Compensation for External Perturbations

A part of the long-loop reflex, Ml, receives somatosensory information that helps compensate for unexpected perturbations during movement. For example, when a hand movement is stopped in progress by an external force, Ml neurons that precede and accompany the movement with a burst of activity renew or increase that activity in response. This signal probably arises from the muscle spindles, which shorten in concert with the extrafusal fibers due to their input from gamma motor neurons. When the limb is stopped, both alpha and gamma motor neurons continue to discharge according to the motor plan. The extrafusual fibers build up force, but the muscle cannot shorten. The muscle spindle fibers continue to contract, which generates a signal comparable to that evoked by a muscle stretch. This signal is relayed to Ml and it causes the motor-control signal to be augmented. The resulting increase in activity serves to compensate, at least partially, for the perturbation, although it may not be adequate to overcome the impediment. This kind of mechanism could account for the achievement of goals upon the initial presentation of a particular perturbation.

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