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Motor neuron - muscle relationships

Motor neuron - muscle relationships. Two types of lower motor neurons in the motor neuron pools Small  motor neurons  motor neurons. Small  motor neurons. Innervate sensory structures in skeletal muscle (muscle spindles) Muscle spindles are embedded in connective tissue

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Motor neuron - muscle relationships

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

  2. 55-485

  3. 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

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

  5. Effect of  motor neurons • Terminate on spindle poles • Stimulate intrafusal contraction • Maintain intrafusal tension • The muscle spindles are able to function. 55-485

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

  7. Motor neurons • Lower motor neurons in the spinal cord • Brainstem motor neurons for head and neck movement in 8 cranial nerves, also called lower motor neurons. 55-485

  8. The motor unit •  motor neurons innervate all the extrafusal muscle fibers • Individual motor neurons synapse broadly over a muscle. • An action potential from a motor neuron brings all of the innervated fibers to threshold 55-485

  9.  motor neurons • Motor units and  motor neurons vary in size. • Small  motor neurons innervate fewer muscle fibers than large  motor neurons. 55-485

  10. Different types of motor units produce movements appropriate for different circumstances. 55-485

  11. Small  motor neurons • Small motor units • Contract slowly • Generate small forces • Lots of myoglobin (red) • Lots of mitochondria • Lots of capillaries (red) • Resistant to fatigue • Called slow (S) motor units 55-485

  12. Large  motor neurons innervate muscle fibers that • Are large and pale • contract forcefully • Have sparse mitochondria • Fatigue easily • Fast fatigable (FF) motor units 55-485

  13. Fast Fatigue-resistant motor units • Motor units of intermediate size • Not as fast as FF units • More resistant to fatigue than FF units • Generate more force than slow units 55-485

  14. Threshold for activation • Small slow motor units have low threshold of activation • Are tonically active (posture) 55-485

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