CranialandSpinalNerves

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MedicalNeuroscience|TutorialNotes

CranialandSpinalNerves

MAPTONEUROSCIENCECORECONCEPTS1

NCC1. Thebrainisthebody'smostcomplexorgan.

LEARNINGOBJECTIVES

Afterstudyoftheassignedlearningmaterials,thestudentwill:

1. Discussthefunctionsofthecranialnervesintermsofthesensoryandmotorsignalsconveyedbyeachnerve.

2. Discusstheorganizationandcompositionofatypicalspinalnerve.

NARRATIVEbyLeonardE.WhiteandNellB.CantDepartmentofNeurologyDepartmentofNeurobiologyDukeUniversitySchoolofMedicineDukeInstituteforBrainSciences Introduction

Afterworkingthroughthistutorial,youshouldbeabletodiscussthecompositionandfunctionofthecranialnerves,andyoushouldbeabletodiscussthegeneralorganizationofspinalnerves.Inthenexttutorial,youwilllearnhowthecranialnervesrelatetograymatterstructuresinthebrainstemthatgrewout the axons in the cranial nerves (motor axons) or receive synaptic input from ganglionic neuronsassociated with the nerves (sensory axons). But before proceeding, you should make sure that youunderstandthebasiclayoutofsensoryandmotorneuronsinthebrainstemandspinalcord.

The centralnervous system interactswith theoutsideworld throughprimary sensoryneurons,whichconveyinformationfromthebodyoritsenvironmentintothebrainandspinalcord,andmotorneurons,which activate striatedmuscles andmodulate the activity of cardiac and smoothmuscles and glands(seeFig. 1belowand/orFigure 1.122).Thecellbodiesofprimarysensoryneuronslieinthedorsal rootganglia or the cranial nerve ganglia. Each neuron gives rise to a peripheral process, which receivesinformationeitherdirectlyor throughassociationwith receptors, anda centralprocess,whichentersthecentralnervoussystemandformssynapseswithsecondorderneurons.Thecellbodiesofsomatic 1 VisitBrainFacts.orgforNeuroscienceCoreConcepts(©2012SocietyforNeuroscience)thatofferfundamentalprinciplesaboutthebrainandnervoussystem,themostcomplexlivingstructureknownintheuniverse.2 FigurereferencestoPurvesetal.,Neuroscience,6thEd.,OxfordUniversityPress,2018.[clickURLorcopyintobrowser:https://global.oup.com/academic/product/neuroscience-9781605353807?q=neuroscience&lang=en&cc=us]

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motor neurons lie in clusters ornucleiwithin the central nervous system and give rise to axons thatinnervatestriatedmusclesinthebodyorhead.Youwillalsobeintroducedtoothermotorneuronsthatarepartofthevisceralmotorsystem(a.k.a.,autonomicnervoussystem)andareindirectlyresponsibleforgoverningcardiacmuscle,smoothmuscleorglands.Bytheconclusionofthisandthenexttutorial,youwilllearnhowtolocate:

1. nuclei thatare thedestinationofallprimarysomaticsensory, visceral sensory, and special sensory inputinto the CNS (i.e., the location of all of thesecond-order neuronal cell bodies that receive theprimary sensory input), except for olfaction andvision.Theolfactorynerveandtheopticnervearenotincluded in this discussion; for several reasons theyareatypical.

2. nuclei that are the origin of all of the somatic andvisceralmotoroutputoftheCNS(i.e.,thelocationofall of the alpha motor neurons and preganglionicvisceralmotorneurons).

Fig. 1. Both the spinal cord and brainstem receive inputfrom primary sensory neurons; the cell bodies of theseneurons lie in sensoryganglia. In addition,both the spinalcord and brainstem give rise to motor output to striatedmuscles and to the autonomic ganglia (ANS, autonomicnervoussystem;synonymouswithvisceralmotor system).(IllustrationbyN.B.Cant)

From the viewpoint of clinical practice, the most important general principle of organization in thecentralnervoussystemisthateachCNS function (e.g.,perceptionofsensorystimuli,controlofmotorbehavior) involves groups of neurons—interconnected through synapses—that are spatiallydistributedthroughoutseveralCNSsubdivisions.Groupsofneuronsthattogethersubserveaparticularfunctionarecalleda ‘system’; forexample, thereare thevisual,motor,andsomaticsensorysystems.Thestructurescontainingtheneuronsandaxonsofaparticularsystemarecollectivelyreferredtoasa‘pathway’. (Theterm ‘system’hasa functionalconnotation,whereas the term ‘pathway’ refers to thestructures involved.)Wewill study several important sensory andmotor pathways in detail in futuretutorials.

