Mechanisms of rhythm generation of the human lumbar spinal cord in response to tonic stimulation without and with step-related sensory feedback

Abstract

Independent studies have shown that the human lumbar spinal networks below a complete spinal cord injury can produce rhythmic motor outputs in response to step- related sensory feedback or sustained electrical spinal cord stimulation (SCS). Here we present our current studies ap- plying lumbar SCS in motor-complete spinal cord injured (SCI) individuals in the supine and supported standing posi- tion, and during assisted treadmill stepping, to explore the mechanisms of spinal rhythm generation. SCS could pro- duce rhythmic EMG activities without step-related sensory feedback; in each case, all muscles were active with a com- mon rhythm frequency. During SCS and passive treadmill stepping, muscle activities were normally synchronized to the imposed step frequency, but activities with an independ- ent rhythm frequency were generated as well. The results suggest rhythmic reflex actions and central spinal generation as two different causes of rhythm generation. Here, we present our current studies that explore the roles of sustained inputs and step-related sensory feedback to the human lumbosacral spinal cord in the generation of rhythmic motor patterns. We applied epidural or transcu- taneous lumbar SCS in motor-complete SCI individuals in the supine position (preventing hip extension and axial limb load that are otherwise critical for rhythm genera- tion), in a supported standing positions with the lower limbs being 'mechanically immobilized' (preventing movements at hip and knee), and during assisted and body-weight supported (BWS) treadmill stepping. Based on their characteristically different signal profiles, the hypothesis was that the non-patterned and patterned inputs to the spinal cord neural networks would reveal different mechanisms of rhythm generation. Methods