SMP Neuro Lab Manual

7/28/2019 SMP Neuro Lab Manual

1/37

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IntrotoNeuroscienceLabManual2013 2

TEACHINGFACULTY

NAME TITLE OFFICE EMAIL

Nabil Azzam, Ph.D. Professor

Neuroscience

WP-08

Research Building

[email protected]

Alessia Bachis, Ph.D. Assistant Professor EG17CResearch Building

[email protected]

Mark Burns, Ph.D. Assistant ProfessorNeuroscience

WP-22AResearch Building

[email protected]

Katherine Conant, MD AssistantProifessor

EP-16Research Building

[email protected]

Hyang-Sook Hoe, Ph.D. Assistant ProfessorNeuroscience

EP-20Research Building

[email protected]

Kathleen Maguire-Zeiss,

Ph.D.

Associate Professor

Neuroscience

EP-08A

Research Building

[email protected]

Italo Mocchetti, Ph.D. ProfessorNeuroscience

WP-13Research Building

[email protected]

Charbell Moussa, PhD Assistant Professor WP-09BResearch Building

[email protected]

J osef Rauschecker, Ph.D. Professor WP-19Research Building

[email protected]

William Rebeck, PhD Professor WP-10Research Building

[email protected]

Max Riesenhuber, Ph.D. Associate Professor

Neuroscience

WP-12

Research Building

[email protected]

Michael Ullman, PhD Professor D237BBuilding D

[email protected]


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INTRODUCTION

Thepurposeof this laboratory is toprovideyouhandsonexperiencewithavarietyof

humancentralnervoussystem(CNS)specimens,andtoreinforcethe importantconceptsof

the lecturecomponentofthecourse. Thesespecimens includewholeandhalfbrains,brain

dissections,plastic

embedded

brain

sections,

Magnetic

Resonance

Images

(MRIs).

LABORATORYFORMATSchedule:

Youwillbeassignedtoonegroup(eitherTuesday,May7th

orWednesday,May8th)who

willbeusingroomsLE2A,LE2B,LF1A,orLF1BofthePreClinicalScienceBuilding. These

roomsareequippedwithsafetyfeaturestohandlehumanmaterial. Thelaboratorywillstart

promptlyat1:30PM,withabriefintroductionbyafacultymember. About3:40PM,youwill

workwith the instructor to review/solve the four questions listed at the end of this lab

manual.

LaboratoryRules:

Eatinganddrinkingarenotpermitted inthe labs. Shoesshouldbeclosed (nosandals,

flipflops,etc). Anyonenotwearingappropriateshoeswillbesentawaytochange.

UsingtheLaboratoryMaterial:

The brains are stored in plastic containers of fixative (50% alcohol). Wash the brains

beforeyouusethem,andreplacetheminthecontainersonceyouaredonewiththem. You

shouldonlyhandlethebrainsifyouarewearinggloves. Don'tforgettoclosethelidonthe

brainbucketafterremovingthebrain.

AdditionalmaterialstobeusedareglassslidesinthewoodenboxandMRIsondisplayon

thelightbox. Specialdissectionswillbepresentedbytheinstructors. Ifyouneedtoreview

specimensoutsidenormallaboratorytimespleasecontactthedirectorsofthecourse.

Requiredtext

There isno requiredtext for this lab. This iswhy themanualcontainsseveralpictures

takenfromspecimensused inthe lab. Inaddition,BACC inthe libraryhasseveraldifferent

typesofteachingaidsthatareavailabletohelpinlearningthematerialthatwecoverinthe

laboratory.TheNeuroscienceDepartment togetherwith the Libraryhavedevelopedaweb

sitecontainingmostoftheslidesandMRIusedinthelaboratoryinwhichnucleiandtractsare

labeled. Weencourageyoutousethesetsofglassslidesprovided. Youcanfindthissiteon

the

Educational

Software

page

of

the

Library:

http://www8.georgetown.edu/dml/educ/neurolab/frameset.html (bestviewedwith Internet

Explorer). SectionsthatarepresentedinthislaboratoryarelabeledGT.

Inaddition totheLibraryWebSite,andtoprovideselfdirectedguides for learning the

three dimensional organization of the CNS, othermaterialwill be available on the library

reserveshelforpostedonBlackboard.


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LABORATORYMANUAL

We encourage you to read thismanual before class. Themanual is devided into 7

sectionsthatprovideadetaileddescriptionofhowtolocalize/identifyagivenstructure. Each

sectioncontainsfiguresofactualspecimensthatarelabeledtohelpyouintheidentification.

Pleasebe aware that specimens in yourbucket couldbedifferent from the figuresof this

manual. Therefore,compareyourspecimenswiththoseofothertables,bothtoappreciate

thevariabilitybetweendifferentbrainsandtofindstructureswhichmaybemissingonyour

specimens. Discussquestionsthatarisewithinyourgroup. Yourparticipation isessential in

order to learn thematerial. Ifyouneedmorehelp,askamemberofstaff. Therearenonavequestionsinneuroanatomy!

Attheendofthemanualyouwill findanappendixcontainingadescriptionofthetwo

main pathways that you need to study after the laboratory. This section is labeled

independentstudiesbecausewebelievethatyoucandoitwithouttheinstructors. These

pathways are important for the overall development of a 3D view of the brain and to

appreciatehowneuroanatomycanhelpclinicians inthediagnosisofneurologicaldisorders.

Theappendixalsocontainspotentialexamquestionsthatwillbereviewedwiththeinstructor

attheendofthelaboratory. Athirdappendixincludesadescriptionofthebasicprinciplesof

MRI. Althoughyouarenotgoingtobetestedontheseprinciples,abasiccomprehensionof

howMRIworksisnecessary.

Laboratoryexam

Therewillbenoexam for the lab. However, the finalexamwill contain at least four

questionsbasedonthematerialthatyouhavelearnedinthelab. Unlessotherwiseindicated,

allterms/structuresinBOLDinthismanualareexaminable. Foryourconveniencewehave

listedthenamesofstructuresthatmustbelearned.

