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Motor Functions of the Spinal Cord - Dr. Taha Sadig Ahmed

Learn about the motor functions of the spinal cord, including reflex arcs and the classification of reflexes. Explore the different properties of reflexes and the spinal centers responsible for various reflexes.

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Motor Functions of the Spinal Cord - Dr. Taha Sadig Ahmed

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  1. Motor Functions of the Spinal Cord المهام الحركية للحبل الشوكيDr. TahaSadig Ahmed طه صادق أحمد

  2. 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 • Ganong’s Review of Medical Physiology , Barrett KE, Barman SM, Boitano S, Brooks HL , edotors . Mc Graw Hill .

  3. صطلحات طبيّة هامة • Spinal cord الحبل الشوكي • Neuron/ Nerve fiber ( one nerve cell) عصبون • Nerve : عصب العصب يتكون من عشرات الآلاف أو مئات الآلاف من العصبونات e.g., Sciatic nerve , median nerve , ulnar nerve • Spinal Reflex منعكس الحبل الشوكي • Afferent ( sensory ) neuron : العصبون الحسّي (\الوارد إلي الحبل الشوكي جالبا الأحساسات ) • Efferent ( Motor ) neuron :العصبون الآمر ( الحركي) (الخارج ( الآمر لعضلة لتنقبض) من الحبل الشوكي • Innervation ( Nerve Supply) : تعصيب • Synapse : مشبك • Monosynaptic reflex : منعكس أحادي المشبك • Polysynaptic reflex : منعكس متعدد المشابك

  4. Upper motor neuron ( UMN) العصبون الحركي العلوي ( • Lower motor neuron (LMN) العصبون الحركي السفلي ( • Spasticity شناج ( بضم الشين ) : فرط التوتر التشنجي • Spastic شناجي ، تشنجي • Muscle tone درجة التوتر العضلي Stretch reflex = Tendon jerk منعكس الشد ، الأنتفاضة العضلية • و مستقبله المغزل العضلي ) muscle spindle ) • Golgi tendon Reflex منعكس قولجي الوتري (و مستقبله عضو قولجي الوتري • Ascending tracts السبل الصاعدة ( حسّية ) • Descending tracts السبل

  5. Functionns of the Spinal Cord • Involves • (1) Carrying sensory information from the receptors to the brain ( through spinal afferent/sensory nerves & ascending/sensory tracts ). • (2) Executing brain motor commands ( via descending/motor tracts & spinal efferent/motor nerves) • (3) Spinal Reflexes : Spinal centers serve toreceive incoming sensory information , integrate it and respond to it by pre-programmed spinal reflexes .

  6. The dorsal rootcontains afferent (sensory) nerves coming from receptors . • The cell body of these neurons is located موجود in dorsal ( posterior ) root ganglion ( DRG) • The ventral root carries efferent (motor) fibers • The cell-body of these motor fibers (AHC, Lower Motor Neuron) is located in the anterior horn of the spinal cord .

  7. Reflex Arc AHC ( Lower Motor Neuron , LMN) Final Common Pathway) • The “ center ” of the reflex comprises the part of the reflex arc inside the spinal cord . • In case of monosynaptic reflexes the afferent neuron synapses directlly on the AHC ; & in case of polysynaptic reflexes , one or more interneuron connects the afferent & efferent neurons . . Consists of : (1) Sense organ (receptor) (2) Afferent ( sensory ) neuron. (3) Motor ( Efferent ) neuron , in the anterior horn of spinal cord  Hence the spinal motor neuron ( or homologous cranial nerve motor neuron ) is called Anterior Horn Cell (AHC) or Lower Motor Neuron ( LMN)

  8. Classification of Reflexes According to the Number of Synapses Present in the Reflex Arc (1) Monosynaptic Reflexes المنعكسات أحادية المشبك: • have one synapse only : The sensory ( afferent ) axon synapse directly on the anterior horn cell. • Therefore , the reflex arc does not contain interneurons . • Examples : The Stretch reflex منعكس الشد ( also called Tendon Jerk ). (2) Polysynaptic reflxesالمنعكسات متعددة المشابك : • Have more than one synapse , therefore contain interneuron(s) between the afferent nerve & AHC . • Examples : Abdominal Reflexes , withdarwal reflex , Plantar response .

  9. Classification of Reflexes According to the Location of the Receptor (A) Superficial Reflexes : Are polysynaptic reflexes . The receptor is in the skin . Examples are abdominal reflexes and plantar reflex , (B) Deep reflexes :The receptor is located in muscle or tendon Examples : (1) Stretch Reflexes (Tendon jerks منعكسات الوتر) , monosynaptic : such as knee-jerk ( patellar reflex ) and ankle jerk . The receptor for all these is the muscle spindle ( which is located within the muscle itself . (2) Inverse Stretch Reflex ( Golgi Tendon organ reflex منعكس قولجي الوتري) , polysynaptic : The receptor is called GolgiTendon Organ , and is present in the muscle tendon .

  10. Types of Muscle Fibers • (1) Extrafusalfibers : • 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 . Lower Motor Neuron (AHC) • (2) Intrafusal fibers : • are tiny , microscopic fibers , present within the muscle spindle (the muscle length detector ) • innervated by Gamma motor neurons

  11. Types of AHC : • (1) Large ones , called Alpha motor neurons  supply extrafusalfibers • Also called Lower Motor Neuron ( LMN) • (2) Small ones , called Gamma motor neurons  supply intrafusalfibers • Inputs to theAHC ( LMN) • 3 sources (1) Primary Afferent ( sensory ) neurons (2) Spinal interneurons (3) Upper motor neurons ( UMN) , ( from Brain )

  12. 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 influences converge on ithe AHC  up to 10000 synapses can be present on one alpha motoneuron . • Q : What is “ Motor Unit ’’ ? • Motor unit comprises  • (1) alpha Motor neuron ( LMN) + • (2) all muscle fibers it innervates • ( remember musculoskeletal block lectures ).

  13. Irradiation & Recruitment • The extent of response ( strength of muscle contraction ) depends on the intensity ( strength ) of the stimulus . 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

  14. Example of a Superficial , Polysynaptic Reflex :Withdrawal reflex(flexor reflex/respnse )

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

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

  17. Sustained After-Dischargein Reverberating Circuits دوائر الصدي prolongs the response • Withdrawal reflex is characterized by after discharge, which prolongs the response and further enhances the protective role of this reflex. • In short, Withdrawal reflex and Crossed Extensor reflex are polysynaptic and show the properties of reciprocal innervation , motor unit recruitment , irradiation and after-discharge .

  18. Important Definitions تعريفات هامة • 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 conduction of impulses along the afferent and efferent nerves. • Minimal Synaptic delay : time taken in one synapse ~ 0.5 ms. • Central Dealy = Total Reflex time –Time spent in conduction of impulses along the afferent and efferent nerves. • Number of synapses = Central Delay /0.5 ms

  19. Thanks !

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