Alpha Gamma Coactivation
Motor Suppy to the Muscle
α (alpha) motor nueron
↓
Supplies extrafusal fibers
↓
Cause muscle contraction
γ (gamma) motor neuron
↓
Supplies intrafusal fibers
↓
Cause contraction of contractile ends of intrafusal fibers
↓
Maintains tension in muscle spindle
Alpha-Gamma Coactivation
As such activation of only α (alpha) motor neuron is required for voluntary muscle contraction.
↓
Yet, the upper motor neuron activates both, α (alpha) and γ (gamma) motor neurons simultaneously for voluntary muscle contraction.
↓
This is called alpha-gamma coactivation
↓
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
↓
Stimulation of extrafusal fibers
↓
Muscle contraction
↓
Shortening of muscle spindle
↓
Sensory endings on the muscle spindle become slack i.e. become loose
↓
In this state the sensory endings are not sensitive to changes in muscle length
↓
Stretch reflex cannot work properly when the muscle is contracted
↓
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
↓
Contraction of contractile ends of intrafusal muscle fibers
↓
Stretches the central region of muscle spindle
↓
Keeps the sensory endings in muscle spindle taut even though the entire muscle is contracted
↓
In this state the muscle spindle continues to be sensitive to stretch
↓
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.
Comments