Listoftermstobelearned

(inalphabeticalorder)

abducensnerve

accessorynerve

agnosia

tactile,visual

Alzheimersdisease

amydgala

angulargyrus

anteriorcommissure

basalforebrain

basalganglia

brainstem

Broca'sspeecharea

Brodmannareas

caudatenucleus

centralsulcus


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cerebellum,

cerebellarlobes

anterior,posteriorandflocculonodularlobes

cerebellarpeduncles

Superior,middle,andinferiorpeduncles

cerebralaqueduct

cerebralcortex

cerebralhemispheres

cerebralpeduncles(orcruscerebri)

cerebrospinalfluid(CSF)

choroidplexus

cingulategyrus

corpuscallosum

corticallobes

frontal,parietal,occipital,andtemporallobes

corticospinaltract(fibers)

corticopontinefibers

cuneus

diencephalon

expressiveaphasia

facialnerve

flocculus

fornix

fourthventricle

frontalgyrus

superior,middle,andinferiorfrontalgyri

globuspallidus

glossopharyngealnerve

hippocampus

hypoglossalnerve

hypothalamus

infundibulum

internalcapsule

lateralfissure

lateralventricles

linguallobe

longitudinal

fibers

mammillarybodies

mediallemniscus(dorsalcolumn)

medulla(medullaoblongata)

midbrain(mesencephalon)

nucleusaccumbens

occipitalgyrus

superioroccipitalandinferioroccipitalgyri


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oculomotornerve

olfactory

olfactorybulbandolfactorytract

olive

optic

opticnerve,

optic

chiasm

and

optic

tract

paracentrallobule

Parkinsonsdisease

pyramidaldecussation

pyramids

postcentralgyrus

precentralgyrus

precuneus

premotorcortex

primarycortex

auditory,motor,somatosensory,visual

putamen

rednucleus

spinalcord

substantianigra

superiorandinferiorcolliculi(orcorporaquadrigemina)

supramarginalgyrus

uncus

telencephalon

temporal

superior,middleandinferiortemporalgyri

thalamus

thirdventricle

transversefibers

trigeminalnerve

troclearnerve

vagusnerve

vestibulocochlearnerve

Wernicke'sspeecharea


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CHAPTERINDEX

1.DefinitionsandTerms

2.SurfaceTopographyoftheCerebralHemispheres

2A.Lateralsurface

2B.Frontallobe2C.Parietallobe

2D.Occipitallobe

2E.Temporallobe

3.VentralSurface

4.Brainstem

4A.Midbrain

4B.PonsandCerebellum

4C.Medulla

4D.Morecranialnerves

5.MidSagittalSection

5A.Corpuscallosumandsagittalgyri

5B.Visualcortex

5C.Lateralventricles

5D.Diencephalon

6. CoronalBrainSections

6A.Sectionthroughtheuncus,optictract,andhypothalamus

6B.Coronalsectionthroughthesupramarginalgyrus,caudalthalamusandpons6C.Magneticresonanceimaging

7.SpinalCord

8A.Generalanatomy

8B.White

Matter

8C.GrayMatter

Appendix

I.Questions

II.Majorpathways

III.PrinciplesofMRI


8/37

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coronal,horizontal(axial),andsagittal. Theseplanesareillustratedinfigure2.

Figure2.Planesofsections.The reason why we are using different plans of sections to study neuroanatomy is

becauseinternalstructures(e.g.thalamus)canbeseenonlybytakingapartaportionofthe

cerebralcortex. Youwillseeseveralexamplesduringthislaboratory.

Another importantsetof termsused inneuroanatomydescribeconnectionswithin the

CNS. TheCNSisgenerallybilaterallysymmetrical,andthetwohalvesarejoinedlargely(but

not exclusively) by nerve fiber bundles referred to as commissures. Commissures

interconnecthomologousareasonthetwosidesofthebrain.

Anothertypeofcrossingisadecussation(Latinfor"xshapedcrossing")thatreferstothe

intercrossingofhomologousaxonalpathwaysaseachcrossestotheoppositesideoftheCNS

during the courseof itsascentordescent through theCNS. Aprojection takesonvarious

namesaccording

to

the

historical

origin

of

their

discovery.

They

may

sometimes

be

called

tract,suchasthepyramidaltract,lemniscustract(e.g.,mediallemniscus),orfasciculus(e.g.

fasciculus gracilis). Each tract/fasciculus contains fibers that originate and go to different

portionoftheCNS. Infreshtissues,myelinatedfibertractshaveawhiteappearance,hence

thenamewhitematter,whereasareasoftheCNSthatcontainpredominatelyneuronalcell

bodiesand theirprocesses (whichconstitute theneuropil)haveagrayappearanceandare

calledgraymatter. Tobetterevidentiategrayandwhitematter,neuroanatomistspreferto

usefixedtissueandspecificdyesthatstainmyelindark. Therefore,infixedtissuethewhite

matterappearsdark(darkblue/darkbrown)whereasthegraymatterappearspale.

2.

SURFACE

TOPOGRAPHY

OF

THE

CEREBRAL

HEMISPHERES

2A.LateralSurfaceandCorticalLobes

Usethewholebrainandexamine its lateralsurface. Theprominentexternalfeatureof

thetelencephalon,thecerebralcortex,iseasilyidentifiedsinceitcontainsverycharacteristic

sulciandgyri(Fig.3). Gyriarebumpsofthecorticalmantleandthegroovesarethesulci,or

if particularly deep, are termed fissures. Gyri, in turn, form four distinct lobes: Frontal,

Parietal, Occipital, and Temporal (Fig. 3). Each lobemay be concernedwith a different


10/37


functionasdescribed later. Thegyriofthehumanbrainare referredtobothbynameand

alsobyanumberingsystemoriginatedbyBrodmannwhosubdividedthecerebralcortexinto

52areasbasedoncytoarchitecturalcriteria. Youarerequiredtoknowonlythosesulciand

gyriandassociatedBrodmannareasthatarespecificallymentionedinlecturesorareinbold

in this manual. In order to understand the localization of higher cortical functions it is

thereforeessential

that

you

develop

asolid

understanding

of

the

anatomical

subdivisions

of

the cerebral cortex. You should be aware that some lobules and gyri are given different

names indifferent textbooks. For this course you areexpected touse thenomenclature

indicatedon the illustrationsand in the text

ofthislabmanual.

Figure3. Nomenclatureofcerebralcortexlobes.

To appreciate the generalnomenclature

and function of gyri and lobes, begin by

locatingthe

following

two

fissures

or

sulci:

thelateralfissure(orsulcus)andthecentral

sulcus(Fig.4).Theseareimportantlandmarks

because they form large portions of the

boundaries of the fourmajor lobes of each

cerebral hemisphere. The lateral sulcus

begins at the inferior surface of the hemisphere and extends in a posterior direction to

partiallydemarcate the frontal,parietaland temporal lobesof the cerebrum (Fig.4). The

central sulcus is a deep (about 2 cm) groove locatedmainly on the lateral aspect of the

cerebralhemisphere,withasmallportionextendingontothemedialsurface. Itbeginsonthe

superiorborderof thehemisphere slightly (about1 cm)behind themidpointbetween the

frontalpole(therostralmosttipofthehemisphere)andtheoccipitalpole(thecaudalmosttipof thehemisphere). It slopesdownward and forward across thehemisphereandends

abovethelateralsulcus. Indoingthis,thecentralsulcusseparatesthefrontalfromparietal

lobes.

