Motor Functions of the Spinal Cord Dr. Taha Sadig Ahmed Reflexes can be (1) Primitive, inherited, or can be (2) ACQUIRED, learned  Conditioned Reflexes ( Pavlov )

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, 23 rd edition. Barrett KE, Barman SM, Boitano S, Brooks HL, edotors. Mc Graw Hill, Boston Pages

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.

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 cord  Hence the spinal motor neuron ( or homologous cranial neuron ( or homologous cranial nerve motor neuron ) is called nerve motor neuron ) is called Anterior Horn Cell (AHC) or Lower Motor Neuron ( LMN) Lower Motor Neuron ( LMN) The “ center ” of the reflex The “ center ” of the reflex comprises the part of the reflex arc comprises the part of the reflex arc inside the spinal cord. In case of monosynaptic reflexes 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.. AHC ( Lower Motor Neuron, LMN) Final Common Pathway) Reflex Arc

Afferent neurons can undergo: Afferent neurons can undergo: Divergence : to spread the effect of a single stimulus to more motoneurons in the same spinal segment, or to adjacent segments, Divergence : to spread the effect of a single stimulus to more motoneurons in the same spinal segment, or to adjacent segments, Convergence : ( e.g. on a motoneuron )to facilitate spatial summation. Convergence : ( e.g. on a motoneuron )to facilitate spatial summation. 5

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 6 (2) Intrafusal fibers : are tiny, microscopic fibers that are present within the muscle spindle, which is the muscle lenght receptor 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 sources 3 sources (1) Primary Afferent ( sensory ) neurons (2) Spinal interneurons (3) Upper motor neurons ( UMN), ( from Brain ) 7

8 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 influences converge on ithe AHC  up to synapses can be present on one alpha motoneuron. All spinal & supraspinal influences converge on ithe AHC  up to synapses can be present on one alpha motoneuron. Q : What is “ Motor Unit ’’ ? Q : What is “ 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 block lectures ). ( remember musculoskeletal block lectures ).

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 9

Classification of Reflexes According to the Location of the Receptor (A (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), 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), polysynaptic : The receptor is called Golgi Tendon Organ, and is present in the muscle tendon. 10

Classification of Reflexes According to the Number of Synapses in the Reflex Arc (A) Monosynaptic Reflexes : – – 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 ( also called Tendon Jerk ). (B) Polysynaptic reflxes : – – Have more than one synapse, therefore contain interneuron(s) between the afferent nerve & AHC. – –Examples : Abdominal Reflexes, withdarwal reflex, Plantar response.

Example of a Superficial, Polysynaptic Reflex : Withdrawal reflex (flexor reflex/respnse ) Example of a Superficial, Polysynaptic Reflex : Withdrawal reflex (flexor reflex/respnse ) 12

Withdrawal reflex (flexor reflex/respnse ) It is a protective reflex It is a protective reflex Stimulation of pain receptors in a limb ( e.g., hand or sole of foot )  Stimulation of pain receptors in a limb ( e.g., 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 muscles anterior horn cells stimulate limb flexor muscles  withdrawal of limb ( moving it away from the injurious agent ).  withdrawal of limb ( moving it away from the injurious agent ). stimulation of flexors muscle accompanied by inhibition of extensors.via inhibitory interneurons  Reciprocal Inhibition الأحباط المتبادل, based on Reciprocal Innervation ). stimulation of flexors muscle accompanied by inhibition of extensors.via inhibitory interneurons  Reciprocal Inhibition الأحباط المتبادل, based on Reciprocal Innervation ). 13

Crossed Extensor Reflex If a stronger stimulus ( than that needed to elicit the Withdrawal Reflex) is delivered  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  Flexion withdrawal of the stimulated limb will be accompanied by extension of the opposite limb  the latter response is called Crossed Extensor Reflex the latter response is called Crossed Extensor Reflex (1) Pushing the entire body away from the injurious agent and (1) Pushing the entire body away from the injurious agent and (2) supporting the body weight against gravity  There fore it is an (2) supporting the body weight against gravity  There fore it is an Antigravity Reflex occurs also in extensor reflex : flexors in the opposite limb are inhibited while extensors are excited  Reciprocal innervations occurs also in extensor reflex : flexors in the opposite limb are inhibited while extensors are excited  14

