PHYSIOLOGY 1 LECTURE 20A SKELETAL MUSCLE SPINAL REFLEXES

MOTOR CONTROL SPINAL REFLEXES Objectives: The student should learn: Objectives: The student should learn: 1. 1.

MOTOR CONTROL SPINAL REFLEXES Introduction: Introduction: –Most movements of the body are controlled at the level of the spinal column by activating a series of preprogrammed spinal reflexes (Walking, running, etc.). This allows the brain to direct motion but not be involved in the necessity of repetitive commands unless an emergency arises.

MOTOR CONTROL SPINAL REFLEXES Thus, the brain initiates the motion and at the end brings it to a stop or changes the pace but is not consciously active for each step. In order to accomplish this spinal cord makes use of reverberatory (oscillatory) circuits leading into the skeletal muscle reflex circuits. Thus, the brain initiates the motion and at the end brings it to a stop or changes the pace but is not consciously active for each step. In order to accomplish this spinal cord makes use of reverberatory (oscillatory) circuits leading into the skeletal muscle reflex circuits.

MOTOR CONTROL SPINAL REFLEXES II. Organization of the Spinal Cord for Motor Function A.Gray matter – integrative area for the spinal cord 1. Posterior root – sensory signals Sensory signals travel two pathways after entering the spinal cord a.Branch one of the sensory nerve terminates almost immediately in the grey matter – local segmental reflexes b.b. Branch two of the sensory nerve transmits signals to higher levels of the nervous system – sensory relay neurons (propriospinal fibers)

MOTOR CONTROL SPINAL REFLEXES 2. Neurons of the Grey Matter 2. Neurons of the Grey Matter a. Anterior Motor neurons – two types a. Anterior Motor neurons – two types 1. Alpha motor neurons – very large control extrafusal fibers (motor units) 1. Alpha motor neurons – very large control extrafusal fibers (motor units) 2. Gamma motor neurons – smaller control intrafusal motor fibers of the spindles 2. Gamma motor neurons – smaller control intrafusal motor fibers of the spindles

MOTOR CONTROL SPINAL REFLEXES b. Interneurons – Small very highly excitable, provide interconnections and are responsible for most reflex control – types b. Interneurons – Small very highly excitable, provide interconnections and are responsible for most reflex control – types 1. Renshaw cells – inhibitory cells – provide for lateral inhibition to sharpen focus and increase central tendency 1. Renshaw cells – inhibitory cells – provide for lateral inhibition to sharpen focus and increase central tendency

MOTOR CONTROL SPINAL REFLEXES 2. Multisegmental connections from one spinal cord level to another – Propriospinal Fibers 2. Multisegmental connections from one spinal cord level to another – Propriospinal Fibers More than half of all spinal nerves – provide for Multisegmental reflexes coordinating simultaneous movements of forelimbs and hind limbs. More than half of all spinal nerves – provide for Multisegmental reflexes coordinating simultaneous movements of forelimbs and hind limbs.

MOTOR CONTROL SPINAL REFLEXES III Skeletal Muscle Reflexes III Skeletal Muscle Reflexes A. Muscle Stretch Reflex – simplest of the muscle reflexes A. Muscle Stretch Reflex – simplest of the muscle reflexes 1. Sudden stretch of a skeletal muscle 1. Sudden stretch of a skeletal muscle 2. Activation of a muscle spindle 2. Activation of a muscle spindle 3. Group 1a fibers from muscle spindle activate alpha motor neuron 3. Group 1a fibers from muscle spindle activate alpha motor neuron 4. Alpha motor neuron contracts stretched skeletal muscle 4. Alpha motor neuron contracts stretched skeletal muscle

MOTOR CONTROL SPINAL REFLEXES 5. Dynamic stretch reflex and static stretch reflex 5. Dynamic stretch reflex and static stretch reflex a. Dynamic stretch reflex – This reflex opposes any sudden change in muscle length and is controlled by the muscle spindle primary sensory endings (1a fibers) a. Dynamic stretch reflex – This reflex opposes any sudden change in muscle length and is controlled by the muscle spindle primary sensory endings (1a fibers) 1] Sudden stretch = contraction 1] Sudden stretch = contraction 2] Sudden unload in contracted state = relaxation 2] Sudden unload in contracted state = relaxation 3] Occurs over a fraction of a second from sudden change 3] Occurs over a fraction of a second from sudden change

MOTOR CONTROL SPINAL REFLEXES b. Static stretch reflex – This is a weaker secondary reflex following the dynamic stretch reflex but continues for up to a minute. It tends to cause the degree of muscle contraction to remain constant unless the nervous system wills otherwise b. Static stretch reflex – This is a weaker secondary reflex following the dynamic stretch reflex but continues for up to a minute. It tends to cause the degree of muscle contraction to remain constant unless the nervous system wills otherwise