2B.FrontalLobe

Thefrontal lobe istheportionofthecortexrostraltothecentralsulcusandsuperiorto

the lateral fissure. The firstmajorgrove rostral (about1.5 cm) to thecentral sulcus is the

precentral sulcus. The gyrus that lies between the precentral and central sulci is the

precentralgyrus. Thisgyrus forms theprimarymotor cortex,orBrodmann'sarea4. The

precentralgyrus

contains

an

orderly

motor

map

of

the

body

(termed

the

homunculus),

with

the head represented laterally and inferiorly and the feet represented on the medial

extensionofprimarymotor cortex (i.e., the feet insideof the longitudinal fissure) and the

tonguerepresentationextendinguptotheanteriorwallofthecentralsulcusneartheinferior

frontalgyrus. Lesionsoftheprecentralgyrusresultinmotordeficitsonthecontralateralside

ofthebody.


11/37

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12/37


orientation. Theinferiorportionoftheparietallobeisformedbytwogyri: thesupramarginal

gyrusandtheangulargyrus(Fig.4). Thesupramarginalgyruscanbeidentifiedbyfollowing

theposteriorportionofthe lateral fissurestartingat thepointwhere itascends; thetissue

surroundingthisfissure isthesupramarginalgyrus. Lesionsofthisgyruscanresultintactile

agnosiawhich is the inability to recognize common objectson thebasisof tactile cuesor

stimuli.Using

tactile

cues

alone,

an

individual

can

recognize

general

features

of

an

object

placed in thehandcontralateralto the lesion (e.g.,metallic,cold,round),butcannotname

theobjectitself.

The angular gyrus (Fig. 4) is adjacent to the supramarginal gyrus and surrounds the

upturnedposteriorendofthesuperiortemporalsulcus(seetemporal lobe). Lesionsofthis

gyrus in the dominant hemisphere can result in visual agnosia. There is impaired, or no,

comprehensionof thewrittenword, although theword canbe seen. Lesionsofboth the

angular and supramarginal gyri lead to Gerstmann syndrome, a neurological disease

characterized by deficiency in the ability to write (dysgraphia), difficulty in learning

mathematics(dyscalculia),inabilitytodistinguishthefingersonthehand(fingeragnosia)and

leftright

disorientation.

Figure 5. Brocas andWernickessareas.

2D.OccipitalLobe

Theoccipital lobe (Fig.3) istheareaofthecerebralhemispherethat liescaudaltothe

parietal

lobe

(on

the

lateral

surface

trace

an

imaginary

line

extending

from

the

preoccipital

notch to the caudal portion of the superior parietal lobe). This lobe contains the visual

processingcenter. Theoccipital lobe iscomposedmainlyofprimaryvisualcortex(area17)

andvisualassociationcortex(areas18and19). Wewillstudymoredetailsaboutthevisual

cortexin5B.

2E.TemporalLobe


13/37


Thetemporal lobe isseparatedfromthefrontalandparietallobesbythelateralfissure

(Fig. 3) and from the occipital lobe by the latter's imaginary rostral border. In the lateral

surface,thetemporal lobe issubdivided intothreeportionsbytwomajorsulci(thesuperior

temporalsulcusandthe inferiortemporalsulcus)thatrunparalleltothe lateralfissure(Fig.

4). These are the superior temporal gyrus, themiddle temporal gyrus, and the inferior

temporalgyrus.

Lesions

in

the

temporal

lobe

cause

temporal

lobe

epilepsy

acondition

which

givesrisetorecurrentseizures.

Hiddenwithintheposterioraspectofthelateralfissureintheinnerbankofthesuperior

temporalgyrusarethetransversetemporalgyriofHeschl(Brodmann'sareas41&42). This

regionconstitutestheprimaryauditorycortex. Theposteriorpartofthesuperiortemporal

gyrus,at its inner intersectionwiththeangularandsupramarginalgyri,containsWernicke's

speecharea(Brodmann'sarea22,Fig.5). LesionsinWernicke'sareaonthedominantsideor

bilaterallyresult inbothadeficit intheunderstandingofthespokenwordandtheabilityto

expressoneselfverbally.

Potentialexamquestionrelatedtothissection. Pleaseseequestion#1inappendix3.VENTRALSURFACE

Thecerebralcortexcontinuesat theventral surface (Fig.6). Observe thegyrus that is

adjacent (medially) tothe inferiortemporalgyrus. This istheparahippocampalgyrus. This

gyrus surrounds the hippocampus (or hippocampal formation), a crucial structure for

declarative memory, and thus the gateway for longterm memory retention. At the

rostromedialaspectof theparahippocampalgyrusyoushouldbeable toseeabulgecalled

theuncus. Beneaththeuncusisanimportanttelencephalicstructure,theamygdala. Some

nucleioftheamygdala(oramygdaloidcomplex)receiveolfactoryprojectionsfromtheventral

cortexsurroundingtheuncus. Thus,thisregionofthetemporallobeistheprimaryolfactory

cortex.

How are odors processed? Examine the ventral surface of the gross brain (Fig. 6).

Identifytheolfactorybulbandtheolfactorytractthatrunthroughthemiddleportionofthe

gyrus rectus. Theolfactory tract contains fibers thatarise from cells in theolfactorybulb,

which in turn, is formed by axons of specialized receptor cells (neurons) located in the

olfactorymucosa. Theolfactorycells form theolfactorynervewhich is responsible for the

senseofolfaction. Follow theolfactory tractsuntil you see abifurcation at the levelof a

regioncalledanteriorperforatedsubstance;this istheolfactorystria. The lateralportionof

the stria reaches the primary olfactory cortex. By this route, olfactory information is

transmitted to the cortex. The anterior perforated substance is gray matter that is

perforatedanteriorlybynumeroussmallbloodvesselsthatsupplyinternalstructures.

Theolfactorynerve is also called cranialnerve I. There are12pairsof cranialnerves

emerging from the brain, each with a different function. They are named or simply

abbreviatedtoCNfollowedbytheLatinnumberingIXII. CNIIistheopticnerve. Thisnerve

may not be present in your specimen because during the removal of the brain from the

cranium, the optic nerve is often cut. However, you can appreciate its location by first

identifyingtheopticchiasm. Lookatthemidportionoftheventralsurface. Youwilleasily


14/37


seetheopticchiasmbecauseithasaXshape(Fig.6). Thetractrostraltotheopticchiasmis

whatremainsoftheopticnerve. Theopticchiasmcontainsapartialdecussationoftheoptic

fibersthatgeneratefromganglioncellsintheretinaandformtheopticnerve. Theaxonsin

theopticchiasmcontinuecaudallyandformtheoptictract.

Figure6. Ventralviewofthecerebralhemisphere. Thepons,cerebellumandmedullahavebeenremoved

Caudaltotheoptictractarethetwomammillarybodies(Fig.6),distinctswellingsthat

belongtothehypothalamus. Thehypothalamusandthethalamus formthediencephalon.

