Mechanisms and neural substrate of spasticity

Mechanisms & NeuralSubstrate of SpasticityDr. Sharon L. Juliano

DefinitionSpasticty - A velocity dependent increase in muscletone ( hypertonia) with exaggerated increase in reflexesreflexes (hyperreflexia).Hypertonia is associated with an increasedresistance to passive stretch.Also associated with the clasp knife phenomenand clonus.Caused by damage to descending pathways thatinfluence gamma or alpha motor neurons.

UMNCerebral CortexBrain StemUMNDesc.TractstoSpinalCordSpinal CordEffectorTissueSkeletal Muscle ofthe BodyInterneuronsLMN(AlphaMotorNeurons)

LOWER MOTOR NEURON LESIONS UPPER MOTOR NEURON LESIONS• CAUSES: CAUSES:••1) Degeneration of Anterior Horn Cells•2) Transection of Ventral Roots1) Degeneration of Nerve Cell Bodies inthe Motor / Somatosensory Cortices•3) Transection of Spinal Nerves 2) Damage to Axons of DescendingSystems particularly CorticospinalFibers•• SYMPTOMS: SYMPTOMS:•1) Flaccid Paralysis•2) Loss of Myotatic Reflexes•3) Hypotonia•4) Muscle Fasciculations•5) Atrophy of Denervated Muscles1) Spastic Paralysis2) Hyperactive Myotatic Reflexes3) Hypertonia4) Clonus5) Muscle Atrophy--if it occurs, it is verylate and results from disuse• 6) Babinski Sign (Corticospinal Damage)• 7) Loss of Superficial Abdominal Reflexand Cremasteristic Reflex (males)

UMNUMN/Descending Tracts LesionA. does NOT denervate target muscleB. results in spastic paralysisDesc.Tract1. resting tension of muscle is(hypertonia)2. hyperactive myotatic reflexesLMNSpinalNerveSkeletal Muscle ofthe Body3. atrophy, if occurs, is a late event-fromdisuse of spastic muscleLMN/Spinal Nerve LesionA. does denervate target muscleB. results in flaccid paralysis1. absent or diminished muscle tone(hypotonia)2. loss of myotatic reflex3. atrophy of denervated muscle/s

cutaneous

IIIIIIIVVIIIIIIVIVIIXVIIIXIXVIII

Flexors=dorsal in Lam. IXDistal MusclesProximal MusclesAxial (Trunk) MusclesExtensors=ventral in Lam. IX

MEDIALANDLATERALDESCENDING SYSTEMS

Medial Descending TractsAnterior FuniculusLateral Descending TractsLateral FuniculusSynapse inLam. VII, VIII &IX (one tract)Innervate axial& proximalmusclesIXIXIIXIIIIIIVVVIVIIVIIIMedialDescendingTractsIIIIIIVVVIVIIVIIIIIXIXLateralDescendingTractsIXSynapse inLam. V, VI, VII,VIII & IX(one tract)InnervatedistalmusclesUE=FlexorsDistalProximalAxialUE=Extensors

RLCORTICOSPINAL TRACTSMotor & Sensory CortexCSTACSTRLLCSTLCSTVIIXVVIIVIIVVIIXIXVIIIVIIIIXACST

RLRUBROSPINAL TRACTRed Nucleus-MidbrainRSTRLIXIXVVIVIIVIIIVIIVIIIVVIIXIX

RLL. VESTIBULOSPINAL TRACTFunction: facilitatesextension of all 4extremitiesLateral VestibularNucleus (Pons)LVSTRLVIXVIVIIVIIVVIIXIXVIIIVIIIIXLVST

RLM. VESTIBULOSPINAL TRACTFunction: controls neckmovements to stabilize headfor eye gaze movementsMedial VestibularNucleus (Medulla)RLMVSTVIXVIVIIVVIVIIIXIXVIIIVIIIIX

RLM. RETICULOSPINAL TRACTFunction: controls muscleatonia during sleepReticularFormation(Medulla)RLMRSTIXVVIVIIVIIVVIIXIXVIIIVIIIIXMRSTMRST

RLP. RETICULOSPINAL TRACTFunction: facilitatesaxial/proximal musclesReticular Formation(Pons)PRSTRLIXIXVVIVIIVIIIVIIVIIIVVIIXIX