MOTOR CONTROL SPINAL REFLEXES 6. Damping function of the stretch reflex (Signal averaging function) – 6. Damping function of the stretch reflex (Signal averaging function) – a. Nervous signals from the motor cortex and spinal cord vary in both frequency and intensity which would produce a very jerky muscle contraction if not modified. a. Nervous signals from the motor cortex and spinal cord vary in both frequency and intensity which would produce a very jerky muscle contraction if not modified. b. The stretch reflexes tend to dampen or average these signals and thus, produce a smooth muscle contraction b. The stretch reflexes tend to dampen or average these signals and thus, produce a smooth muscle contraction

MOTOR CONTROL SPINAL REFLEXES 7. Role of Muscle Spindle in Voluntary Motor Activity 7. Role of Muscle Spindle in Voluntary Motor Activity a. Coactivation Alpha motor neuron and gamma motor neuron a. Coactivation Alpha motor neuron and gamma motor neuron 1] Both extrafusal and intrafusal muscle fibers contract simultaneously 1] Both extrafusal and intrafusal muscle fibers contract simultaneously 2] Sensitivity of the muscle spindles is maintained 2] Sensitivity of the muscle spindles is maintained 3] Damping function is enhanced 3] Damping function is enhanced 4] Keeps muscle within ideal operating lengths 4] Keeps muscle within ideal operating lengths

MOTOR CONTROL SPINAL REFLEXES b. Stabilizes body Position during tense muscle action b. Stabilizes body Position during tense muscle action 1] Reflex contracts both agonist and antagonist muscles in response to muscle spindle activation which tends to lock joints in a stable position to best support the body 1] Reflex contracts both agonist and antagonist muscles in response to muscle spindle activation which tends to lock joints in a stable position to best support the body 2] In performing a delicate task requiring fine motor control the reflex locks major joints providing a solid stable platform for the execution of the task 2] In performing a delicate task requiring fine motor control the reflex locks major joints providing a solid stable platform for the execution of the task

MOTOR CONTROL SPINAL REFLEXES 8. Clinical Applications of the stretch reflex 8. Clinical Applications of the stretch reflex a. Determine degree of spinal facilitation of the spinal cord centers a. Determine degree of spinal facilitation of the spinal cord centers 1] Knee jerk reflex – (Patellar reflex) 1] Knee jerk reflex – (Patellar reflex) a] Muscle jerk exaggerated – upper level facilitation high a] Muscle jerk exaggerated – upper level facilitation high b] Muscle jerk low – upper level facilitation low b] Muscle jerk low – upper level facilitation low

MOTOR CONTROL SPINAL REFLEXES 2] Clonus – Oscillation of muscle jerks – Given certain circumstances the muscle jerks can oscillate 2] Clonus – Oscillation of muscle jerks – Given certain circumstances the muscle jerks can oscillate a] Ankle clonus – a person standing on tip toes suddenly drops to a flat footed stand stretching the gastrocnemius muscles stretch reflex which then causes contraction of the muscle to lift the body again this can repeat several times a] Ankle clonus – a person standing on tip toes suddenly drops to a flat footed stand stretching the gastrocnemius muscles stretch reflex which then causes contraction of the muscle to lift the body again this can repeat several times b] Clonus usually only occurs when the cord is highly facilitated in the first place b] Clonus usually only occurs when the cord is highly facilitated in the first place

MOTOR CONTROL SPINAL REFLEXES B. Golgi Tendon Reflex B. Golgi Tendon Reflex 1. Golgi tendon organ senses muscle tension and tends to redistribute load over the entire muscle group limiting chance of injury to any small group of fibers 1. Golgi tendon organ senses muscle tension and tends to redistribute load over the entire muscle group limiting chance of injury to any small group of fibers 2. Dynamic and static responses 2. Dynamic and static responses a. Dynamic response – initiatial response to increased tension is an increase in firing rate (Initiatial motor units bear the entire load) a. Dynamic response – initiatial response to increased tension is an increase in firing rate (Initiatial motor units bear the entire load)

MOTOR CONTROL SPINAL REFLEXES b. Static response - once the initial response is over firing rates tend to drop back toward normal as the load is distributed over the entire muscle group b. Static response - once the initial response is over firing rates tend to drop back toward normal as the load is distributed over the entire muscle group c. Mechanism - Golgi reflex is inhibitory – thus when fibers which are not involved in the original lift are not inhibited and become stimulated due to stretch then they pick up part of the load – redistribution c. Mechanism - Golgi reflex is inhibitory – thus when fibers which are not involved in the original lift are not inhibited and become stimulated due to stretch then they pick up part of the load – redistribution

