Objectives At the end of this lecture the student should :

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 , 23rd edition . Barrett KE, Barman SM, Boitano S, Brooks HL , edotors . Mc Graw Hill, Boston 2010 . • Pages 157-165 .

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. 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 .

6. 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)

7. 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 .

8. 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 .

9. 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

10. 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 )

11. 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 ).

12. 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

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

14. 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 ).

15. 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 

16. 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 .

17. 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

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