Collapsed Tracts

8/3/2019 Collapsed Tracts

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FasciculusGracilis: (T7down)present allthroughout thespinal cord

FasciculusCuneatus: (T6up)situated laterallyin the upperthoracic andcervicalsegments of the

cord

Septumseparates thesefasciculi

Sense musclespindles and

tendon organs

horn, internuncialneurons andanterior horncellsintersegmentalreflex

Long ascendingfibers: alsoinvolved inintersegmentalreflex but ascendas the fasciculusgracilis andcuneatus

Ipsilateralascent!!!

internal arcuatefibers, cross themedian plane at

the great sensorydecussation at

the inferior/caudal

half of themedulla)

Fibers ascend asthe MEDIALLEMNISCUS

PosteriorSpinocerebellar

UnconsciousMuscle JointSense

Feedback controlfor motorperformance

Musclespindles,

tendon organsand joint

receptors of thetrunk andlower limbs

Posterior rootganglion

in sacral, lumbar

and lowerthoracic levels

(axons enteringthe spinal cord

from the posteriorroots of the lower

lumbar andsacral segments

ascend in the

posterior whitecolumn until they

reach L3/L4,where they enter

the nucleusdorsalis)

Nucleus Dorsalis(ClarkesColumn)

Lamina VII

Axons enter thelateral whitecolumn and

ascendipsilaterally to the

medulla

--- NONE Cerebellarcortex via the

inferiorcerebellar

peduncles(terminate asmossy fibers)

Information isused by

cerebellar cortexin the

coordination oflimb movements

and maintenanceof posture

AnteriorSpinocerebellar

Double-crossingfibers


Information fromskin and

Musclespindles,

tendon organs

and jointreceptors of the

upper and


in lumbar and

sacral segments

Nucleus Dorsalis(ClarkesColumn)

Majority of axons:cross to opposite

--- Axons of N2 crossto contralateral

side at the

nucleus dorsalisat the same level

of entry

Cerebellarcortex via the

superior

cerebellarpeduncles

(terminate as


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Comparatorbetween action ofinhibitory &excitatory inputsto spinal motorneurons andinterneurons

superficial fascia

Providecerebellum withinformation aboutthe state of motorneuron excitation

lower limbs,skin and

superficialfascia

side and ascendcontralaterally

BUT these axonscross back in the

cerebellum

Minority of axons:ascend

ipsilaterally in thelateral white

column

The same fiberscross back to

original side in thecerebellum

mossy fibers)

Cuneocerebellar

Fibers of thefasciculus

cuneatus thatproceed to thecerebelluminstead of BA 312


Musclespindles,

tendon organsand joint

receptors of theupper limband upper

part of thorax


in cervical andupper thoracic

region

Lateral/AccessoryNucleus

Cuneatus ofmedulla

Axons leave forthe cerebellum as

the posteriorexternal arcuate

fibers

--- NONE Cerebellarcortex via the

inferiorcerebellar

peduncles(terminate asmossy fibers)

Spinotectal Afferentinformation forspinovisual

reflexes bringsabout movementof eyes and headtoward source ofstimulation


Unknown

But fibers cross

median plane atthe same level ofentry

--- N2 fibers crossthe median plane

SuperiorColliculus of

Midbrain

Spinoreticular Influences level ofconsciousness

Responsible forspread of painsensationthroughout brain


Unknown

Axons ascend inlateral white

columns mixedwith the spinal

lemniscus

Most fibersremain

UNCROSSED

ReticularFormation in themedulla, ponsand midbrain

Spino-olivary Information fromcutaneous andproprioceptiveorgans


Unknown but inposterior gray

column

Axons ascend inthe contralateral

anterolateral

white column ofthe cord

Inferior OlivaryNuclei (Medulla)

Axons crossmidline back

toward inferiorcerebellar

peduncle onsame side of

entry into the cord

N2 fibers crossthe midline

N3 fibers crossmidline back

toward originalside

Cerebellarcortex via the

inferiorcerebellarpeduncles


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VisceralSensory

Information fromthorax andabdomen

Visceral pain:Caused by

ischemia,chemicaldamage, spasmof smooth muscleand distension

Pain andstretch

receptorendings in

viscera

Visceralafferent fibersusually takepart in reflex

activity


N2 in posterior orlateral gray

columns

Axons arebelieved to join

the spinothalamictracts as they

ascend

Ventralposterolateralnucleus of the

thalamus

SomestheticArea (BA 312) ofcerebral cortex(post-central

gyrus)

