Corticospinal Tract Co-Activation of Both Alpha (α) and Gamma (γ) Lower Motor Neurons May Help with Small Automatic Corrections OfVoIuntary Movements

As noted earlier, α-γ co-activation is a principle common to the excitation of lower motor neurons by upper motor neurons. It has been suggested that such co-activation may permit the muscle spindle to function as an “automatic error correction system” when voluntary movement against a load results in a small deviation from the intended result.

As discussed in Chapter 8, the activation of γ motor neurons along with a motor neurons ensures that the intra­fusal muscle fibers remain taut enough to transduce stretch even as the muscle reaches a shorter length on contraction of the extrafusal fibers. The / motor neuron activation tightens the intrafusal fibers by causing contraction of their polar ends, resulting in adjustment of the muscle spindle sensitivity to the new length of the muscle. It is thought that the α-y co- activation resulting from a voluntary motor command is meant to produce a contraction of intrafusal fibers that is con­cordant with extrafusal fiber contraction, such that the muscle spindle is made just sensitive enough to transduce stretch at the new muscle length. Under these circumstances, if the load is more than expected, the α motor neuron activity will not have produced enough extrafusal fiber contraction to shorten the muscle to the new desired length. However, the γ motor neuron activity will have produced the appropriate intrafusal fiber contraction to adjust the muscle spindle sensitivity for the new desired length. This mismatch, where the spindle sensitivity has been adjusted for the new muscle length but the extrafusal fibers have not contracted enough to reach that length, results in a stretching of the muscle spindle and activation of segmental stretch reflex mechanisms. That is, the stretching of the muscle spindle results in more excitatory postsynaptic potentials (EPSPs) on the α motor neurons to the muscle, increasing their action potential firing and increasing the extrafusal fiber contraction to assist with reaching the new desired length.

This type of error correction, in which segmental stretch reflex mechanisms help to accomplish the intended muscle shortening when the corticospinal pathway has not produced the sufficient α motor neuron activity, is called a servo-assist function. Thought to result from α-χco-activation, this servo­assist function is analogous to the power steering in a car, where a compressor in the motor adds power to the drivers turning of the steering wheel when significant resistance is encountered by the tires.