Propriospinal input to shoulder 1 Task - dependent modulation of propriospinal inputs to human shoulder

In the human upper limb a proportion of the descending corticospinal command may be relayed through cervical propriospinal premotoneurons. This may serve to co-ordinate movements involving multiple joints of the arm, such as reaching. The present study was conducted to determine if a shoulder stabilising muscle, infraspinatus (INF), is functionally integrated into the putative cervical propriospinal network, and if there is task-dependent modulation of the network. Fourteen healthy adults participated in this study. Responses in the right INF were evoked by transcranial magnetic stimulation (TMS) over the motor cortex and compared to responses conditioned by ulnar nerve stimulation. Interstimulus intervals were chosen to summate inputs at the level of the premotoneurons. Participants performed a forearm and shoulder muscle co-contraction task or a grip-lift task that also coactivated forearm and shoulder muscles. During the co-contraction task, INF motor evoked potentials (MEPs) were significantly facilitated by ulnar nerve stimulation at low intensities and suppressed at higher intensities. Only facilitation reached significance during the griplift task. We have shown for the first time that propriospinal pathways may connect the hand to the rotator cuff of the shoulder. The modulation of facilitation/suppression during the grip-lift task suggests that inhibition of propriospinal premotoneurons is downregulated in a task dependent manner to increase the gain in the feedback reflex loop from forearm and hand muscles as required. Propriospinal input to shoulder 3