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Motor neuron - muscle relationships

Explore the two types of lower motor neurons, α and γ, and their roles in innervating muscles, including muscle spindles. Learn how motor units function in generating movements and maintaining posture. Discover the different types of motor units and their characteristics.

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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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