VIIXIXVVIIVIIIR LR LVVIIXIXVIIIReticular Formation(Pons)PRSTVII

RLP. RETICULOSPINAL TRACTXPRSTReticular Formation(Pons)RLIXIXVVIVIIVIIIVIIVIIIVVIIXIXX

••AREA DAMAGED: Transection of the Spinal Cord•STRUCTURES & TRACTS INVOLVED:• 1) Disruption of all Spinal Cord TractsSPINAL CORD TRANSECTION•NEUROLOGICAL DEFICITS:• 1) Period of Spinal Shock (Avg = 3 wks)• 2) Termination of Spinal Shock = Appearance ofbilateral Babinski signs at ~ 3 wks• 3) Period of Minimal Reflex Activity (3-6 wks)• a) muscles flaccid, no myotatic reflexes• b) weak flexor responses that begin distally inresponse to nocioceptive stimuli• 4) Flexor Spasms (6-16 wks)• a) Increasing tone in flexors and strongerflexor responses involving more proximalmuscles in response to nocioceptive stimuli• b) Triple Flexion Response is first seen andconsists of flexion of the lower extremity at the hip, knee and ankle inresponse to mild nocioceptive stimulus• c) Mass Reflex--powerful triple flexion reflex inresponse to non-specific stimulus• d) Paradoxical sweating below lesion level• 5) Alternate Flexor & Extensor Spasms (4 mo-1 yr)• 6) Predominant Extensor Spasms (> 1 yr)• a) marked extensor tone (spasticity & claspknifephenomenon)• b) clonus• c) bilateral Babinski signs• d) loss of sensation below the lesion level• e) increased myotatic reflexes• f) bowel, bladder and sexual functiondisturbances• g) reflex spinal sweating

InhibitoryInterneuronD. RootAfferents[1]ExcitatoryInterneuronDescending TractAxonAlpha Motor Neuron

InhibitoryInterneuronD. RootAfferentsExcitatoryInterneuronDescending TractAxonAlpha Motor Neuron

XInhibitoryInterneuronD. RootAfferentsExcitatoryInterneuronDescending TractXAxonAlpha Motor Neuron

Changes at the level of the spinal cord:oAlpha motor neurons appear to be the most involved.oDecrease in the amount of presynaptic inhibition of 1a afferents.oPost activation repression of 1a afferents is decreased.oReciprocal 1a inhibition is reduced.oControl of inhibitory interneurons impaired.oIntraspinal sprouting.

XKatz, 1999Decrease in the amount of presynaptic inhibition of 1a afferents.Lack of this inhibition can contribute to increased stretch reflexes.

Reciprocal 1a inhibition is reduced

Reciprocal 1a inhibition is reduced.Gracies, 2005

Lesion at Spinal CordLesion aboveSpinal CordIntraspinal sprouting.

Lack of corticospinal controlAbnormal descending influenceAbnormal afferent influenceChanges in inhibitory circuitry

• Lack of corticospinal control• Abnormal descendinginfluence• Abnormal afferent influence• Changes in inhibitorycircuitry

The End!

Reciprocal 1a inhibition is reduced

Muscle SpindlesMuscle spindles consist of intrafusalfibers & specialized sensory and motornerve fibers. These specialized fibersare anchored to the extrafusal musclefibers and are stretched when themuscle is stretched. Responsible for themyotatic reflex.Types of intrafusal fibers:Nuclear bag – responds to velocity &changes in muscle length (dynamic).Nuclear chain – responds to changes inmuscle length (static).Nerve supply:Sensory – Group 1a -Bag & Chain(dynamic).Group II – Bag & Chain – staticMotor – Gamma motor neurons –innervate ends of intrafusal fibers.

DIFFERENCESFUNICULUSTARGET CELLSPATTERN OFINTERNEURONPROJECTIONSMUSCLESINNERVATEDMUSCLE ACTIONFUNCTIONSMEDIALDESCENDINGSYSTEMANTERIORFUNICULUSMEDIAL VII-IXLONG AXONSMANY SC SEG.MUSC. GROUPSAXIAL &PROXIMALFL.=UE;EXT.=LEPOSTURELATERALDESCENDINGSYSTEMLATERALFUNICULUSLATERAL V-IXSHORT AXONSFEW SC SEG.INDIV. MUSC.DISTAL LIMBMUSCLESEXT.=UE;FL.=LESKILLED MOVE.

Gracies, 2005Post activation repression of 1a afferents is decreased.