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