SimpletestsofcranialnervefunctionprovidecluesforlocalizationofneurologicalinjuryanddiseaseOnemeansforreinforcingyourunderstandingofthefunctionalsignificanceofthecranialnerves istoactuallytesttheir functions inyourselfandawillingfriendorfamilymember.ReviewTableA2below(fromPurvesetal.,Neuroscience,6thEd.),whichliststhecranialnervegangliaandnucleifromwhichthesensory andmotor components of each nerve arise. Then, consider themeans by which you would

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assessthefunctionalintegrityofthecranialnerves.Actually,thereareanumberoftestsofcranialnervefunction that can be done with very simple materials. These tests provide considerable informationaboutthepresenceorabsenceofnormalfunctioninthebrainstemandthenervesthemselves.Someofthesearedescribedonthesenextfewpagestogiveyouanideaofthetypesofteststhatcanbeusedandwhyafoundationalunderstandingoffunctionalneuroanatomyiscriticalforclinicalpractice3.

CranialnervesIII,IV,andVI

Thesenervesaretestedasaunit,sinceallsupplymusclesforeyemovement.Theoculomotornervealsosupplies the levator muscle that elevates the lids and the smoothmuscles that constrict the pupils.Rangeofocularmovementischeckedbyaskingthepatienttofollowthemovementsoftheexaminer’sfingers as they are moved in all directions of gaze. With involvement of the oculomotor nerve, thepatientwillnotbeabletolookup,downormediallywiththeaffectedeye.Therewillalsobedilatationof thepupilanddroopiness (ptosis)orclosureof the lidontheaffectedside. If thetrochlearnerve isaffected,thepatientwillbeunabletolookdownwardwhentheeyeisadducted.Iftheabducensnerveisaffected,thepatientwillnotbeabletolooklaterallywiththeinvolvedeye.Inanyofthesecases,thepatientmaycomplainofdoublevision.(Itisimportanttorememberthateitherthenervesthemselvesortheirnucleiinthemidbrainandponsmaybeinvolved.)ExaminationofthepupillaryreflexesinvolvingnervesIIandIIIwillbeexploredinalatercoursesession.

CranialnerveV

The examiner first checks for the presence of the several types of sensation and then determineswhether both sides of the face are equally sensitive. Failure to feel wisps of cotton touching theforehead,cheeksandjawindicatesanesthesiatolighttouch.Differencesinresponseonthetwosidesof

3 Formoreoncranialnerveexams,visitneuroexam.com[http://neuroexam.com/neuroexam/]andexplorevideosthatshowtestsofcranialnervefunction,withaccompanyingexplanationbyDr.HalBlumfeld,MD,PhD(authorofNeuroanatomythroughClinicalCases;SinauerAssoc.,Inc.).

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the face indicate increasedordecreased sensitivity to light touch. The sameprocedure is followed intestingfordegreeofsensitivitytopinpricksandtowarmandcoldobjects.

Themasseterandtemporalismuscles (musclesofmastication innervatedby themotorcomponentofthe fifth nerve) are examined by palpating them when the jaws are clamped tightly together. Theexaminershouldnotewhetherthereisdeviationofthejawwhenthemouthisopened.

CranialnerveVII

Thepatient is asked to imitate theexaminer asheor she looks at the ceiling,wrinkles the forehead,frowns,smiles,showsteeth,andraisestheeyebrows.Anyasymmetryofthefaceisnoted.Totestthestrengthof theeyelidmuscles, thepatient isaskedtokeephisorhereyesclosedwhile theexaminerattempts to open them. The sensory portion of the facial nerve can be tested by having the patientidentifythetasteofsugarorsaltplacedontheanteriorpartofthetongueoneachside.

