Motor neuron muscle relationships Two types of lower

Motor neuron - muscle relationships • Two types of lower motor neurons in the motor neuron pools – Small motor neurons – motor neurons 1/12/06 55 -485 1

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Small motor neurons • Innervate sensory structures in skeletal muscle (muscle spindles) • Muscle spindles are embedded in connective tissue – Are called intrafusal muscle fibers 1/12/06 55 -485 3

Small motor neurons • In Intrafusal muscle fibers – Sets the intrafusal muscle fiber to an appropriate length for the sensory neuron to function. 1/12/06 55 -485 4

Effect of motor neurons • Terminate on spindle poles • Stimulate intrafusal contraction • Maintain intrafusal tension • The muscle spindles are able to function. 1/12/06 55 -485 5

motor neurons • Large • Innervate extrafusal muscle fibers. These generate force for movement and posture. 1/12/06 55 -485 6

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. 1/12/06 55 -485 7

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 1/12/06 55 -485 8

motor neurons • Motor units and motor neurons vary in size. • Small motor neurons innervate fewer muscle fibers than large motor neurons. 1/12/06 55 -485 9

• Different types of motor units produce movements appropriate for different circumstances. 1/12/06 55 -485 10

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 1/12/06 55 -485 11

Large motor neurons innervate muscle fibers that • • • Are large and pale contract forcefully Have sparse mitochondria Fatigue easily Fast fatigable (FF) motor units 1/12/06 55 -485 12

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 1/12/06 55 -485 13

Threshold for activation • Small slow motor units have low threshold of activation – Are tonically active (posture) 1/12/06 55 -485 14