The thalamuswillbe studied laterwhenwewill lookat thehemisectedbrain. The region

bounded by themammillary bodies, optic chiasm, and the beginning of the optic tract is

known as the tuber cinereum, which is the coneshaped protrusion from which the

infundibulumextends.

The

pituitary

gland

is

attached

to

the

infundibulum

(the

pituitary

is

oftenmissinginyourspecimen).

Caudaltothemammillarybodiesisadeepspacecalledinterpeduncularfossa(Fig.6).The

name derives from the fact that it is limited on the right and left by the two cerebral

peduncles (or crus cerebri). Whithin the interpeduncula fossa,you shouldbeablealso to

locate the oculomotor nerve, or CN III (Fig. 7). The oculomotor nerve innervates several

extraocular somatic muscles (inferior oblique, medial rectus, superior rectus and inferior


15/37

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17/37


axonsthatterminateinthespinalcord.

4B.PonsandCerebellum

The pons is especially large in humans because its fibers either run longitudinally

(longitudinalfibers)orhorizontally(transversefibers). Toseethesefibersyouneedtouse

thesections

mounted

on

glass

slides

present

in

your

wooden

box

(glass

slide

#7)

and

the

illustrationbelow (Fig.9). Examplesof longitudinal fibersare thecorticospinal tractat the

baseoftheponsandthemedial lemniscusabovethecorticospinaltract. Thecorticospinal

tract carries axons that generate in themotor cortex and innervatemotorneurons in the

spinalcord. Themediallemniscuscarriessomatosensoryinformationfromthespinalcordto

thecerebralcortex(seeappendix).

Examplesoftransversefibersarethosegeneratedfromthepontinenuclei(Fig.9),small

groupsofcellsscatteredamongthe longitudinalandtransverse fasciculi,thatreceiveaxons

fromthemotorcortexand,inturn,projecttothecontralateralcerebellum. Thus,themotor

cortex influencestheactivityofthecerebellumthroughtherelay inthepontinenuclei. The

tractthat

carries

these

fibers

is

called

the

middle

cerebellar

peduncle

(MCP)

which

can

be

locatedalongthe lateralaspectof theponsconnecting thepons tothecerebellum (Fig.9).

Additional peduncles that connect the cerebellum to the brain stem are the superior and

inferiorcerebellarpeduncles. Thesepedunclesalsocarryfiberstoandfromthecerebellum.

Cerebellarpedunclesalsohelpformingthelateralwallsofthefourthventricle(Fig.9)whichis

anexpansionofthecerebralaqueductinitscaudalportion(Fig.7). Wewilldiscussthefourth

ventricleinmoredetaillater.

Figure9 Leftpanel.Ventralviewofthebrain.Theblacklinethroughtheponsillustratestheplaneof

cutusedtoobtainthetransversesectionshownon

the right. Right panel: A=descending longitudinal fibers; B=medial lemniscus; C=pontinenuclei;D=middlecerebellarpeduncle;E=fourthventricle;F=pontocerebellarfibers.


18/37


TheponsalsocontainsseveralCNnuclei. Wewillnotbe learningthe locationofthese

nucleiinthiscourseduetothetimeconstraint. However,beawareofthefactthatimpaired

functionsofcranialnerveareoftenusedinclinicalsettingstodiagnoseneurologicaldiseases.

Locatethecerebellum.Inthegrossbrain,thecerebellumcanbeeasilyrecognizedinthe

caudaldorsalportionofthewholebrainforitstypicaltransversesulciandconvolutioncalled

folia.There

is

no

need

for

learning

the

names

of

these

sulci.

However,

it

is

important

to

recall

thethreemainsubdivisionsofthecerebellarlobesbecausetheyareassociatedwithdifferent

functions. From rostral to caudal we consider three lobes: anterior, posterior and

flocculonodular lobes(Fig.10). Thelobesofcerebellumarelocalizedbyfirstidentifyingthe

primaryfissure,whichformstheboundarybetweentheanterior lobeofthecerebellumand

theposterior lobe.Theposterior lobe is the largestareaof thecerebellum. This lobehas

majorconnectionwiththecerebralcortexandisimportantinplanningvoluntarymovements.

Figure10.Ventralviewofthecerebellum.The flocculonodular lobe is made of the two lateral paired flocculi and the median

nodulus. The flocculus can be seen on the ventral surface of the cerebellum projecting

laterallyforabout2cmoneachsideofthemedulla(Fig.10). Thislobe ismainlyassociated

withvestibularfunctionsandisconcernedwithequilibrium.

Now lookat theventralportionofthegrossbrain. In the lateralportionof thebasilar

pons,approximatelyatthemidline,youshouldbeabletoseethetrigeminalnerve(orcranial

nerveV, Figs. 9 and 10). This is a very largenerve and containsbothmotor and sensory

components. Thesensoryportioncarriesalltypesofsensationsfromtheregionoftheface.

Themotorpartinnervatesthemusclesofmastication.

4C.Medulla

Themedulla (ormedullaoblongata) is a cone shape structurewhich is about3/4 inch

long. It containsmany groups of fibers and nuclei that regulate the cardiac, respiratory,

vomiting and vasomotor activities and dealwith involuntary functions, such as breathing,

heart rate and blood pressure. It begins after the pons at a region known as the

pontomedullaryjunction,aprominentsulcusthatrunstransversely(Fig.10). Themedulla is

easily recognizable for its most prominent features located in the ventral surface: the

pyramids (Fig. 10). These are separated along themedian plane by the anteriormedian

fissure. Thepyramidscontainthecorticospinaltract. Canyoulocalizethistractonglassslide


19/37


#4? Lateraltothepyramidsisanelongatedprotuberancecalledtheolive(Fig.11). Theolive

isactuallyformedbytheolivarynucleus,a largegroupofneuronsthatplayarole inmotor

learning. Thisnucleuscanbeeasilyseenonlyinstainedsections(Fig.11).

Figure11.Sectionoftheopenmedullathroughtheolive. Leftpanel, ventral viewof thebrain stem.The blue line showswhere the cutwas done to

obtain the transverse section on the right panel.

A=corticospinal trat; B=Olivary nucleus; C=medial

lemniscus;D=fourthventricle.

At its rostral end, themedullawidens and helps form the caudal floor of the fourth

ventricle(seethehemisectedbraininFig.7). Thisventriclecontainstwoapertures(foramen

of Luska and Magendie) that allow the CSF to enter the subarachnoid space (the space

between the arachnoidmembrane and piamater). From the arachnoid space the CSF is

absorbedintothevenousdrainage.

The caudalportionof themedulladoesnot contain the4th

ventricleandappearsoval

resemblingtheshapeofthespinalcord(Fig.11). Seealsoglassslide#3. Indeed,thecaudal

portionof themedulla turns into the spinalcordafter thepyramidaldecussation (Fig.11).