MOTOR CONTROL SPINAL REFLEXES C. Flexor Reflex and the Withdrawal Reflexes C. Flexor Reflex and the Withdrawal Reflexes 1. Flexor reflex – usually elicited by stimulation of pain endings (Pin prick, heat, or a wound). 1. Flexor reflex – usually elicited by stimulation of pain endings (Pin prick, heat, or a wound). a. Stimulation of pain or pressure endings on the limbs a. Stimulation of pain or pressure endings on the limbs b. Activation of interneurons in spinal cord b. Activation of interneurons in spinal cord 1] Multisegmental connections – stimulation of flexor Alpha Motor neurons and interconnections to higher levels of the nervous system 1] Multisegmental connections – stimulation of flexor Alpha Motor neurons and interconnections to higher levels of the nervous system 2] Renshaw cells – Inhibition of the extensor alpha motor neurons 2] Renshaw cells – Inhibition of the extensor alpha motor neurons

MOTOR CONTROL SPINAL REFLEXES c. Activation of flexor muscles and inhibition of extensor muscles – The withdrawal pattern depends upon the sensory nerve stimulated (principal of local sign – the body part afflicted will be withdrawn from the pain stimulus using whichever nerves and muscles which can best complete the job) c. Activation of flexor muscles and inhibition of extensor muscles – The withdrawal pattern depends upon the sensory nerve stimulated (principal of local sign – the body part afflicted will be withdrawn from the pain stimulus using whichever nerves and muscles which can best complete the job) d. Limb withdraws from contact d. Limb withdraws from contact

MOTOR CONTROL SPINAL REFLEXES 2. Crossed Extensor Reflex - 2. Crossed Extensor Reflex - Very shortly after a flexor reflex the opposite limb begins to extend in order to push the offending object away from the body the crossed extensor reflex Very shortly after a flexor reflex the opposite limb begins to extend in order to push the offending object away from the body the crossed extensor reflex

MOTOR CONTROL SPINAL REFLEXES D. Reflexes of Posture and Locomotion D. Reflexes of Posture and Locomotion 1. Positive Supportive Reaction - Pressure on the footpad causes the limb to extend against the pressure 1. Positive Supportive Reaction - Pressure on the footpad causes the limb to extend against the pressure a. Complex interneuron activation result in activation of both flexors and extensors straighten the limb and lock the joint a. Complex interneuron activation result in activation of both flexors and extensors straighten the limb and lock the joint

MOTOR CONTROL SPINAL REFLEXES b. Magnet reaction – pressure on one side of the foot causes the limb to extend toward that side helping to keep the animal upright b. Magnet reaction – pressure on one side of the foot causes the limb to extend toward that side helping to keep the animal upright

MOTOR CONTROL SPINAL REFLEXES 2. Cord Righting Reflexes – An animal placed on its side will make uncoordinated movements until the spinal cord is in the upright position. 2. Cord Righting Reflexes – An animal placed on its side will make uncoordinated movements until the spinal cord is in the upright position.

MOTOR CONTROL SPINAL REFLEXES 3. Stepping and Walking Movements – The act of walking is a rhythmical set of motions that are initiated by the motor cortex but then continued in the spinal cord by reverberatory (oscillatory) circuits and mutually inhibitory circuits between limbs. 3. Stepping and Walking Movements – The act of walking is a rhythmical set of motions that are initiated by the motor cortex but then continued in the spinal cord by reverberatory (oscillatory) circuits and mutually inhibitory circuits between limbs.

MOTOR CONTROL SPINAL REFLEXES a. Motor cortex command a. Motor cortex command b. Reverberatory circuit firing initiated b. Reverberatory circuit firing initiated c. Leading leg flexor alpha motor neuron and gamma motor neurons are excited while extensor alpha motor neuron and gamma motor neurons are inhibited. c. Leading leg flexor alpha motor neuron and gamma motor neurons are excited while extensor alpha motor neuron and gamma motor neurons are inhibited.

MOTOR CONTROL SPINAL REFLEXES d. Leading leg lifts and moves forward d. Leading leg lifts and moves forward e. Trailing leg extensor alpha motor neuron and gamma motor neurons are excited while flexor alpha motor neuron and gamma motor neurons are inhibited. e. Trailing leg extensor alpha motor neuron and gamma motor neurons are excited while flexor alpha motor neuron and gamma motor neurons are inhibited. f. Trailing leg extends and moves backward f. Trailing leg extends and moves backward g. Body moves forward g. Body moves forward