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Motor Systems

Motor Systems. Motor Unit. Motoneuron + muscle fibers it innervates. Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of fibers (e.g. digits) To thousands of fibers (e.g. trunk and major limb segments). Smaller motor units yield more refined control

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Motor Systems

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  1. Motor Systems

  2. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of fibers (e.g. digits) To thousands of fibers (e.g. trunk and major limb segments) Smaller motor units yield more refined control a motor “fovea”

  3. sarcomere

  4. Excitation of a Muscle Fiber • An action potential in an alpha motoneuron axon. • ACh is released by the axon terminal at the neuromuscular junction (synapse between motoneuron and muscle. • Nicotinic receptor channels open, Na+ enters and the membrane depolarizes (EPSP). • Voltage-gated Na+ channels open and an action potential is propagated down the muscle fiber. • Depolarization produces Ca2+ release from intracellular stores.

  5. Contraction of a Muscle Fiber • Ca2+ binds to tropinin. • Myosin binding sites are exposed. • Myosin heads bind actin. • Myosin cross-bridges pivot. • Myosin heads disengage at the expense (ATP-dependent). • Recycle as long as Ca2+ and ATP are present.

  6. Relaxation of a Muscle Fiber • As EPSPs end, membrane returns to resting potential. • Free Ca2+ is sequestered by metabolically driven pump. • Myosin binding sites covered.

  7. α-, γ-motoneuron co-activation α-motoneuron activation at rest Co-activation of α-motoneurons and γ-motoneurons

  8. Meissner’s Merkel’s Pacinian Ruffini’s Free nerve ending

  9. Monosynaptic stretch reflex Golgi tendon organ reflex

  10. somatosensory afferents motor afferents

  11. (Aδ) (C)

  12. Muscle Spindles (intrafusal fibers) respond to muscle length, or stretch insensitive to tension 1a afferents for dynamic spindles Group II afferents for postural spindles monosynatic stretch reflex (excitatory) Golgi Tendon Organs respond to muscle tension insensitive to muscle length, or stretch 1b afferents disynaptic reflex (inhibitory)

  13. Problem of Locomotion Degrees of freedom problem (Bernstein) Dozens of joints controlled by hundreds of muscles Muscles can contract in any combination, in any order, for any duration, and with any force. Possibilities are virtually limitless, making control of complex movements at the level of individual muscles impossible

  14. Reciprocal inhibition between motor units of antagonistic muscles

  15. Crossed extensor reflex

  16. oscillators = “legos” of movement CPGs = what you can build with legos

  17. Central Pattern Generators vs. Reflexes Central Pattern Generators Fast Act as “clocks” Flexible--<100 oscillators necessary Reflexes Refine timing Respond to the unexpected Acquisition of new skills

  18. Descending motor pathways Lateral and ventromedial pathways Pyramidal and extra-pyramidal systems

  19. Supplementary Motor Area (SMA) important for instituting motor programs activated prior to voluntary movement activated during mental rehearsal Premotor cortex sensory guided movement cooperatively with basal ganglia mirror neurons Pre-frontal cortex decisions for actions long-term planning

  20. Cortex Caudate- putamen (striatum) Thalamus and STN Globus Pallidus Substantia Nigra Red Nucleus (nucleus ruber) Reticular Formation Cerebellum Rubrospinal tract Reticulospinal tract

  21. Cortex Caudate- putamen (striatum) cortex pontine nuclei cerebellum thalamus Thalamus and STN Globus Pallidus Substantia Nigra Red Nucleus (nucleus ruber) Pontine nuclei and Reticular Formation Cerebellum Rubrospinal tract Reticulospinal tract

  22. Cortex Caudate- putamen (striatum) cortex striatum globus pallidus thalamus Thalamus and STN Globus Pallidus Substantia Nigra Red Nucleus (nucleus ruber) Pontine nuclei and Reticular Formation Cerebellum Rubrospinal tract Reticulospinal tract

  23. Cortex Caudate- putamen (striatum) cortex striatum substantia nigra thalamus Thalamus and STN Globus Pallidus Substantia Nigra Red Nucleus (nucleus ruber) Pontine nuclei and Reticular Formation Cerebellum Rubrospinal tract Reticulospinal tract

  24. Cortex Caudate- putamen degenerate in Huntington’s disease Thalamus and STN Globus Pallidus Substantia Nigra Red Nucleus (nucleus ruber) Reticular Formation Cerebellum Rubrospinal tract Reticulospinal tract

  25. Cortex Caudate- putamen degenerates in Parkinson’s disease Thalamus and STN Globus Pallidus Treatments: 1. l-DOPA to compensate for lost DA projections 2. Deep brain stimulation of striato-pallidal projections 3. Surgical destruction of cells in medial pallidum Substantia Nigra Red Nucleus (nucleus ruber) Reticular Formation Cerebellum Rubrospinal tract Reticulospinal tract

  26. Subthalamus pedunculopontine nucleus (PPN) reticulospinal tractspinal CPGs

  27. ALS: Upper motoneuron disease Polio: Spinal motoneuron disease Apraxias: Frontal cortical damage

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