usually
provokedbysomesensorystimulus,suchasbrushingteethor
evenapassingbreezehittingtheface.Thoughttobecausedbya
loopofarteryorveinthatcompressesthetrigeminalnervenear
itsexitfromthebrainstem.
Analgesicsandcarbamazepine(ananticonvulsant)areonly
partiallyeffective.Inseverecases,surgeryrelievestheagony
eitherbymovingthecompressingvesselorbydestroyingthe
nerve.
Nervedestructionresultsinlossofsensationonthatsideofface

20. VI AbducensNerves
Originandcourse:Fibersleaveinferiorponsand
enterorbitviasuperiororbitalfissuretoruntoeye.
Function:Primarilymotor;supplysomaticmotor
fiberstolateralrectusmuscle,anextrinsicmuscleof
theeye.Conveyproprioceptorimpulsesfromsame
muscletobrain.
Clinicaltesting:Testincommonwithcranialnerve
III(oculomotor).
AppliedAnatomy:Inabducensnerveparalysis,eye
cannotbemovedlaterally.Atrest,eyeballrotates
medially(internalstrabismus).

22. VII Facial Nerves
Originandcourse:Fibersissuefrompons,justlateraltoabducensnerves
entertemporalboneviainternalacousticmeatus,andrunwithinbone(and
throughinnerearcavity)beforeemergingthroughstylomastoidforamen.
Nervethencoursestolateralaspectofface.
Function:Mixednervesthatarethechiefmotornervesofface.Fivemajor
branches:temporal,zygomatic,buccal,mandibular,andcervical
Conveymotorimpulsestoskeletalmusclesofface(musclesoffacial
expression),exceptforchewingmusclesservedbytrigeminalnerves,and
transmitproprioceptorimpulsesfromsamemusclestopons.
Transmitparasympathetic(autonomic)motorimpulsestolacrimal(tear)
glands,nasalandpalatineglands,andsubmandibularandsublingual
salivaryglands.Someofthecellbodiesoftheseparasympatheticmotor
neuronsareinpterygopalatineandsubmandibulargangliaonthe
trigeminalnerve.
Conveysensoryimpulsesfromtastebudsofanteriortwo-thirdsoftongue;
cellbodiesofthesesensoryneuronsareingeniculateganglion.

23. Clinicaltesting:Testanteriortwo-thirdsoftongue
forabilitytotastesweet(sugar),salty,sour
(vinegar),andbitter(quinine)substances.Check
symmetryofface.Asksubjecttocloseeyes,smile,
whistle,andsoon.Assesstearingwithammonia
fumes.
AppliedAnatomy:-Bell'spalsyischaracterizedby
paralysisoffacialmusclesonaffectedsideand
partiallossoftastesensation.

25. VIII VestibulocochlearNerves
Originandcourse:Fibersarisefromhearingandequilibriumapparatus
locatedwithininnerearoftemporalboneandpassthroughinternalacoustic
meatustoenterbrainstematpons-medullaborder.Afferentfibersfrom
hearingreceptorsincochleaformthecochleardivision;thosefromequilibrium
receptorsinsemicircularcanalsandvestibuleformthevestibulardivision
(vestibularnerve).Thetwodivisionsmergetoformvestibulocochlearnerve.
Function:Mostlysensory.Vestibularbranchtransmitsafferentimpulsesfor
senseofequilibrium,andsensorynervecellbodiesarelocatedinvestibular
ganglia.Cochlearbranchtransmitsafferentimpulsesforsenseofhearing,and
sensorynervecellbodiesarelocatedinspiralganglionwithincochlea.Small
motorcomponentadjuststhesensitivityofsensoryreceptors.
Clinicaltesting:Checkhearingbyairandboneconductionusingtuningfork.
AppliedAnatomy:-Lesionsofcochlearnerveorcochlearreceptorsresultin
central,ornerve,deafness.Damagetovestibulardivisionproducesdizziness,
rapidinvoluntaryeyemovements,lossofbalance,nausea,andvomiting.

