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Spinal Control of Movement

Spinal Control of Movement. Lecture 18. Spinal Mechanisms Of Movement. Ventral Spinal Cord motor neurons Striate muscle voluntary movement & reflexes Feedback sensory cells in muscle propioception safety mechanism postural maintenance ~. Spinal Cord Circuits. Output: motor neurons

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Spinal Control of Movement

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  1. Spinal Control of Movement Lecture 18

  2. Spinal Mechanisms Of Movement • Ventral Spinal Cord • motor neurons • Striate muscle • voluntary movement & reflexes • Feedback • sensory cells in muscle • propioception • safety mechanism • postural maintenance ~

  3. Spinal Cord Circuits • Output: motor neurons • Ventral Horns • muscle contraction • Input: sensory neurons • Dorsal Horns • feedback • Integration • interneurons ~

  4. Alpha Motor Neurons • Or lower motor neurons • control striate muscles • Uninterrupted to muscle fibers • final common pathway • Only excitatory input to muscles • Inhibition at spinal cord ~

  5. Alpha Motor neuron Dorsal + Ventral

  6. 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 ~

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

  8. Striate Muscle • Extrafusal Muscle Fibers • muscle cells • input from a motor neurons • contraction • SC inhibition  relax • Force for limb movements • flexion - closes joint • extension - opens joint ~

  9. Muscle Contraction • a motor neuron excitation • AP in muscle fiber • Ca++ released from internal stores • Muscle fiber contracts • continues while Ca++ & ATP available • Relaxation • Ca++ sequestered by active transport ~

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

  11. Motor end-plate Terminal Button Muscle Fiber

  12. Myasthenia Gravis • Autoimmune disorder • body develops antibodies for ACh-R • weakness & rapid fatigue • Most common: women in 30s • Risk of respiratory paralysis • Treatment • AChE inhibitors • Immunosupressants ~

  13. Movement of Limbs • Flexors and extensors are ANTAGONISTIC • reciprocal innervation • Limb flexion • flexors excited & extensors inhibited • Limb extension • extensors excited & flexors inhibited • Disynaptic inhibition • in spinal cord ~

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

  15. Motor Units & Motor Pools • Motor Unit • Single alpha motor neuron • & all the muscle fibers it controls • 1:3 to 1:100 • fewer fibers  finer control • Motor Pool • all alpha motor neurons • that control a single muscle (e.g., biceps) ~

  16. 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 ~

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

  18. + + + Polysynaptic withdrawal reflex R

  19. Golgi Tendon Organ • Gauges muscle tension • Stretch receptor • safety mechanism • controlled contraction • Inhibits alpha motor neurons • disynaptic inhibition ~

  20. GTO + - Dorsal Inhibits alpha motor neuron + Ventral

  21. Monosynaptic Stretch Reflex • Sensory neuron  alpha motor neurons • monosynaptic excitation • disynaptic inhibition • e.g., Knee jerk reflex • Postural adjustments • Muscle tonus ~

  22. Monosynaptic Stretch Reflex • Muscle-Spindle (MS) • Muscle length detectors • Parallel w/ extrafusal fibers • Low threshold • Too little muscle tone •  tension • MS  sensory neuron  motor neuron • And inhibition of antagonistic muscle ~

  23. M S + + - + Dorsal + + Ventral

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