Alpha Gamma Coactivation
Motor Suppy to the Muscle
α (alpha) motor nueron
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Supplies extrafusal fibers
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Cause muscle contraction
γ (gamma) motor neuron
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Supplies intrafusal fibers
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Cause contraction of contractile ends of intrafusal fibers
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Maintains tension in muscle spindle
Alpha-Gamma Coactivation
As such activation of only α (alpha) motor neuron is required for voluntary muscle contraction.
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Yet, the upper motor neuron activates both, α (alpha) and γ (gamma) motor neurons simultaneously for voluntary muscle contraction.
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This is called alpha-gamma coactivation
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It keeps the muscle spindle sensitive to stretch over a wide range of muscle length.
Without Concurrent Activation of γ (gamma) Motor Neuron
Activation of only α (alpha) motor neuron
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Stimulation of extrafusal fibers
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Muscle contraction
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Shortening of muscle spindle
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Sensory endings on the muscle spindle become slack i.e. become loose
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In this state the sensory endings are not sensitive to changes in muscle length
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Stretch reflex cannot work properly when the muscle is contracted
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This is prevented by concurrent activation of γ (gamma) motor neuron along with α (alpha) motor neuron
With Concurrent Activation of γ (gamma) Motor Neuron
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Contraction of contractile ends of intrafusal muscle fibers
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Stretches the central region of muscle spindle
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Keeps the sensory endings in muscle spindle taut even though the entire muscle is contracted
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In this state the muscle spindle continues to be sensitive to stretch
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Stretch reflex can work properly when the muscle is contracted.
Thus alpha-gamma coactivation helps maintain normal stretch reflex over a wide range of initial muscle length.