Thepyramidaldecussationcontainsthemajorityofcorticospinalfibersthatdecussategoing

into the lateralportionof the spinalcord,hence thename lateralcorticospinal tract. This

pathwayisinvolvedwithvoluntarycontroloflimbs.

Potentialexamquestionrelatedtothissection.Pleaseseequestion#2inAppendix.4D. Morecranialnerves(CN)

Returntotherostralmedullaatthelevelofthepontomedullaryjunction. Youshouldbe

abletonoticeaseriesofrootsthatmarkvariousCNs. ThemostmedialistheabducensorCN

VI (Figs.10and11). Thisnerve innervatesthe lateralrectusmuscleoftheeye. Followthe

junction laterally,youwillencounteradditionalCNs. Theseare the facial (CNVII) and the


20/37


Vestibulocochlear(CNVIII)nerves(Fig.12). Atfirstglancetheymayappearasonlyone. The

moremedialnerveisCNVIIandthelargerlateralnerveistheCNVIII. CNVIIinnervatesthe

musclesoffacialexpression. CNVIIalsocontainsbothautonomiccomponentstotheglands

andsensoryfibersfortastefromtheanteriortwothirdsofthetongue. Thevestibulocochlear

nerveisaspecialsensorynervewithauditory,hearingandvestibular,balancecomponents.

Figure 12. Representative section of closedmedulla. Leftpanel, ventral view of the brain.Thebluelineillustrateswherethesectiononthe

rightwasobtained.Rightpanel,A=Corticospinal;B=mediallemniscus;C=cuneatenucleus;D=

gracilisnucleus.

Followthelateralaspectofthemedullaandmovingcaudallyalongthe lateralborderof

theoliveyouwillfindtherootsoftheglossopharyngeal(CNIX)nerve. Thisisamixednerve

containingbothmotorand sensorycomponents. Thevagusnerve (CNX)consistsofmany

rootlets which also exit from the posterolateral sulcus immediately caudal to the

glossopharyngealnerve. CNIXandCNXarefunctionallyverysimilar. Theycarrythesenseof

tasteandothertypesofsensationfromthemouthregion. Theyalsohavemotorcomponents

thatinnervatethemusclesinvolvedinspeakingandswallowing.

Additionalnerverootletsinserieswiththevagusnerverootlets,butplacedmorecaudally

form

the

cranial

component

of

CN

XI

or

Accessory

Nerve

(Fig.

12).

The

spinal

component

emergesasaseriesof rootlets fromthe first fivecervicalsegmentsofthespinalcord. The

spinal component of CN XI innervates two skeletal muscles in the neck, the

sternocleidomastoidandthetrapezius. The lastnerve,theHypoglossalNerve (CNXII)exits

therostralmedullaoblongataattheanterolateralsulcus locatedbetweentheoliveandthe

pyramid(Fig.12). Thisnerveinnervatesthemusclesofthetongue.


21/37


5.MIDSAGITTALSECTION

5A.Corpuscallosumandsagittalgyri

Nowthatyouhavestudiedtheexternalsurfaceofthebrain,wewillconcentrateonthe

internalstructures.Wewillbeginwithamidsagittalcutofthebrain (Fig.13). Examine the

halfbrain. Thecerebralcortexconsistsof twohemispheresthatare interconnectedby the

corpuscallosum.

The

corpus

callosum

has

aCshape

structure

and

is

made

of

cortical

fibers

thatconnectthetwohemispheres. Thegyrusjustaboveandsurroundingentirelythecorpus

callosum is the cingulate gyrus. This gyrus plays a role in different functions, including

attention,motivationandemotional responses. In its rostralportion,thecingulategyrus is

cappedbythesuperior frontalgyrus. Thisgyrusappearstobe involved inhighercognitive

functions (workingmemory and selfawareness). The gyrus caudal to the superior frontal

gyrusandabovethemidportionofthecingulategyrusistheparacentrallobule. Therostral

portionof this lobule contains theprimarymotor cortexwhereas the caudal region is the

primary somatosensory cortex. This region of the cortex regulates movement of the

contralateral lower limbs. The gyrus immediately caudal to the paracental lobule is the

precuneus,an

area

of

the

parietal

cortex

that

is

involved

in

visuo

spatial

imagery,

episodic

memoryretrievalandselfprocessingoperations.

Figure13. Midsagittalviewoftheleftcerebralhemisphere.5B.VisualCortex

Thegyricaudaltotheprecuneusbelongtotheoccipitalpole. Asmentionedbefore,this

isthevisualcortex. Youneedtolocalizetwoimportantareasofthevisualcortex:thecuneus

whichistheareaoftheprimaryvisualcortex,andthelingualgyrus. Thecuneusisthegyrus


22/37


immediately caudal to theprecuneus. It is separated from theprecuneusby theparietal

occipital sulcus which begins in the lateral surface approximately 45 cm in front of the

occipitalpoleandslopesmediallydownwardandforwardalmosttouchingtheposteriorend

of thecorpus callosum. Approximatelyat themidlineof this sulcus you shouldbeable to

identifyanothersulcusthatrunshorizontallyandjoinstheparietaloccipitalsulcus. Thisisthe

calcarinefissure.

The

area

ventral

to

the

calcarine

fissure

is

the

lingual

gyrus.

The

cuneus

receives visual information from the contralateral superior retina representing the inferior

visual field,and the lingualgyrus receives information from thecontralateral inferior retina

representing the superior visual field. Lesions of both primary visual cortices result in

completeblindnessbilaterally. Destructionofoneprimaryvisualcortexresultsinblindnessin

thecontralateralvisualfield. Lesionsinvisualassociationcortex,whichspareprimaryvisual

cortex, result invisualagnosia (the inability to identifyanobjectbynameordetermine its

functionthroughvisualcuesalone).

5C.LateralVentricles

The hemisected brain allows you to further understand the relationship between the

telencephalon,diencephalon andmidbrain (ormesencephalon) and theventricular system.

Beginby locatingagain thecorpuscallosumwhich forms the roofof the lateral ventricles.

The lateral ventricles are the largest of the CSF filled ventricular cavities that follow the

naturalcurvatureofthetelencephalonandare,therefore,essentiallyCshaped(Fig.14). The

paired lateral ventricles are divided into three branches or horns: the anterior (frontal),

posterior(occipital)andinferior(temporal)horns,andabodyregion. Youshouldbeableto

appreciate the size of the lateral ventricles by removing the septum pellucidum, the thin

membranethatformsthemedialwallofthisventricle(itmaybemissingortorn insomeof

the hemisected brains). In the floor of this ventricle, you should be able to identify the

choroidplexus,aseriesofepithelialcells,capillariesandlooseconnectivetissuethatproduce

theCSF.

