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Ch. 15: Lower Motor Neuron Circuits and Motor Control

Ch. 15: Lower Motor Neuron Circuits and Motor Control. - a model for understanding the nervous system. Neural centers responsible for movement. 4 interactive subsystems contribute to motor control. Neural centers responsible for movement. Gray matter of spinal cord and brainstem

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Ch. 15: Lower Motor Neuron Circuits and Motor Control

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  1. Ch. 15: Lower Motor Neuron Circuits and Motor Control - a model for understanding the nervous system 55-485

  2. Neural centers responsible for movement • 4 interactive subsystems contribute to motor control 55-485

  3. Neural centers responsible for movement • Gray matter of spinal cord and brainstem - local motor neurons innervate skeletal muscle cells - motor neurons of cranial nerves from cell bodies in the brain stem - local circuit neurons are the major source of synaptic input to the lower motor neurons. 55-485

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  5. Neural centers responsible for movement • Upper motor neurons - cell bodies in brainstem or cerebral cortex 55-485

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  8. Neural centers responsible for movement 3) Cerebellum - regulate activity of upper motor neurons 55-485

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  10. Neural centers responsible for movement • 4) Basal Ganglia • Regulate upper motor neurons 55-485

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  12. Basal ganglia - caudate, putamen, globus pallidus -subthalamic nucleus, substantia nigra 55-485

  13. 1) Gray matter of spinal cord and brainstem • Lower motor neurons send axons out of the brain stem and spinal cord to innervate skeletal muscles of the head and body. 55-485

  14. Gray matter of spinal cord and brainstem • Local circuit neurons • Synaptic input to lower motor neurons 55-485

  15. Gray matter of spinal cord and brainstem • Lower motor neurons convey voluntary and reflex commands to skeletel muscle 55-485

  16. Gray matter of spinal cord and brainstem • Local circuit neurons • Receive descending projections from higher brain centers • Also receive sensory input from higher brain centers 55-485

  17. Gray matter of spinal cord and brainstem • Circuits provide co-ordination between muscle groups for movement • Are the final common path for movement. 55-485

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  19. 2) Upper motor neurons • Cell bodies in the brainstem or cerebral cortex • Axons descend to synapse with local circuit neurons, sometimes directly onto lower motor neurons 55-485

  20. 2) Upper motor neurons • In the cortex • Initiate voluntary movements • Initiate complex spatiotemporal sequences of skilled movements • E.g. descending projections from the frontal lobe (primary motor cortex and lateral pre-motor cortex) 55-485

  21. 2) Upper motor neurons • In the brainstem • Regulate muscle tone for orienting eyes, head and body wrt sensory information (balance, somatic, sound, vision input) • Imp. For movement and posture 55-485

  22. 3) Cerebellum • Does not directly access local circuit neurons or lower motor neurons • Regulate the upper motor neurons • Located on the dorsal suface of the pons 55-485

  23. 3) Cerebellum • Detects the difference between an intended movement and the movement that is actually performed (servomechanism) • Reduces movement error in short term and in long term (learning) eg. Riding a bike. 55-485

  24. 4) Basal ganglia • No direct access to local circuit neurons or to lower motor neurons • Regulate upper motor neurons • Suppress unwanted movement • Prepare upper motor neurons for movements to start 55-485

  25. How does thought lead to movement? • Still poorly understood • Clear picture at the level of muscles themselves. • Start with the lower motor neurons and their targets. 55-485

  26. Motor neuron - muscle relationships • Neuronal tracer injections into the cell body show the pathway on motor neuron axons • motor neuron pools are all of the motor neurons that innervate a muscle. 55-485

  27. Motor neuron - muscle relationships • Motor neuron pools run together in the spinal cord. • An orderly arrangement between the motor neuron pools and the muscles that they innervate*** - a rule for the nervous system (mapping, topography) 55-485

  28. Motor neuron - muscle relationships • Fig. 15.3 • Topography • Neurons for postural muscles are medial in the cord • Neurons for the shoulders are next most lateral • Neurons for upper arms are next lateral. • Neurons for distal extremeties are most lateral. 55-485

  29. Motor neuron - muscle relationships • Spatial organization gives information about functions 55-485

  30. Motor neuron - muscle relationships • Two types of lower motor neurons in the motor neuron pools • Small  motor neurons •  motor neurons 55-485

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  32. Small  motor neurons • Innervate sensory structures in skeletal muscle (muscle spindles) • Muscle spindles are embedded in connective tissue • Are called intrafusal muscle fibers 55-485

  33. Small  motor neurons • In Intrafusal muscle fibers • Sets the intrafusal muscle fiber to an appropriate length for the sensory neuron to function. 55-485

  34.  motor neurons • Large • Innervate extrafusal muscle fibers. These generate force for movement and posture. 55-485

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