Motor Functions of the Spinal Cord

الشوكي للحبل الحركية المهامDr. Taha Sadig Ahmed

أحمد صادق طه

• Objectives• At the end of this lecture the student should :• (1) appreciate the two-way trafiic along the spinal cord .• (2) describe the reflex arc . • (3) classify reflexes into superficial and deep ; monosynaptic & polysynaptic , give

examples of them , and show how they differ from each other .• (4) describe the general properties of reflexes and their synaptic pools such as

convergence , divergence , irradiation , recruitment , reverberating circuits ,after-discharge , minimal synaptic delay, central delay and reflex time .,

• (5) be able to describe the spinal centers of biceps , triceps , knee , ankle , abdominal and plantar reflexes .

• Refernce Book Refernce Book

• Ganong’s Review of Medical Physiology , 23rd edition . Barrett KE, Barman SM, Boitano S, Brooks HL , edotors . Mc Graw Hill, Boston 2010 .

• Pages 157-165 .

هامة صطلحات 9ة طبّي• Spinal cord الشوكي الحبل• Neuron/ Nerve fiber ( one nerve cell) عصبون• Nerve : عصب

العصبونات من اآلالف مئات أو اآلالف عشرات من يتكون العصبe.g., Sciatic nerve , median nerve , ulnar nerve • Spinal Reflex الشوكي الحبل منعكس• Afferent ( sensory ) neuron :األحساسات )\ ( جالبا الشوكي الحبل إلي الوارد الحس9ي العصبون• Efferent ( Motor ) neuron : ) الحركي ) اآلمر العصبون

) الشوكي) ) الحبل من لتنقبض لعضلة اآلمر الخارج • Innervation ( Nerve Supply) : تعصّيب• Synapse : مشبك • Monosynaptic reflex : المشبك أحادي منعكس• Polysynaptic reflex : المشابك متعدد منعكس

• Upper motor neuron ( UMN) العلوي الحركي ) العصبون• Lower motor neuron (LMN) السفلي الحركي ) العصبون• Spasticity : ) ( التشنجي التوتر فرط الشّين بضم شناج• Spastic تشنجي ، شناجي• Muscle tone العضلي التوتر Stretch reflex = Tendon درجة

jerk العضلّية األنتفاضة ، الشد منعكس• العضلي المغزل مستقبله ( muscle spindle ( و• Golgi tendon Reflex ( عضو مستقبله و الوتري قولجي منعكس

الوتري قولجي• Ascending tracts ) ( ّية حس9 الصاعدة السبل• Descending tracts السبل

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The dorsal rootcontains afferent (sensory) nerves coming from The dorsal rootcontains afferent (sensory) nerves coming from receptors . receptors .

The cell body of these neurons is located The cell body of these neurons is located موجودموجود in dorsal ( posterior ) in dorsal ( posterior ) root ganglion ( DRG)root ganglion ( DRG)

The ventral root carries efferent (motor) fibersThe ventral root carries efferent (motor) fibers

The cell-body of these motor fibers The cell-body of these motor fibers (AHC, Lower Motor Neuron(AHC, Lower Motor Neuron) is ) is located in the anterior horn of the spinal cord .located in the anterior horn of the spinal cord .

Consists of :Consists of :

(1) (1) Sense organ (receptor)Sense organ (receptor)

(2) Afferent ( sensory ) neuron. (2) Afferent ( sensory ) neuron.

(3) Motor ( Efferent ) neuron ,(3) Motor ( Efferent ) neuron ,

in the anterior horn of spinalin the anterior horn of spinal

cord cord Hence the spinal motor Hence the spinal motor

neuron ( or homologous cranialneuron ( or homologous cranial

nerve motor neuron ) is callednerve motor neuron ) is called

Anterior Horn Cell (AHC) orAnterior Horn Cell (AHC) or

Lower Motor Neuron ( LMN) Lower Motor Neuron ( LMN)

The “ center ” of the reflexThe “ center ” of the reflex

comprises the part of the reflex arccomprises the part of the reflex arc

inside the spinal cord .inside the spinal cord . In case of In case of monosynaptic reflexesmonosynaptic reflexes

the afferent neuron synapses directllythe afferent neuron synapses directlly

on the AHC ; & in case of on the AHC ; & in case of polysynapticpolysynaptic

reflexes reflexes , one or more interneuron, one or more interneuron

connects the afferent & efferent connects the afferent & efferent neurons . neurons .