CranialnerveVIII

The eighth nerve is divided into two parts, the cochlear or auditory nerve and the vestibular nerve.Special equipment is required to examine the vestibular nerve and it is not tested routinely. (If thepatientgivesahistoryofvertigoordisturbedbalance,thepossibilityofvestibulardysfunctionshouldbeconsideredandthepatientcanbegivenacalorictest,whichisdescribedinPurvesetal.,Neuroscience6thEd.,Chapter14,ClinicalApplications).Preliminarytestsofhearingcanbedonewithatuningfork,butdetailedauditorytestingisdonebyanaudiologist.

CranialnervesIXandX

Thepharyngeal gag reflex is testedby touching each sideof thepharynxwith a tonguedepressor orapplicatorstick.Thepalatalreflexistestedbystrokingeachsideofthemucousmembraneoftheuvula.Thesidetouchedshouldrise.Normalfunctionofthevagusnerveisrevealedbythepatient’sabilitytoswallowandtospeakclearlywithouthoarseness,bysymmetricalmovementsofthevocalcords,andbysymmetricalmovementsofthesoftpalatewhenheorshesays“Ahhh.”

CranialnerveXI

The examiner 1) palpates and notes the strength of the trapezius muscle while the shoulders areshruggedagainstresistance,and2)palpatesandteststhesternocleidomastoidmuscleforstrength.

CranialnerveXII

Anylateraldeviationofthetonguewhenitisprotrudedisnoted.Theexamineralsolooksforatrophyortremorofthetongue.Thestrengthofthetongue istestedbyaskingthepatienttoprotrude itandtomoveitfromsidetosideagainstatonguedepressor.

ThespinalcordThe spinal cord extends caudally from the brainstem, running from the medullary-spinal junction atabout the level of the first cervical vertebra to about the level of the twelfth thoracic vertebra. Thevertebral column (and the spinal cordwithin it) is divided into cervical, thoracic, lumbar, sacral, andcoccygealregions.Theperipheralnerves(calledthespinalorsegmentalnerves)thatinnervatemuchofthebodyarisefromthespinalcord’s31pairsofspinalnerves.Oneachsideofthemidline,thecervicalregion of the cord gives rise to eight cervical nerves (C1–C8), the thoracic region to twelve thoracic

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nerves(T1–T12),thelumbarregiontofivelumbarnerves(L1–L5),thesacralregiontofivesacralnerves(S1–S5), and the coccygeal region to one coccygeal nerve. The segmental spinal nerves leave thevertebral column through the intervertebral foramina that lie adjacent to the respectively numberedvertebralbody.Sensory informationcarriedbytheafferentaxonsofthespinalnervesentersthecordviathedorsalroots,andmotorcommandscarriedbytheefferentaxonsleavethecordviatheventralroots.Once thedorsalandventral roots join, sensoryandmotoraxons (with someexceptions) traveltogetherinthesegmentalspinalnerves.

Two regions of the spinal cord are enlarged to accommodate the greater number of nerve cells andconnections needed to process information related to the upper and lower limbs. The spinal cordexpansionthatcorrespondstothearmsiscalledthecervicalenlargementandincludesspinalsegmentsC3–T1;theexpansionthatcorrespondstothelegsiscalledthelumbarenlargementandincludesspinalsegmentsL1–S2.Becausethespinalcordisconsiderablyshorterthanthevertebralcolumn,lumbarandsacralnervesrunforsomedistanceinthevertebralcanalbeforeemerging,thusformingacollectionofnerve roots known as the cauda equina. This region is the target for an important clinical procedurecalleda“lumbarpuncture”thatallowsforthecollectionofcerebrospinalfluidbyplacinganeedleintothespacesurroundingthesenervestowithdrawfluidforanalysis.Inaddition,localanestheticscanbesafelyintroducedtoproducespinalanesthesia;atthislevel,theriskofdamagetothespinalcordfromapoorlyplacedneedleisminimized.

STUDYQUESTIONS

Q1. IdentifytheCORRECTpairingofcranialnervetofunction.

A. hypoglossalnerve/movementoffacialmusclesforexpressionB. trigeminalnerve/somaticsensationfromfaceC. opticnerve/eyemovementsD. abducensnerve/medialeyemovement(eyeadduction)E. spinalaccessorynerve/vocalarticulation

Q2. Whichofthefollowingstructuresassociatedwiththespinalcordcontainsthecellbodiesofprimarysomaticsensoryneurons?

A. dorsalcolumnB. ventralhornC. dorsalhornD. dorsalrootgangliaE. sympatheticchainganglia