NOTES:

In the thalamus, the lateral spinothalamic tract is joined by the posterior white

column pathway (fasciculus gracilis and cuneatus) which mediates position-vibration sensem simple touch and conscious proprioception

FIBERS for position-vibration sense will NOT GO TO GRAY MATTER butwill terminate in the posterior columns (fasciculus gracilis and cuneatus)

The periaqueductal gray area is where pain is masked (area of painmodulation)

LESIONSMIDLINE LESIONS in the SPINOTHALAMIC TRACTS cause BILATERALDEFICITS because fibers cross at the ventral gray commissure!!! Cervical

fibers are in the medial aspect while sacral fibers are in the most lateral

aspect of the tracts so upper body is usually affected by these lesions

Before entry to the thalamus, the MEDIAL LEMNISCUS and the LATERALSPINOTHALAMIC TRACT are almost joined. It is only in the VPLN thatthere is COMPLETE FUSION OF THESE TWO TRACTS!!!

CEREBELLAR LESIONS always manifest on the ipsilateral side!!!

Lesions at POSTERIOR FOSSA LEVEL: ipsilateral head and contralateral

body deficits loss of pain and temperature sensations!

THALAMIC SYNDROME: Loss of all modalities of sensation on thecontralesional side!!!

General Somatosensory System: Sensory tracts for anterior part of the faceAscending Tract Sensation

ConveyedReceptor N1 N2 N3 Decussation(s) Destination (N4)

From CNV1,CNV2, CNV3 ofTrigeminalNerve

Pain &temperature fromthe anterior partof the head

Free nerveendings

IpsilateralSemilunar

(Gasserian)Ganglion

Ipsilateral MainSensory Nucleusof the 5

thcranial

nerve (sensorynucleus of the

trigeminal)

Ventralposteromedialnucleus of the

thalamus

SomestheticArea (BA 312) ofcerebral cortex(post-central

gyrus)


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Direct Activation Pathway: Motor tracts for the initiation and control of voluntary muscle activity Pyramidal TractsDescending

TractMovement Controlled N1

(Cerebral Cortex)N2

(Spinal Cord)N3

(Lower MotorNeuron)

Decussation(s) Branches

CorticospinalTract

Voluntary, discrete &skilled movements,

especially those of thedistal part of the limbs

Majority of fibers aremyelinated and arerelatively SMALL, SLOW-CONDUCTING fibers

Form the pathway that

confers speed and agilityto voluntary movements used in performingrapid, skilled movements

Pyramidal (BETZ)cells in the 5

thlayer

of the cortex in BA4

Fiber origins:1/3: from BA 41/3: from BA 61/3: from BA 312

Descending fibersconverge on the

corona radiata

Posterior limbofinternal capsule

Middle 3/5 of basispedunculi of the

midbrain

Tract is broken inthe pons by the

transversepontocerebellar

fibers

Fibers regroup inthe anterior of the

medulla as thepyramids

2/3 of fibers cross

midline at thedecussation of

pyramids and enterthe lateral whitecolumns to form

lateralcorticospinal tract

Internuncialneurons

Alpha motor andGamma motor

neurons in anteriorhorn cells

Largestcorticospinal fibers

can synapsedirectly with themotor neurons

Act on all motorneuron pools but

especially on lateralportions of theventral horns

(which then controlthe distal muscles

of the arm andhand)

Fibers ventralhorn cells

via glutamate(excitatory)

But ventral horncell axons

muscle fibers viaACh

Junction of themedulla and spinal

cord at thedecussation of the

pyramids

Lesion abovedecussation

contralateral deficit

Lesion below

decussation ipsilateral deficit

Anteriorcorticospinal tract(those uncrossed at

the pyramidaldecussation) cross

over at the level ofdestination

(1) Early fibers thatreturn to the

cortex to inhibitactivity in theadjacent corticalregions

(2) Branches to thecaudate andlentiform nuclei,red nuclei, and

the olivary nucleiand the reticularformation keep thesubcorticalregions informedabout corticalmotor activity