..

AHC ( Lower Motor Neuron , LMN) Final Common Pathway)

Reflex Arc

Classification of Reflexes According to the Number of Synapses Present in the Reflex Arc

(1) Monosynaptic Reflexes (1) Monosynaptic Reflexes أحادية أحادية المنعكسات المنعكسات : :المشبكالمشبك

– have one synapse only : The sensory ( afferent ) have one synapse only : The sensory ( afferent ) axon synapse directly on the anterior horn cell.axon synapse directly on the anterior horn cell.–Therefore , the reflex arc does not contain Therefore , the reflex arc does not contain interneurons .interneurons .–Examples : The Stretch reflex Examples : The Stretch reflex الشد الشد منعكس also ( also ) منعكسcalled Tendon Jerk ).called Tendon Jerk ).

(2) Polysynaptic reflxes (2) Polysynaptic reflxes المشابك متعددة المشابك المنعكسات متعددة المنعكسات::

– Have more than one synapse , therefore contain Have more than one synapse , therefore contain interneuron(s) between the afferent nerve & AHC .interneuron(s) between the afferent nerve & AHC .–Examples : Abdominal Reflexes , withdarwal reflex Examples : Abdominal Reflexes , withdarwal reflex , Plantar response , Plantar response ..

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Classification of Reflexes According to the Location of Classification of Reflexes According to the Location of the Receptorthe Receptor

(A) Superficial Reflexes :Are polysynaptic reflexes . The receptor is in the skin . Are polysynaptic reflexes . The receptor is in the skin .

Examples are Examples are abdominal reflexes and plantar reflex ,

(B) Deep reflexes : The receptor is located in muscle or The receptor is located in muscle or tendon Examples : tendon Examples :

(1) Stretch Reflexes (Tendon jerks (1) Stretch Reflexes (Tendon jerks الوتر , ( , ( منعكساتmonosynaptic monosynaptic :: such as knee-jerk ( patellar reflex ) such as knee-jerk ( patellar reflex ) and ankle jerk .and ankle jerk .

The receptor for all these is the The receptor for all these is the muscle spindle ( which is located within the muscle itself .is located within the muscle itself .

(2) Inverse Stretch Reflex ( Golgi Tendon organ reflex (2) Inverse Stretch Reflex ( Golgi Tendon organ reflex الوتري قولجي الوتري منعكس قولجي polysynapticpolysynaptic : : The receptor is The receptor is , ( , (منعكس

called called GolgiGolgi Tendon OrganTendon Organ , and is present in the , and is present in the muscle tendon . muscle tendon .

Types of Muscle Fibers• (1) Extrafusal

fibers : • are the contractile

units of the muscle , which constitute the muscle bulk ,

• and which are responsible for the actual shortening and force generation by the muscle

• Innervated by Alpha motor neurons .

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(2) Intrafusal fibers : are tiny , microscopic fibers ,

present within the muscle spindle (the muscle length detector )

innervated by Gamma motor neurons

Lower Motor Neuron (AHC)

Types of AHC : (1) Large ones , called Alpha motor neurons supply

extrafusal fibers Also called Lower Motor Neuron ( LMN) (2) Small ones , called Gamma motor neurons supply intrafusal fibers

Inputs to theAHC ( LMN) Inputs to theAHC ( LMN) 3 sources3 sources

(1) Primary Afferent ( sensory ) neurons(1) Primary Afferent ( sensory ) neurons

(2) Spinal interneurons (2) Spinal interneurons

(3) Upper motor neurons ( UMN) , ( from Brain )(3) Upper motor neurons ( UMN) , ( from Brain )

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Q : What is the Q : What is the Final Common Pathway

It is the Alpha motor neuron (AHC)

It constitutes he only output of CNS on muscle i.e.,

All spinal & supraspinal All spinal & supraspinal influences converge on ithe AHC influences converge on ithe AHC up to 10000 synapses can be up to 10000 synapses can be present on one alpha present on one alpha motoneuron .motoneuron .

