The descending influence of body orientation on force feedback in the decerebrate cat
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Humans and animals interact with a variety of terrain with sloped surfaces being on the of the most fundamental. As the biomechanical requirements for walking on sloped surfaces changes with the angle of incline and direction of travel, the nervous system must have an efficient means to tune the motor output to meet these variable requirements. We hypothesized that this tuning is achieved by altering the gains of intermuscular force dependent spinal reflexes. We tested this hypothesis using the decerebrate cat model with vestibular labyrinthectomy which allowed for the simulation of sloped support surfaces by manipulating the angle of the neck. We measured the changes in intermuscular force feedback by stretching individual ankle extensor muscles in various combinations during background motor patterns such as stepping and crossed extensor reflex. We also measured the changes in endpoint stiffness of the cat hindlimb using a newly developed robotic system. We found that certain pathways of inhibitory force feedback, such as between gastrocnemius and flexor hallucis longus, were enhanced in the downhill cases and somewhat diminished in the uphill conditions. These findings corresponded to a measured decrease in hindlimb stiffness during downhill conditions; however, the uphill cases did not exhibit any trend of altered stiffness. Both the increased inhibition and decreased stiffness for downhill matched our expectations as a more compliant limb is needed to provided braking action rather than producing propulsive force. The greater variability in our uphill experiments possibly indicates that multiple successful strategies exist under these conditions or that the patterns are very similar to level conditions and the differences are too subtle to resolve with our methods.