Once alerted,subcortical regions

can send their ownnervous sinals tothe alpha andgamma motorneurons by otherdescendingpathways

CorticobulbarTract

Innervate voluntarymuscles of the larynx,pharynx, etc

Supranuclear fibers

occupy a more

Internuncialneurons

Brainstem motornuclei of cranial

nerves

Same level as exit


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Indirect Activation Pathway: Motor tracts for involuntary muscle control of posture, coordination and equilibrium Extrapyramidal TractsDescending

TractMovement Controlled N1 N2

(Spinal Cord)N3

(Lower MotorNeuron)

Decussation(s) Branches

Medial Indirect Activation Pathways

MedullaryReticulospinalTracts

Depends ondescendingstimuation formsupretentorialmotor structures

Inhibition of extensormotor neurons

Excitation of flexors

Inhibition of tendonreflexes

Hypothalamus controlssympathetic & SACRAL

parasympathetic outflows

Inhibitory Area ofthe ReticularFormation in the

Ventromedial Partof the Caudal

Medulla(lower medulla)

descend in lateralwhite column

Medial interneurons Posterolateral partof anterior hornalpha and gamma

motor neurons

Crosses over atvarious levels Give off multiplebranches as theydescend

TectospinalTracts

Reflex posturalmovements in responseto visual stimuli

Superior Colliculusof Midbrain

cross midline soonafter origin

descends throughthe anterior white

columns close toanterior median

fissure

does not extendbelow the cervical

levels

Internuncialneurons in anteriorgray column in the

upper cervicalsegments

Anteromedialportion of anteriorhorn on both alphaand gamma motor

neurons

Cross midline soonafter origin

?

VestibulospinalTracts

Excitation of extensormotor neurons

Inhibit flexors

Maintains balance

Lateral VestibularNuclei

(beneath the floorof the 4th

ventricle)

descendsuncrossed through

medulla in theanterior white

column

Medial interneuronsof the anterior gray

column

Anteromedialportion of anterior

horn on both alphaand gamma motorneurons

Most descenduncrossed

Receive input frominner ear through

vestibular nerveand from thecerebellum

Lateral Indirect Activation Pathways

PontineReticulospinalTracts

Excitation of extensormotor neurons

ExcitatoryDorsolateral

Reticular Formation

Lateralinterneurons

Anteromedialportion of anteriorhorn on both alpha

Descenduncrossed in themedial part of the

Multiple branchesas they descend


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Cerebellar Afferent Fibers

Cerebellar Afferent Fibers from the Cortex- Important in control of voluntary movements- Information on initiation of movement transmitted from cerebral cortex to the cerebellum so that movement can be monitored and

muscle adjustments can be made

Pathway Origin Cerebellar

Afferent fibers

(from the

cortex)

Descends

through

Terminates where? Further pathway Function

Corticopontocerebellar

Pathway

Nerve cells in thefrontal, parietal,

temporal, & occipital

lobes of cerebral cortex

Corticopontinefibers

coronaradiata

and

internal

capsule

pontine nuclei andmossy fibers to

cerebellar cortex

Pontine nuclei give rise totransverse fibers of the pons

(will cross the midline and

enter opposite cerebellar

hemisphere as the middle

cerebellar peduncle)

Converyscontrol form

cerebral cortex

Cerebro-

olivocerebellar Pathway

Nerve cells in the

frontal, parietal,temporal, & occipital

lobes of cerebral cortex

Cortico-olivary

fibers

corona

radiataand

internal

capsule

Terminate

bilaterally on theinferior olivary

nuclei

Inferior olivarynuclei give rise

to fibers that will cross themidline & enter opposite

cerebellar hemisphere

through inferior cerebellar

peduncle (these fibers

terminate as climbing fibers in

the cerebellar cortex)

Converys

control formcerebral cortex

Cerebroretic

ulocerebellarPathway

Nerve cells from

sensorimotor areas ofcerebral cortex

Corticoreticula

r fibers

Reticular formation

on the same &opposite side in the

pons and medulla

Cell in reticular formation give

rise to reticulocerebellarfibers and will enter the

cerebellar hemisphere on the

same side through inferior

and middle cerebellar

peduncles

Converys

control formcerebral cortex

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Collapsed Tracts