Q : What is Q : What is “ Motor Unit ’’ ?“ Motor Unit ’’ ? Motor unit comprises Motor unit comprises (1) alpha Motor neuron ( LMN) +(1) alpha Motor neuron ( LMN) + (2) all muscle fibers it innervates (2) all muscle fibers it innervates ( remember musculoskeletal ( remember musculoskeletal

block lectures block lectures ). ).

Irradiation & Recruitment The extent of response ( strength of musclecontraction ) depends on theintensity ( strength ) of thestimulus .This is because (1) Increased stimulation

intensity irradiation to other segments of the spinal cord

(2) Progressive recruitment of more and more motor units) stronger contraction

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Example of a Superficial , Polysynaptic Example of a Superficial , Polysynaptic Reflex :Reflex :

Withdrawal reflexWithdrawal reflex

(flexor reflex/respnse ) (flexor reflex/respnse )

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Withdrawal reflex (flexor reflex/respnse ) Withdrawal reflex (flexor reflex/respnse )

It is a superficial , polysynaptic , protective It is a superficial , polysynaptic , protective reflex reflex Stimulation of Stimulation of pain receptorspain receptors in a limb ( e.g., in a limb ( e.g., hand or sole of foot ) hand or sole of foot ) impulses to spinal cord via A or C fibres impulses to spinal cord via A or C fibres interneurons interneurons anterior horn cells stimulate limb flexor anterior horn cells stimulate limb flexor musclesmuscles withdrawal of limb ( withdrawal of limb ( moving it away from moving it away from the injurious agent ) the injurious agent ) . . stimulation of stimulation of flexors muscleflexors muscle accompanied by accompanied by inhibition of extensorsinhibition of extensors.via inhibitory .via inhibitory interneuronsinterneurons Reciprocal Inhibition, based on Reciprocal Inhibition, based on Reciprocal Innervation ).Reciprocal Innervation ).

Crossed Extensor ReflexCrossed Extensor Reflex If a stronger stimulus ( than If a stronger stimulus ( than that needed to elicit the that needed to elicit the Withdrawal Reflex) is delivered Withdrawal Reflex) is delivered Flexion withdrawal of the Flexion withdrawal of the stimulated limb will be stimulated limb will be accompanied by accompanied by extensionextension of of the the opposite limb opposite limb the latter the latter rresponse is called esponse is called Crossed Extensor ReflexCrossed Extensor Reflex(1) Pushing the entire body (1) Pushing the entire body away from the injurious agent away from the injurious agent and and (2) (2) supporting supporting the body the body weight against gravity weight against gravity There There fore it is an fore it is an Antigravity Reflex Reciprocal innervations occurs also in occurs also in extensor reflex : extensor reflex : flexors in the opposite limb are flexors in the opposite limb are inhibited while extensors are inhibited while extensors are excited excited

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Sustained Sustained After-DischargeAfter-Discharge inin Reverberating Reverberating CircuitsCircuits الصدي الصدي دوائر prolongs the response prolongs the response دوائرWithdrawal reflex is characterized by Withdrawal reflex is characterized by after after dischargedischarge, which prolongs the response and , which prolongs the response and further enhances the further enhances the protective roleprotective role of this reflex. of this reflex. In short, Withdrawal reflex and Crossed In short, Withdrawal reflex and Crossed Extensor reflex are polysynaptic and Extensor reflex are polysynaptic and show the show the properties of reciprocal innervation , motor unit properties of reciprocal innervation , motor unit recruitment , irradiation and after-discharge .recruitment , irradiation and after-discharge .

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Important Definitions Important Definitions هامة هامة تعريفات تعريفات Reflex Time :Time that elapses between application

of the stimulus and appearance of the response . Central Delay : Time taken in spinal cord synapses .. i.e., Reflex Time = = Central Delay + Time spent in + Time spent in

conduction of impulses along the afferent and conduction of impulses along the afferent and efferent nerves.efferent nerves.

Minimal Synaptic delay : : time taken in one synapse time taken in one synapse ~ 0.5 ms.~ 0.5 ms.

Central Dealy Central Dealy = Total Reflex time –Time spent in = Total Reflex time –Time spent in conduction of impulses along the afferent and conduction of impulses along the afferent and efferent nerves.efferent nerves.

Number of synapses Number of synapses = = Central Delay / /0.5 ms

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Thanks !Thanks !

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