Figure 14. General view of theventricles. The direction of flow of

CSF in the lateral ventricles is from

the inferior horn through the body

andout the interventricular foramen

into the third ventricle, the cerebral

aqueduct into the fourth ventricle.

Please note the location of the

choroidplexus.


23/37


5D.Diencephalon

TheCSFof the lateralventricleempties into the third ventricle (Figs.13and14)viaa

structurecalledtheforamenofMonro(interventricularforamen). Thethirdventriclemarks

the beginning of the diencephalon, that we previously described containing two major

structures,thethalamusandhypothalamus. Thethalamusformsthelateralwallsofthethird

ventricle(Fig.

13).

The

thalamus

is

arelay

center

for

sensory

and

motor

signals

to

the

cerebral

cortex. It also controls sleep and awaken states. Ventrally to it, you will find the

hypothalamuswhichalsohelpsformingaportionofthelateralwallsandthefloorofthethird

ventricle. The hypothalamus controls certain homeostatic processes, including hormonal

releaseandvariousactivitiesoftheautonomicnervoussystem.

ThethintissueofwhitematterthatformstheroofoftheforamenofMonroisthefornix

(Fig. 13), an important fiber tract that carries signals from the hippocampus to the

hypothalamus. Thehippocampal/hypothalamiccircuit,via itsconnectionwiththecingulate

cortex,isinvolvedinthecontrolofemotions.

6.

CORONAL

BRAIN

SECTIONS

6A.Sectionthroughtheoptictract,hypothalamusanduncus

Wewillnowspendsometimelookingatvariousinternalstructuresofthetelencephalon

and diencephalon that are visible in the coronal sections of the gross brain specimens

(sectionsarealreadypreparedbecausewedonothaveenoughbrainspecimenstoallowyou

todissect themyourself). Theobjectiveof thisportionof the laboratory is toexpandyour

threedimensionalunderstandingofthebrainbyevaluating internalstructures. Beawareof

thefactthatthesestructuresareoftendamagedbystroke,tumors,aging,orothertypesof

lesions. Thus,theknowledgeoftheiranatomyhasaclinicalimportance.

Locateasectionthat isclosely represented inFig.15. Ask the instructor ifyoucannot

locateit.

The

section

should

have

been

cut

through

the

optic

tract,

hypothalamus

and

uncus

and it should contain various structures of the telencephalon (e.g. cortex and caudate

nucleus)aswellasofthediencephalon(e.g.thalamusandhypothalamus).

Startattheventrolateralportionofthecortex. Thisisthetemporallobeandtheuncusis

itsmostmedialportion. Embeddedintheuncusyouwillfindtheamygdala. Theamygdalais

associatedwithemotions,learningandmemoryfunctions. Thetwoamygdalaeareconnected

to each other by the anterior commissure. You should be able to see this commissure

spanningventrallyfromonetemporallobetotheotherone(Fig.15). Nevertheless,itmaybe

missingon same specimensdue to the angleofdissection. Similarly,dependingupon the

planeof section, theamydgalamaybemissingandeither thehippocampusor the inferior

portionof

the

lateral

ventricle

is

present

instead

of

it.

The

area

below

the

anterior

commissurebelongstothebasalforebrain. NeuronallossinthisareacanleadtoAlzheimers

disease.

Themidventralportionof the sectionjustdorsalof theoptic tract isoccupiedby the

hypothalamus. The space surroundedby thehypothalamus is the third ventricle. Moving

superiorlyyoumay find thehypothalamicadhesion (ormassa intermedia)whichmarks the

beginningofthethalamus. Thethalamusformsthefloorofthelateralventriclesthatinthis


24/37


sectionhaveabutterflyshape. Thechoroidplexus lieontopofthethalamus. Thewallsof

the lateralventriclesare formedbythecaudatenucleuswhereas the roof is formedbythe

corpus callosum. You can clearly see that the two lateral ventricles are separatedby the

septumpellucidumandthefornix.

Figure15.Coronalsectionthroughthehypothalamus,optictractanduncus.Gobackto thecaudatenucleus. ThecaudatenucleushasaCshapestructurewithan

enlargementintherostralportion(headofthecaudatenucleus)andatailattheend(tailin

latiniscauda,thereforethenamecaudatenucleus). ThesectionshowninFig.15isthrough

thebodyofthecaudate.

Thewhitematter immediately lateraltothecaudate isthe internalcapsule,agroupof

fibersthatcarryinformationtoandfromthecerebralcortex. Adjacenttotheinternalcapsule

slightlyventral to thecaudatenucleus is theputamen. In this section the internalcapsule

separatesthe

body

of

the

caudate

from

the

more

ventral

putamen

and

globus

pallidus.

The

caudate,putamenandglobuspallidusarecomponentsofthebasalganglia,subcorticalnuclei

thatcontrolvoluntarymovements.

The caudate nucleus, in its ventral position (ventral striatum), contains a subdivision

termednucleusaccumbens. Thenucleusaccumbens istheportionoftheCNSthatoften is

called the pleasure center. When someone craves a substance (including food), neural

activity increases in this nucleus in anticipation of future pleasure. Thus, it has been


25/37

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26/37


At the dorsocaudal border of the thalamus is located the posterior commissure. The

posterior commissure is a convenient landmark for localizing the border between the

diencephalonandthemesencephalon. It isbelievedthatthiscommissureconnectsvarious

structures involved in bilateral pupillary light reflex. The small opening ventral to the

posterior commissure is the cerebral aqueductwhich, in turn,marks thebeginningof the

midbrain.Therefore,

you

should

be

able

to

identify

the

substantia

nigra

lateral

to

the

cerebralaqueduct. Therednucleusandthecruscerebriwillbejustaboveandlateraltothe

substantianigra,respectively. Intheventrolateralsurfaceofthetemporallobeyoushouldbe

abletoseethehippocampus(fromlatinseahorse),cappingtheinferiorportion(horn)ofthe

lateralventricle.

Figure17.Coronalsectionthroughthesupramarginalgyrus,caudalthalamusandpons.

Potentialexamquestionrelatedtothissection.Pleaseseequestion#3inAppendix.6C.MagneticResonanceImaging(MRI).

Tohelpyou todevelopamoreprecise threedimensional representationof subcortical

structuresandhowtheyarerelatedtoeachotheranatomically,wehaveincludedMRIfrom

thesameperspective(additionalMRIscansareondisplayinthelaboratory). MRIisamedical

imagingtechniqueusedinradiologytovisualizedetailedinternalstructuresofourbody. The

basicprinciplesofMRIarepresentedintheappendix. MRIisespeciallyusefulinimagingthe

brainbecauseitprovidesgoodcontrastofthewatercontentbetweengrayandwhitematter.


