The sensory mechanism of servo action in human muscle.

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Abstract
Anesthesia of the thumb suppresses servo action in the long flexor of the thumb during movements of the terminal phalanx. It required a greater subjective effort to flex an anesthetized thumb than a normal one. Anesthesia of other digits was without effect on the thumb flexor. In an anesthetic thumb, without servo responses, the changes in force exerted when a mechanical perturbation was applied, which were due purely to the passive mechanical properties of the muscle could be measured. Subtraction of these gave the active components of servo action in the normal thumb and thus an estimate of the mechanical gain of the servo. Giving the subject a visual tracking task could partially restore servo action when the thumb is anesthetic. After some years subjects became resistant to the effect of peripheral anesthesia. Peripheral anesthesia had no detectable effect on servo responses in the long flexor of the great toe, in infraspinatus or in pectoralis major. Servo action in these muscles was presumably based predominantly on muscle receptors. Tendon jerks were not apparently influenced by peripheral anesthesia. For the thumb, muscle afferents may co-operate with a somatic and a visual input in a servo loop via the cerebral cortex.