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Effects of γ-Motor Neuron Discharge

المؤلف:  Kim E. Barrett, Susan M. Barman, Heddwen L. Brooks, Jason X.-J. Yuan

المصدر:  Ganongs Review of Medical Physiology

الجزء والصفحة:  25th E, P232

2026-11-11

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 Stimulation of γ-motor neurons produces a very different picture from that produced by stimulation of the α-motor neurons. Stimulation of γ-motor neurons does not lead directly to detectable contraction of the muscles because the intrafusal fibers are not strong enough or plentiful enough to cause shortening. However, stimulation does cause the contractile ends of the intrafusal fibers to shorten and therefore stretches the nuclear bag portion of the spindles, deforming the endings, and initiating impulses in the Ia fibers (Figure 1). This in turn can lead to reflex contraction of the muscle. Thus, muscles can be made to contract via stimulation of the α-motor neurons that innervate the extrafusal fibers or the γ-motor neurons that initiate contraction indirectly via the stretch reflex.

Fig1. Effect of various conditions on muscle spindle discharge. When the whole muscle is stretched, the muscle spindle is also stretched and its sensory endings are activated at a frequency proportional to the degree of stretching (“loading the spindle”). Spindle afferents stop firing when the muscle contracts (“unloading the spindle”). Stimulation of γ-motor neurons cause the contractile ends of the intrafusal fibers to shorten. This stretches the nuclear bag region, initiating impulses in sensory fibers. If the whole muscle is stretched during stimulation of the γ-motor neurons, the rate of discharge in sensory fibers is further increased.

If the whole muscle is stretched during stimulation of the γ-motor neurons, the rate of discharge in the Ia fibers is further increased (Figure 1). Increased γ-motor neuron activity thus increases spindle sensitivity during stretch.

In response to descending excitatory input to spinal motor circuits, both α- and γ-motor neurons are activated. Because of this “α–γ coactivation,” intrafusal and extrafusal fibers shorten together, and spindle afferent activity can occur throughout the period of muscle contraction. In this way, the spindle remains capable of responding to stretch and reflexively adjusting α-motor neuron discharge.