~BN19 motor spinal control.ppt

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~BN19 motor spinal control.ppt

  1. 1. Spinal Control of Movement Lecture 19
  2. 2. Anatomy <ul><li>Ventral Spinal Cord </li></ul><ul><ul><li>Topographic organization </li></ul></ul><ul><li>Alpha motor neurons </li></ul><ul><li>Spinal interneurons </li></ul><ul><li>Striate muscle </li></ul><ul><ul><li>extrafusal fibers ~ </li></ul></ul>Proximal limbs Distal limbs
  3. 3. Alpha Motor Neurons <ul><li>Or lower motor neurons (Class I) </li></ul><ul><ul><li>Cell body in ventral horn </li></ul></ul><ul><ul><li>Emerge from ventral root </li></ul></ul><ul><li>Innervate extrafusal fibers </li></ul><ul><li>Uninterrupted to muscle fibers </li></ul><ul><ul><li>final common pathway </li></ul></ul><ul><li>Only excitatory input to muscles </li></ul><ul><li>Inhibition at spinal cord ~ </li></ul>
  4. 4. ACh Dorsal Ventral Extrafusal Fibers Alpha Motoneuron
  5. 5. Input to Alpha Motor Neurons <ul><li>3 sources only </li></ul><ul><li>1. DRG neurons </li></ul><ul><ul><li>sensory neurons (proprioception) </li></ul></ul><ul><ul><li>feedback from muscle spindles </li></ul></ul><ul><li>2. Upper motor neurons </li></ul><ul><ul><li>primarily from M1 </li></ul></ul><ul><li>3. Spinal interneurons </li></ul><ul><ul><li>largest input (excitatory & inhibitory) </li></ul></ul><ul><ul><li>generation of motor programs ~ </li></ul></ul>
  6. 6. Inputs to Alpha Motor Neurons Dorsal Ventral Spinal interneurons DRG Sensory neurons Upper motor neurons - M1
  7. 7. Neuromuscular Junction <ul><li>Synapse between neuron & effector </li></ul><ul><li>Cholinergic (ACh) </li></ul><ul><ul><li>nicotinic receptors </li></ul></ul><ul><li>Motor end-plate </li></ul><ul><ul><li>postsynaptic membrane </li></ul></ul><ul><ul><li>folds packed with receptors </li></ul></ul><ul><ul><ul><li>increased surface area ~ </li></ul></ul></ul>
  8. 8. Motor end-plate Terminal Button Muscle Fiber
  9. 9. <ul><li>Motor Units </li></ul><ul><ul><li>Single alpha motor neuron and all the muscle fibers that it innervates </li></ul></ul><ul><ul><li>1:3 to 1:100 </li></ul></ul><ul><ul><li>fewer fibers  finer control </li></ul></ul><ul><li>Motor Pool </li></ul><ul><ul><li>all alpha motor neurons that innervate a single muscle ~ </li></ul></ul>Neuromuscular Organization
  10. 10. Graded Control of Muscle Contraction <ul><li>Highly reliable synapse </li></ul><ul><ul><li>1 presynaptic AP  1 postsynaptic AP </li></ul></ul><ul><ul><li> 1 twitch (contract/relax) </li></ul></ul><ul><li>temporal summation </li></ul><ul><ul><li> tension & sustained contraction </li></ul></ul><ul><li>Recruitment </li></ul><ul><ul><li> # motor units   tension </li></ul></ul><ul><ul><li>order: smallest  largest ~ </li></ul></ul>
  11. 11. Extrafusal Muscle Fibers <ul><li>Striate muscle </li></ul><ul><li>Force for limb movements </li></ul><ul><ul><li>flexion - closes joint </li></ul></ul><ul><ul><li>extension - opens joint </li></ul></ul><ul><li>Contract or relax ~ </li></ul>
  12. 12. Muscle Contraction <ul><li>AP generated in muscle fiber (cell) </li></ul><ul><ul><li>Ca++ released from internal stores </li></ul></ul><ul><li>Muscle fiber contracts </li></ul><ul><ul><li>continues while Ca++ & ATP available </li></ul></ul><ul><li>Relaxation </li></ul><ul><ul><li>Ca++ sequestered by active transport ~ </li></ul></ul>
  13. 13. Movement of Limbs <ul><li>Flexors and extensors are ANTAGONISTIC </li></ul><ul><ul><li>muscles and are reciprocally innervated </li></ul></ul><ul><li>Limb flexion </li></ul><ul><ul><li>flexors excited & extensors inhibited </li></ul></ul><ul><li>Limb extension </li></ul><ul><ul><li>extensors excited & flexors inhibited </li></ul></ul><ul><li>Disynaptic inhibition ~ </li></ul>
  14. 14. + Upper Motor Neurons Dorsal Ventral + - + Alpha Motoneurons +
  15. 15. Withdrawal Reflex <ul><li>Flexion </li></ul><ul><ul><li>remove limb from noxious stimulus </li></ul></ul><ul><li>Polysynaptic reflex </li></ul><ul><ul><li>sensory neuron </li></ul></ul><ul><ul><li>interneurons </li></ul></ul><ul><ul><li>motor neuron </li></ul></ul><ul><li>2 or more synapses </li></ul><ul><ul><li>slower than monosynaptic ~ </li></ul></ul>
  16. 16. R + Polysynaptic withdrawal reflex + + + + -
  17. 17. Generation of Rhythmic Motor Patterns
  18. 18. Central Pattern Generators <ul><li>Half-center Model </li></ul><ul><ul><li>alternating activity in flexor & extensor </li></ul></ul><ul><li>Step-cycle has 2 phases </li></ul><ul><ul><li>swing phase </li></ul></ul><ul><ul><ul><li>foot off ground & flexing upward </li></ul></ul></ul><ul><ul><li>stance phase </li></ul></ul><ul><ul><ul><li>foot planted & leg extending </li></ul></ul></ul><ul><li>Each limb has own pattern generator ~ </li></ul>
  19. 19. Half-center Model   Tonic input Flexor Extensor + + + + + + + +
  20. 20. Rhythmic Patterns: Sensory Feedback <ul><li>Not necessary for locomotion </li></ul><ul><ul><li>but slower, less coordinated </li></ul></ul><ul><li>Stumble correction reaction </li></ul><ul><ul><li>during swing phase </li></ul></ul><ul><ul><li>tactile stimulus on dorsal foot  flexion </li></ul></ul><ul><li>Reflex reversal </li></ul><ul><ul><li>override during extension </li></ul></ul><ul><ul><li>flexion would cause collapse ~ </li></ul></ul>

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