A NEW POSTURAL MOTOR RESPONSE TO SPINAL CORD STIMULATION: POST-STIMULATION REBOUND EXTENSION
Spinal cord stimulation (SCS) has emerged as a therapeutic tool for improving motor function following spinal cord injury. While many studies focus on restoring locomotion, little attention is paid to enabling standing which is a prerequisite of walking. In this study, we fully characterize a new type of response to SCS, a long extension activated post-stimulation (LEAP). LEAP is primarily directed to ankle extensors and hence has great clinical potential to assist postural movements. To characterize this new response, we used the decerebrate cat model to avoid the suppressive effects of anesthesia, and combined EMG and force measurement in the hindlimb with intracellular recordings in the lumbar spinal cord. Stimulation was delivered as five-second trains via bipolar electrodes placed on the cord surface, and multiple combinations of stimulation locations (L4 to S2), amplitudes (50-600 uA), and frequencies (10-40 Hz) were tested. While the optimum stimulation location and frequency differed slightly among animals, the stimulation amplitude was key for controlling LEAP duration and amplitude. To study the mechanism of LEAP, we performed in vivo intracellular recordings of motoneurons. In 70% of motoneurons, LEAP increased at hyperpolarized membrane potentials indicating a synaptic origin. Furthermore, spinal interneurons exhibited changes in firing during LEAP, confirming the circuit origin of this behavior. Finally, to identify the type of afferents involved in generating LEAP, we used shorter stimulation pulses (more selective for proprioceptive afferents), as well as peripheral stimulation of the sural nerve (cutaneous afferents). The data indicates that LEAP primarily relies on proprioceptive afferents and has major differences from pain or withdrawal reflexes mediated by cutaneous afferents. Our study has thus identified and characterized a novel postural motor response to SCS which has the potential to expand the applications of SCS for patients with motor disorders.