Inhibitory pathways from Golgi tendon organs project widely between muscles crossing different joints and axes of rotation. Evidence suggests that the strength and distribution of this intermuscular inhibition is dependent on motor task and corresponding signals from the brainstem. The purpose of the present study was to investigate whether this sensory network is altered after spinal cord hemisection as a potential explanation for motor deficits observed after spinal cord injury (SCI). Force feedback was assessed between the long toe flexor and ankle plantarflexor (flexor hallucis longus), and the three major ankle extensors, (combined gastrocnemius, soleus, and plantaris muscles) in the hind limbs of unanesthetized, decerebrate, female cats. Data were collected from animals with intact spinal cords (control) and lateral spinal hemisections (LSHs) including chronic LSH (4–20 weeks), subchronic LSH (2 weeks), and acute LSH. Muscles were stretched individually and in pairwise combinations to measure intermuscular feedback between the toe flexor and each of the ankle extensors. In control animals, three patterns were observed (balanced inhibition between toe flexor and ankle extensors, stronger inhibition from toe flexor to ankle extensor, and vice versa). Following spinal hemisection, only strong inhibition from toe flexors onto ankle extensors was observed independent of survival time. The results suggest immediate and permanent reorganization of force feedback in the injured spinal cord. The altered strength and distribution of force feedback after SCI may be an important future target for rehabilitation.
One of the major clinical problems associated with spinal cord injury (SCI) is the lack of animal's ability to maintain weight support during standing or walking. We hypothesize that following SCI there is reorganization of the postural limb reflexes driven by stretch and load receptors in addition to loss of excitatory inputs from supraspinal structures to spinal interneurons that could result in altered intermuscular interactions. In control decerebrate animals there is a proximal to distal gradient in the strength of inhibitory force feedback among the limb muscles. We propose that pathways mediating this inhibition may remain inhibitory after spinal hemisection but their strengths may change to a distal to proximal gradient in the animal resulting in a loss of weight support. We also propose that ventral horn pathways are important for regulating the distribution of intermuscular force feedback. To test our hypothesis we are studying two types of SCI, namely dorsal and lateral hemisections, at both acute and chronic time points post‐injury. We measured intermuscular force feedback using combinations of stretches applied to pairs of muscles. Our most recent evidence supports our hypothesis and shows that the intermuscular force feedback is reversed after lateral but not dorsal hemisections.This work is supported by NIH grant HD32571, 1 I01 RX000243‐01A1, State of Florida BSCIRTF
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