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Spinal Control of Movement

Spinal Control of Movement. Lesson 19. Anatomy. Ventral Spinal Cord Topographic organization Alpha motor neurons Spinal interneurons Striate muscle extrafusal fibers ~. Distal limbs. Proximal limbs. Alpha Motor Neurons. Or lower motor neurons (Class I) Cell body in ventral horn

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Spinal Control of Movement

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  1. Spinal Control of Movement Lesson 19

  2. Anatomy • Ventral Spinal Cord • Topographic organization • Alpha motor neurons • Spinal interneurons • Striate muscle • extrafusal fibers ~ Distallimbs Proximallimbs

  3. Alpha Motor Neurons • Or lower motor neurons (Class I) • Cell body in ventral horn • Emerge from ventral root • Innervate extrafusal fibers • Uninterrupted to muscle fibers • final common pathway • Only excitatory input to muscles Inhibition at spinal cord ~

  4. Dorsal Extrafusal Fibers Alpha Motor neuron Ventral ACh

  5. Input to Alpha Motor Neurons • 3 sources only 1. DRG neurons • sensory neurons (proprioception) • feedback from muscle spindles 2. Upper motor neurons • primarily from M1 3. Spinal interneurons • largest input (excitatory & inhibitory) • generation of motor programs ~

  6. Inputs to Alpha Motor Neurons Sensory neurons Dorsal DRG Spinal interneurons Upper motor neurons - M1 Ventral

  7. Neuromuscular Junction • Synapse between neuron & effector • Cholinergic (ACh) • nicotinic receptors • Motor end-plate • postsynaptic membrane • folds packed with receptors • increased surface area ~

  8. Motor end-plate Terminal Button Muscle Fiber

  9. Neuromuscular Organization • Motor Units • Single alpha motor neuron and all the muscle fibers that it innervates • 1:3 to 1:100 • fewer fibers  finer control • Motor Pool • all alpha motor neurons that innervate a single muscle ~

  10. Graded Control of Muscle Contraction • Highly reliable synapse 1 presynaptic AP  1 postsynaptic AP  1 twitch (contract/relax) • temporal summation • ­ tension & sustained contraction • Recruitment • ­ # motor units ­ tension • order: smallest  largest ~

  11. Extrafusal Muscle Fibers • Striate muscle • Force for limb movements • flexion - closes joint • extension - opens joint • Contract or relax ~

  12. Muscle Contraction • AP generated in muscle fiber (cell) • Ca++ released from internal stores • Muscle fiber contracts • continues while Ca++ & ATP available • Relaxation • Ca++ sequestered by active transport ~

  13. Movement of Limbs • Flexors and extensors are ANTAGONISTIC • muscles and are reciprocally innervated • Limb flexion • flexors excited & extensors inhibited • Limb extension • extensors excited & flexors inhibited • Disynaptic inhibition ~

  14. Dorsal + + - + Alpha Motor neurons Ventral Upper Motor Neurons +

  15. Withdrawal Reflex • Flexion • remove limb from noxious stimulus • Polysynaptic reflex • sensory neuron • interneurons • motor neuron • 2 or more synapses • slower than monosynaptic ~

  16. + + - Polysynaptic withdrawal reflex + + + R

  17. Generation of Rhythmic Motor Patterns

  18. Central Pattern Generators • Half-center Model • alternating activity in flexor & extensor • Step-cycle has 2 phases • swing phase • foot off ground & flexing upward • stance phase • foot planted & leg extending • Each limb has own pattern generator ~

  19. + + + + + + + + Half-center Model Flexor a Tonic input a Extensor

  20. Rhythmic Patterns: Sensory Feedback • Not necessary for locomotion • but slower, less coordinated • Stumble correction reaction • during swing phase • tactile stimulus on dorsal foot  flexion • Reflex reversal • override during extension • flexion would cause collapse ~

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