Altered intermuscular force feedback after spinal cord injury in cat

Abstract

Bipeds and quadrupeds are inherently unstable and their bodies sway during quiet stance and require complex patterns of muscle activation to produce direction-specific forces to control the body’s center of mass. The relative strength of length and force feedback within and across muscles collectively regulates the mechanical properties of the limb as a whole during standing and locomotion (Bonasera and Nichols 1994; Ross and Nichols 2009). Loss of posture control following spinal cord injury (SCI) is a major clinical challenge. While much is known about intermuscular force feedback during crossed extension reflex (XER) and locomotion in decerebrate cats, these have not been well characterized in animals with spinal cord injury. In this study, we mapped the distribution of heterogenic force feedback in hindlimb ankle extensor muscles using muscle stretch (natural stimulation) in intercollicular, non-locomoting, decerebrate cats with chronic lateral spinal hemisection (LSH). We also, determined the time of onset of redistribution of heterogenic force feedback following LSH by collecting force feedback data from cats with acute sci. In addition we revisited heterogenic force feedback between ankle extensors in decerebrate non-locomoting cats during mid-stance to ascertain whether these cats with intact spinal cord depict a certain pattern of force feedback. The goal was to ascertain whether the patterns and strength of feedback was different between the two states (cats with intact spinal cord and cats with SCI). We found that heterogenic feedback pathways remained inhibitory in non-locomoting decerebrate cats in two states. The latencies of inhibition also corresponded to those observed for force feedback from Golgi tendon organs. We observed variable patterns of force feedback between ankle extensors in decerebrate/control cats. On the other hand we observed consistent results in cats with chronic LSH exhibiting very strong distal to proximal pattern of inhibition from 2 weeks to 20 weeks following chronic LSH. The same results were obtained in acute LSH cats suggest that the change in neuromuscular system appears immediately after SCI and persists even after the animal start walking following SCI. The observed altered pattern of force feedback after spinal cord injury suggests either presence of a pattern intrinsic to the spinal cord or a unique pattern exhibited by the damaged spinal cord. The results are important clinically because even with vigorous rehabilitation attempts patients do not regain posture control after SCI even though they regain ability to walk. Therefore, to effectively administer treatment and therapy for patients with compromised posture control, a complete understanding of the circuitry is required.