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NS30

NS30. Motor unit, muscle spindle, spinal reflexes. Motor system summary. LMNs. UMNs. alpha motor neurons, gamma motor neurons. Muscle spindles, GTOs. LMNs. LMNs. Types Alpha Gamma Tracts of UMNs that influence LMNs Corticospinal Corticonuclear Rubrospinal Reticulospinal

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NS30

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  1. NS30 Motor unit, muscle spindle, spinal reflexes

  2. Motor system summary LMNs UMNs alpha motor neurons, gamma motor neurons Muscle spindles, GTOs LMNs

  3. LMNs • Types • Alpha • Gamma • Tracts of UMNs that influence LMNs • Corticospinal • Corticonuclear • Rubrospinal • Reticulospinal • Vestibulospinal • Vestibulospinal • MLF

  4. NMJ • TEMs of neuromuscular junction

  5. NMJ • Presynaptic element ACh • Postsynaptic membrane & AChR • EPP & muscle contraction • acetylcholinesterase

  6. Motor units • Small • 10 to 100 fibers / LMN • Low levels of force • Rapid contraction • Fine control • Large • 600 to 1000 fibers / LMN • High levels of force • Small motor units recruited 1st with voluntary movements • One motor unit contains either fast or slow twitch fibers • All or none contraction

  7. Ib GTOs • Myotendinous junction • Respond to changes in tendon (muscle) tension • Innervated by type Ib sensory neurons • These synapse on inhibitory interneurons • These synapse on alpha motor neurons to the same muscle

  8. Muscle spindles • Within skeletal muscles • Contains intrafusal muscle fibers • Myotendinous junction • Respond to changes in muscle length

  9. Nuclear chain fibers • Nuclei single row • Innervated by type II sensory neurons (predominantly) • Respond to change in muscle length • Innervated by static gamma motor neurons

  10. Nuclear bag fibers • Nuclei clustered in central region of cell • Innervated by type Ia sensory neurons • Respond to change in muscle length • Innervated by static gamma motor neurons static

  11. Dynamic nuclear bag fibers • Nuclei clustered in central region of cell • Innervated by type Ia sensory neurons • Respond to change in muscle length • Innervated by dynamic gamma motor neurons

  12. Ia sensory neurons • Passively stretch muscle • Stimulate alpha motor neurons • Stimulate alpha & gamma motor neurons • Gamma innervation allows muscle spindle to monitor length even if muscle is shortened with contraction

  13. Gamma loop sequence of events • Supraspinal input activates gamma motor neurons (versus alpha) • Intrafusal fibers contract • Type Ia sensory neurons increase activity • Alpha motor neurons to same muscle increase activity • Muscle contracts • Critical for maintenance of stretch reflexes & muscle tone

  14. Alpha gamma coactivation • Supraspinal input activates alpha & gamma motor neurons (versus alpha) • When extrafusal fibers shorten, so do intrafusal fibers • Gamma input maintains intrafusal fibers at nearly constant tension

  15. Neurotransmitters • Glutamate • Type Ia • Type Ib • Type II • etc • Glycine • ACh

  16. Generalized spinal nerve

  17. Reflex terms • Intrasegmental • Reflex confined to 1 spinal cord level • Intersegmental • Hyperreflexia • Hyporeflexia • Areflexia • Grading scale • Clonus

  18. Autogenic inhibition • Ib sensory neurons of GTO respond to tension by increasing AP firing rate as tension increases • Ib sensory neurons synapse on Ib inhibitory interneurons • Ib inhibitory interneurons synapse on alpha motor neurons that innervate the muscle being contracted

  19. Tendon reflex • Ia & II sensory neurons respond to change in muscle length & rate of change in length by increasing their AP firing rate as muscle length increases • Ia & II neurons synapse directly on alpha motor neurons which innervate muscle being stretched • With patellar tendon reflex quadriceps contract

  20. Reciprocal inhibition • Tendon reflex & reflex inhibition can occur together • Ia & II sensory neurons also synapse on Ia & II inhibitroy interneurons • These synapse on alpha motor neurons that inneravte the antagonistic muscle of the stretched muscle

  21. Withdrawal reflex • Aδ & C sensory neurons respond to pain • These synapse on excitatory & inhibitory interneurons of several spinal cord segments • Excitatory interneurons s\ynapse on alpha motor neurons which innervate flexor muscles & stimulate their contraction • Inhibitory interneurons synapse on alpha motor neurons which innervate contralateral flexor muscles & inhibit their contraction

  22. Crossed extension reflex • Occurs with withdrawal reflex • Aδ & C neurons synapse on interbeurons that cross to contralateral side & synapse on excitatory & inhibitory interneurons of several spinal cord segments • Excitatory interneurons synapse on alpha motor neurons that stimulate contralateral extensor muscles to contract • Inhibitory interneurons synapse on alpha motor neurons that inhibit contraction of contralateral flexor muscles

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