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Pleasenote thatMRI ismore sensitive tochanges in tissuewatercontent, therefore itcan

expose subtle pathological changes that escape detectionwith othermethods of analysis.

You should appreciate the degree of resolution of brain structures availablewith theMRI

technique. For instance, in Fig. 18, you should be able to locate the following: corpus

callosum,caudatenucleus,internalcapsule,lateralventricle,putamen,anteriorcommissure,

hippocampus,thalamus,

pons.

Ask

the

instructor

ifyou

have

problems!

Figure18.T1weightedMRIobtainedapproximatelyatthelevelofthebrainillustratedinFigs15and17. White=fiberstracts,gray=nuclei,black=spaces.MRI isapoverful technique todetectabnormalities inhumanbrain. The followingpicture

(Fig.19)illustratesthekeyfeaturesofaneurologicaldiseaseobservedina45yearoldmale

individual with impaired cognitive function. Can you identify at least three major

abnormalitiesbylookingatthefollowingT1weighetdMRI?


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Figure19. CoronalMRIofapatientwith???..showingatrophyof..???.....

7.SPINALCORD

7A.Generalanatomy.

Theclosedmedullaendsatthe levelofthepyramidaldecussationbutfibersandnuclei

continueandhelpformthespinalcord. Intheaverageadultthespinalcordisapproximately

4245cminlengthandoccupiesabouttheupper2/3ofthevertebralcanal. Itscaudalendis

usually located at the levelof the intervertebraldiscbetween the firstand second lumbar

vertebrae(Fig.20).Thespinalcord isanchoredtothemeningealsheathcoveringthespinal

cordbyafinethreadcalledthefilumterminale(Fig.20).

Thespinalcordisthemainpathwayforinformationconnectingthebrainandperipheral

nervoussystembythespinalnerves.Thereare31pairsofspinalnervesand31spinalcord

segmentsas

follows:

8cervical

(C1

8),

12

thoracic

(T1

12),

5lumbar,

5sacral,

1coccygeal.

Theanatomical relationshipbetween spinal levels and vertebral levels is an importantone

clinically. The spinal nerves leave the vertebral canal through the intervertebral foramina

accordingtothefollowingpattern: ThefirstcervicalnerveemergesABOVEthefirstcervical

vertebra. The8thcervicalnerveemergesfromtheintervertebralforamenbetweenvertebrae

C7andT1. Alloftheremaining(morecaudal)spinalnervesemergefromthe intervertebral

foramenBELOWthevertebraofthesamenumber. Inspiteofthefactthatthespinalcordis


29/37


notas long as thevertebral column, the rootsof the spinalnervesexit from thevertebral

canal through the intervertebral foramen at the proper vertebral level. This makes it

necessary for the roots to become longer and longer at more caudal levels of the cord

resultinginthelargenumberofnerverootsbelowthelevelofthecordthatformastructure

showninFig.20calledthecaudaequina(fromtheLatin,meaninghorse'stail). Thus,atthe

levelsof

lumbar

vertebra

4/5,

clinicians

can

insert

aneedle

without

worrying

about

puncturing the spinal cord. This iswhy the lumbar puncture is the preferredmethod of

injectingmedicationsintotheCNSorwithdrawingCSFforanalysis.

Figure20.Spinalcordcaudaequina. Themeningiescoveringthespinalcordwerecutopentorevealthespinalcordandnerves,andthefilumterminale.

To better comprehend the importance of the spinal cordwewill review some of the

anatomicalpathways

using

across

section.

More

sections

are

available

in

the

laboratory.

7B.SpinalCordWhiteMatter

Thespinalcordisdividedintoanouterregionofwhitematter,calledfuniculi,containing

longitudinallyrunningnervefibers(myelinatedaxons)surroundinganHshapedcentralregion

of graymatter (Fig. 21). The vertical limbs of theHshaped graymatter form dorsal and

ventralhorns. Thesehornshelpseparatethefuniculi intothreeregions:dorsal (posterior),

lateral,andtheventral(anterior)funiculi(Fig.21).

These funiculi contain three types of fiber tracts: ascending (sensory), descending

(motor) and segmental (providing sensorymotor connectionswithin the spinal cord). The

whitematter

in

the

region

of

the

central

canal

is

subdivided

into

anterior

(ventral)

and

posterior (dorsal)whitecommissuresoverlying thegraycommissuralareas. Inaddition to

these longascendinganddescendingpathways,all funiculiwithin thespinalcordcontaina

propriospinal tract that is located immediatelyexternal to the graymatter. Thispathway

contains axons that form the short intersegmental projections arising from interneurons

withinthegraymatter.

Majorascendingpathwaysarethedorsalcolumnmediallemniscalsystem(MLF)andthe


30/37


anterolateral system (ALS). TheMLFmediates anumberofproprioceptive and cutaneous

sensations including two point discrimination, stereognosis and limb position sense. The

dorsalcolumnsareformedbythefasciculusgracilis(gracilefasciculus,Fig.21)whichcontrols

the lowerhalfof thebodyand the fasciculus cuneatus (cuneate fasciculus,Fig.21),which

controlstheupperhalfofthebody. TheALScarriesinformationonpainandtemperatureas

wellas

some

varieties

of

touch

for

the

body.

Motor descending pathways are localized in the lateral funiculus (lateral corticospinal

and rubrospinal tracts)aswellas in theventral funiculus (reticulospinalandvestibulospinal

tracts). The lateralcorticospinalandrubrospinaltractscontaindecussatedfibersthathave

generatedinthemotorcortexandrednucleus,respectivelythatinnervatemotorneuronsin

the gray matter (see below). These are themost important fibers controlling voluntary

movementsinhumans. Reticulospinalandvestibulospinaltractsoriginatefromnucleiinthe

ponsandmedullaandarethemainsourcescontrolling involuntarymovements,equilibrium

andmuscletone.

Figure21.Coronalsectionofcervicalspinalcord.7C.SpinalCordGrayMatter

Lookattheslidesorpicturesofspinalcordsections(Fig.21). Notethatthegraymatter

ofthespinalcordhastheshapeofabutterfly,centeredatthecentralcanal. Thedorsal(or

posterior)columns

("horns")

of

the

gray

matter

are

usually

smaller

than

the

ventral

(or

anterior) columns ("horns"). This isbecause theventralhorncontains largemotorneurons

that innervate themuscles. This isparticulary true in thecervicaland lumbarenlargement

portionsofthespinalcordastheycontainneuronsthatinnervatetheupperandlowerlimbs,

respectively. Theareabetween the twohornsandsurrounding thecentralcanal is further

termedtheanteriorandposteriorgraycommissures.

Potentialquestionrelatedtothissection.Pleaseseequestion#4inAppendix.


31/37

Introt

Questi

Th

A.

B.

C.

D.

E.

Neuroscie

n#1

structurei

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32/37

Introt

Th

A.

