Directional Response Properties of Muscle Proprioceptors to Postural Disturbances

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Martin, Ramaldo S.
Nichols, T. Richard
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The somatosensory system has been implicated in the compensatory response of the nervous system to postural perturbations in humans and cats. The approach elicited - dubbed the Force Constraint Strategy - through a possible combined action of proprioceptive and cutaneous feedback, determines, and adjusts for, horizontal disturbances in various directions of a supporting surface. To understand the mechanisms underlying this strategy, we asked whether the response patterns of muscle proprioceptors correspond to those of electromyographic recordings (EMG) in the aforementioned Force Constraint experiments. The mechanical properties of the musculoskeletal system may also play a role in the restoration of stability. Thus, we also hypothesize that a proximal muscle would be relatively tightly tuned spatially whereas the distal muscle would exhibit a more diffuse spatial response distribution. To this end we selected the medial gastrocnemius (MG) and biceps femoris (BF) muscles to serve as our proximal and distal models respectively. Cats anesthetized using pentobarbital were set in a stereotaxic frame with the right leg positioned on a servo-controlled platform. The platform was shifted horizontally in 16 different directions according to a ramp-and-hold waveform. Intra-axonal recordings of activity from Ia afferents of BF and MG muscles were taken. Results indicate that the strategy makes use of information from the muscle proprioceptors. However, there is no differential response in tuning breadth with respect to muscle architecture. By characterizing the role of muscle proprioceptors in the mediation of corrective responses to perturbations of balance and stability, the results from this experiment can be used to verify biomechanical models, as well as further elucidate the underlying mechanisms of motor control.
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