Neural mechanisms underlying upper extremity heteronymous multijoint reflexes post stroke

A central problem concerning movement control in chronic stroke patients with spastic hemiparesis is how muscles across multiple joints act in a coordinated manner to produce a desired movement. One of the major concerns in this area is abnormal muscle \u27synergies\u27, or the grouping of multiple muscles into functional units, reported during volitional movement in people post-stroke. One mechanism that could contribute to synergies is reflex coupling of muscle groups. Reflex coupling may occur via direct monosynaptic connection from muscle afferents to motoneurons of the different limb muscles or through interneuronal pathways in the spinal cord. Supraspinal processing of sensory feedback may also result in intralimb coupling. The intent of our overall study was to understand the reflex mechanisms contributing to discoordination of the arm post stroke. We hypothesized that motor dysfunction of the upper arm is affected by reflex coupling of muscles crossing multiple joints, which is enhanced post stroke. This reflex coupling could contribute to impaired quality of arm movement in hemiparetic subjects. The hypothesis was tested by analyzing the torque and EMG responses of the paretic arm during relaxed constant velocity passive and active constant velocity isokinetic movements of the paretic elbow and to tendon tap perturbations of the elbow flexors and extensors. The results from the study demonstrated differences in reflex activation of shoulder muscles during passive and volitional movement of the paretic elbow. The results from the tendon tap experiments provided further insight into the homonymous and heteronymous pathways involved in the multijoint responses. The results suggest both monosynaptic and oligosynaptic connections between the afferents of the elbow and the muscles of the shoulder and wrist