Feedback Regulation

As mentioned previously, feedback regulation of movement was a central component of the framework developed by Sherrington and followers. As is well-known, the stretch reflex involves monosynaptic excitation from primary muscle spindle afferents (as well as disynaptic inhibition arising from Golgi tendon organ afferents) onto the homonymous muscle. It has long been recognized that this arrangement provides for a regulation of muscle length through spindle feedback and for a regulation of force by means of tendon organ afferent feedback. Furthermore, since stiffness is the ratio of length to tension, the combination of length and force feedback could provide a means for stiffness regulation. Thus, as has long been recognized, this reflex potentially provides for feedback regulation of posture. In the 1950s, it was postulated that it could also provide a mechanism for a feedback control of movement. The fusimotor innervation of muscle spindles (from g motoneurons) can change the spindles' rest length. Accordingly, it was hypothesized that the gamma innervation provided a reference length (the desired kinematics) and that spindle afferents provided an error signal proportional to a deviation from the desired kinematics.

Thus, in this classical view, the stretch reflex has the possibility of affording movement control on amuscle-by-muscle basis. Whether or not it does so effectively depends on the reflex gain. This value has been notoriously difficult to estimate because of the inherent nonlinearities in muscle mechanics. However, the best available estimates assign a relatively modest value to the reflex gain. Furthermore, there is good experimental evidence that feedback regulation is not on a muscle-by-muscle basis. As mentioned previously, the control of motion in a limb with two or more joints often involves eccentric contractions of muscles that are agonists for the movement. Similarly, when perturbations evoke rotations at more than one joint, reflex excitation of muscles that shorten consequent to the perturbation has been observed, contrary to the classical view. Spindle afferents are known to project not only to motoneurons of homonymous and syner-gistic muscles but also, either directly or indirectly, to motoneurons ofmuscles spanning distant joints. Thus, the reflex actions observed when a perturbation affects more than one joint could be purely spinal. However, there is also experimental support for "long-loop reflexes'' involving supraspinal structures, including the cerebral cortex and the cerebellum.

In the classical view, feedback regulation via the stretch reflex would compensate primarily for external perturbations, but it is equally conceivable that feedback could compensate for inaccuracies in the motor commands sent to the muscles. In fact, this supposition has experimental support. The effective moment of inertia of the arm varies with the direction of arm movement, and Claude Ghez and colleagues suggested that the initial descending motor commands do not take into account this inertial anisotropy. For movements of equal amplitude, the initial acceleration of the arm is largest for movements in directions for which the effective inertia is smallest. Nevertheless, the final amplitude of the movement is equally accurate for all directions. These investigators suggested that this was achieved by means of feedback compensation, a conclusion based on the additional observation that movement amplitude in deafferented patients varies in proportion to the variation in the initial acceleration of the arm. In other words, these patients do not exhibit any compensation for inertial anisotropies.

A final point should be made concerning feedback regulation of movements. The previous discussion considered the effects of kinesthetic feedback on the control of the evolving movement (i.e., an on-line regulation of the movement). There is also evidence that afferent inflow of information during one movement alters the execution of a subsequent movement. A simple, everyday example illustrates this point. Consider lifting a suitcase of unknown weight. If the suitcase is much heavier than it appears, the movement is likely to be hypometric, or it may even be aborted. However, the next attempt at lifting the suitcase will most likely be successful, implying that the motor commands have been modified based on previous experience. Visual feedback can also provide both on-line and trial-to-trial regulation.

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