(3) Moreover, the response is prolonged and may continue for some time after cessation of due to the sustained After-Discharge stimulation  due to the sustained After-Discharge in Reverberating Circuits دوائر الصدي

Irradiation & Recruitment Occur in the Crossed Extensor Response :The extent of the response ( strength of muscle contraction in a reflex depends on the intensity ( strength ) of the stimulus. Occur in the Crossed Extensor Response :The extent of the response ( strength of muscle contraction in a reflex depends on the intensity ( strength ) of the stimulus. The more intense the stimulus is, the greater is the spread ( irradiation ) of activity in adjacent & other spinal cord segments, leading to The more intense the stimulus is, the greater is the spread ( irradiation ) of activity in adjacent & other spinal cord segments, leading to  Recruitment of more and more motor units  stronger contraction & more widespread to other muscle groups  Recruitment of more and more motor units  stronger contraction & more widespread to other muscle groups Example : when the sole of the foot is stimulated by a weak painful stimulus, only the big toe is flexed. Example : when the sole of the foot is stimulated by a weak painful stimulus, only the big toe is flexed. A stronger stimulus will cause reflex flexion of the big toe, other toes, plus the ankle. A stronger stimulus will cause reflex flexion of the big toe, other toes, plus the ankle. The strongest stimulus will cause withdrawal of the whole leg by causing reflex flexion of the big toe, ankle, knee and hip. The strongest stimulus will cause withdrawal of the whole leg by causing reflex flexion of the big toe, ankle, knee and hip. Impulses may also cross to the other side of the spinal cord to cause extension of the other leg. Impulses may also cross to the other side of the spinal cord to cause extension of the other leg.

Important Definitions تعريفات هامة Reflex Time زمن الأستجابة :Time that elapses between application of the stimulus and appearance of the response. Reflex Time زمن الأستجابة :Time that elapses between application of the stimulus and appearance of the response. الزمن الذي إنقضي بين إعطاء التحفيز و ظهور الأستجابة الزمن الذي إنقضي بين إعطاء التحفيز و ظهور الأستجابة و طبعا هو مجموع التأخير في العصبونين ( الوارد و الخارج ) + التأخير داخل المشابك Central Delay و طبعا هو مجموع التأخير في العصبونين ( الوارد و الخارج ) + التأخير داخل المشابك Central Delay يعني الوقت الذي استغرقته الرحلة في العصبونات زائدا الوقت الذي استغرققه التأخير داخل المشابك (التي هي بين العصبونات) يعني الوقت الذي استغرقته الرحلة في العصبونات زائدا الوقت الذي استغرققه التأخير داخل المشابك (التي هي بين العصبونات) التأخير داخل مجموع المشابك : Time taken in spinal cord synapses. Central Delay التأخير داخل مجموع المشابك : Time taken in spinal cord synapses. 17

= + Time spent in conduction of impulses along the afferent and efferent nerves. i.e., Reflex Time = Central Delay + Time spent in conduction of impulses along the afferent and efferent nerves. : التأخير داخل المشبك الواحد ( time taken in one synapse) ~ 0.5 ms. Minimal Synaptic delay : التأخير داخل المشبك الواحد ( 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. Central Dealy = Total Reflex time –Time spent in conduction of impulses along the afferent and efferent nerves. لأنه لو طرحنا الوقت الذي استغرقته الرحلة في العصبونات من كل وقت التأخير المركزي نتوصل إلي التأخير داخل المشابك التي هي بين العصبونات لأنه لو طرحنا الوقت الذي استغرقته الرحلة في العصبونات من كل وقت التأخير المركزي نتوصل إلي التأخير داخل المشابك التي هي بين العصبونات Number of synapsesعدد المشلبك = / Number of synapsesعدد المشلبك = Central Delay / 0.5 ms 18

Thanks ! Thanks ! 19