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Neuroscie

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Superiorcol

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Uncus

lfactorybu

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32


33/37

Introt

Th

A.

B.

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Neuroscie

structurei

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ortex

basalgangli

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34/37

Introt

Al

A.

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Neuroscie

sioninthe

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structurein

pointdiscri

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34


35/37


APPENDIXII(Independentstudiesofmajorpathways)

Because all ascending and descending pathways are funneled into the spinal cord,

lesionsinthespinalcordproduceseveremotorandsensorydeficits. Basedonthesiteofthe

lesion (levelofthespinalcord)apatientmay loosetheabilityofmovingandsensing inthe

upperor

lower

part

of

the

body.

However,

motor

and

sensory

abnormalities

can

also

be

seen

inlesionsofthebrain. Thebestwaytodiagnoseaneurologicaldiseaseistoobservewhether

deficitsofmotorandsensoryfunctionsarepresentandwhichportion(s)ofthebodyhasbeen

affected. Abasicunderstandingofthemajormotorandsensorypathwaysisnecessaryfora

properdiagnosis. The following isanappendixthatreviewstheoriginsandterminationsof

twopathways:thedorsalcolumnmediallemniscalsystemandthelateralcorticospinaltract.

The dorsal columnmedial lemniscal system originates with the central processes of

dorsalrootgangliathatenterthedorsalrootsinthemedialdivision. Theygiveoffcollaterals

that synapse in the dorsal horn and others that ascend in the ipsilateral dorsal funiculus

towards thebrainstem. Fibers from the lowerhalfof thebody give rise to the fasciculus

gracilis(gracile

fasciculus,

Fig.

20);

axons

from

upper

thoracic

and

cervical

ganglia

form

the

fasciculuscuneatus(cuneatefasciculus,Fig.20). Theaxonsformingbothofthesepathways

terminateonnucleibearing thesamenames,nucleusgracilisandnucleuscuneatus, in the

caudalmedulla(Fig.12).

Axonsofprojectionneuronsinthesenuclei(thesecondorderneuronsforthispathway)

formtheinternalarcuatefibersthatcrosstothecontralateralsideofthebrainstemtoform

themediallemniscus. Attheleveloftheopenmedulla,themediallemniscuscanbeseenas

averticallyelongatedmassofheavilymyelinatedaxonsadjacenttothemidline inthe lower

tegmentum,justabovethepyramidsandmedialtotheinferiorolivarynuclei(Fig.11). Inthe

pons, the medial lemniscus is seen as a horizontally elongated fiber mass in the lower

tegmentumextending

laterally

from

the

midline

(Fig.

9).

Axons

representing

the

lower

portion of the body are in themost lateral portion of the lemniscus. At the level of the

superiorcolliculusinthemidbrain,themediallemniscusliesjustlaterallytotherednucleus.

At thejunctionof themidbrainwith thediencephalon,axonsof themedial lemniscuspass

into the thalamus and synapse on the third order neurons of this system. Axons from

projectionneuronsofthethalamustravelintheinternalcapsuletoreachthesomatosensory

cortexofthepostcentralgyrus.

The lateral corticospinal tract arises from axons of neurons located in layer V of the

motorcortex. Axonsarecollectedandfunnelledwithintheposteriorportionoftheinternal

capsule.

In

the

midbrain

these

fibers

help

forming

the

central

portion

of

the

cerebral

peduncles(Fig.8). Atthelevelofthepons,thesefiberscanbeseenaslongitudinalfibersat

thebasisofthepons(Fig.9). Inthemedulla,theselongitudinalfibersbecomethepyramids.

The lateral corticospinal tract decussates at the level of the pyramidal decussation and

occupiesthe lateralfuniculus inthespinalcord(Fig.20)wheretheygiveoffcollateralsthat

synapsewithmotorneuronswithintheventralhorns.


36/37


APPENDIXIII(PrinciplesofMRI)

WrittenbyJ.VanMeter

Themagnetic field inanMRIscanner isgeneratedbysurroundingacoilofsuperconductive

wire with super cooling fluids (liquid helium) lowering the temperature to about 10oK.

Medical MRI takes advantage of the high prevalence of hydrogen in the body and the

magneticpropertiesoftheprotoninahydrogenatom.Protonsinduceasmallmagneticfield

duetotheirspinthatcanbemeasuredinMRI.

TheMRImeasurementconsistsofthefollowingprocesses:

1. Alignmentof theprotons in thebodywith the largemagnetic fieldof theMRIscanner.

After a few seconds in the scanner the protons in the patient are aligned with the

magneticfield.

2. Aradiofrequency(RF)pulseisusedtotiptheprotonsoutofalignmentwiththescanners

magneticfield.

3. OnceoutofalignmentthemagneticmomentofthehydrogenprotonscanbemeasuredastheyrotatepastmeasurementRFcoils(loopsofwire)inducinganelectricalcurrent.

4. Theprotonsarepulledback intoalignmentwiththemainmagnetic fielddecreasing the

measurablesignal. TherateatwhichthisoccursdeterminestheT1propertiesofatissue.

Iftheprotonsinatissuereturntoalignmentfasterthanallothertissuesthenthistissue

willbebrightestonaT1weightedscan.

5. While rotating theprotonsgraduallybecomeoutofphasewithoneanotherdecreasing

the measurable signal. The rate at which this dephasing occurs determines the T2

propertiesofatissue. Iftheprotons inatissueremain inphasewithoneanother longer

thanallothertissuesthenthistissuewillbebrightestonaT2weightedscan.

6. Aproton

density

(PD)

scan

minimizes

both

T1

and

T2

contrast

to

produce

an

image

in

whichbrightnessisdeterminedbythenumberofprotonsinavoxel.

Twocontrolsdetermine tissuecontrast:TR (repetition time)andTE (echo time)of the

scan.RepetitiontimeisthetimebetweensuccessiveRFpulses.Alongrepetitiontimeallows

theprotons in allof the tissues to relaxback into alignmentwith themainmagnetic field

minimizingT1contrast.Ashort repetition timewill result in theprotons fromsome tissues

nothavingfullyrelaxedbackintoalignmentbeforethenextmeasurementismadedecreasing

thesignalfromthistissue.Echotimeisthetimeatwhichtheelectricalsignalinducedbythe

spinningprotonsismeasured.Alongechotimeresultsinreducedsignalintissueslikewhite

matter

and

gray

matter

since

the

protons

are

more

likely

to

become

out

of

phase.

Protons

in

a

fluidwillremaininphaseforalongertimesincetheyarenotconstrainedbystructuressuch

asaxonsandneurons.Ashortechotimereducestheamountofdephasingthatcanoccurin

tissue like white matter and gray matter thus minimizing T2 contrast. The relationship

betweenTRandTEandtissuecontrastareshowninthefigurebelow.


37/37

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