27. IX GlossopharyngealNerves
Originandcourse:Fibersemergefrommedullaandleaveskullviajugular
foramentoruntothroat.
Function:Mixednervesthatinnervatepartoftongueandpharynx.Provide
somaticmotorfibersto,andcarryproprioceptorfibersfrom,asuperior
pharyngealmusclecalledthestylopharyngeus,whichelevatesthepharynxin
swallowing.Provideparasympatheticmotorfiberstoparotidsalivaryglands.
Sensoryfibersconducttasteandgeneralsensory(touch,pressure,pain)
impulsesfrompharynxandposteriortongue,fromchemoreceptorsinthe
carotidbody(whichmonitorO2andCO2levelsinthebloodandhelpregulate
respiratoryrateanddepth),andfrombaroreceptorsofcarotidsinus(which
monitorbloodpressure).Sensoryneuroncellbodiesarelocatedinsuperior
andinferiorganglia.
Clinicaltesting:Checkpositionofuvula;checkgagandswallowingreflexes.
Asksubjecttospeakandcough.Testposteriorthirdoftonguefortaste.
AppliedAnatomy:-Injuredorinflamedglossopharyngealnervesimpair
swallowingandtaste.

29. X VagusNerves
Originandcourse:Theonlycranialnervestoextendbeyondheadandneck
region.Fibersemergefrommedulla,passthroughskullviajugularforamen,
anddescendthroughneckregionintothoraxandabdomen.
Function:Mixednerves.Nearlyallmotorfibersareparasympatheticefferents,
exceptthoseservingskeletalmusclesofpharynxandlarynx.Parasympathetic
motorfiberssupplyheart,lungs,andabdominalvisceraandareinvolvedin
regulatingheartrate,breathing,anddigestivesystemactivity.Transmitsensory
impulsesfromthoracicandabdominalviscera,fromtheaorticarch
baroreceptors(forbloodpressure)andthecarotidandaorticbodies
(chemoreceptorsforrespiration),andtastebudsontheepiglottis.Carry
proprioceptorfibersfrommusclesoflarynxandpharynx.
Clinical testing: As for cranial nerve IX.
Applied Anatomy:-Since laryngeal branches of the vagusinnervate nearly all
muscles of the larynx, vagalnerve paralysis can lead to hoarseness or loss of
voice.
Other symptoms are difficulty swallowing and impaired digestive system motility.

31. XI Accessory Nerves
Originandcourse:Uniqueinthattheyformfromrootletsthat
emergefromthespinalcord,notthebrainstem.Theserootletsarise
laterallyfromsuperiorregion(C1–C5)ofspinalcord,passupward
alongspinalcord,andentertheskullastheaccessorynervesvia
foramenmagnum.Theaccessorynervesexitfromskullthroughjugular
foramentogetherwiththevagusnerves,andsupplytwolargeneck
muscles.
Function:Mixednerves,butprimarilymotorinfunction.Supplymotor
fiberstotrapeziusandsternocleidomastoidmuscles,whichtogether
moveheadandneck,andconveyproprioceptorimpulsesfromsame
muscles.
Clinicaltesting:Checkstrengthofsternocleidomastoidand
trapeziusmusclesbyaskingpersontorotateheadandshrugshoulders
againstresistance.
AppliedAnatomy:-Injurytooneaccessorynervecausesheadtoturn
towardinjurysideasresultofsternocleidomastoidmuscleparalysis.
Shruggingthatshoulder(roleoftrapeziusmuscle)becomesdifficult.

33. XII Hypoglossal Nerves
Originandcourse:hypoglossalnervesmainlyservethetongue.
Fibersarisebyaseriesofrootsfrommedullaandexitfromskull
viahypoglossalcanaltotraveltotongue.
Function:Mixednerves,butprimarilymotorinfunction.Carry
somaticmotorfiberstointrinsicandextrinsicmusclesoftongue,
andproprioceptorfibersfromsamemusclestobrainstem.
Hypoglossalnervecontrolallowstonguemovementsthatmix
andmanipulatefoodduringchewing,andcontributeto
swallowingandspeech.
Clinicaltesting:Asksubjecttoprotrudeandretracttongue.
Noteanydeviationsinposition.
AppliedAnatomy:-Damagetohypoglossalnervescauses
difficultiesinspeechandswallowing.Ifbothnervesare
impaired,thepersoncannotprotrudetongue.Ifonlyonesideis
affected,tonguedeviatestowardaffectedside.