Sympathetic-Somatomotor Coupling in Human Spinal Cord Injury

The regulation of cardiovascular systems during muscle activity is poorly understood in people with incomplete spinal cord injury (SCI). In the past, disruptions in somatomotor and sympathetic control have been investigated separately in SCI. We propose to investigate the coupling of sympathetic and somatomotor control because it is relevant to exercise training paradigms that are designed to improve somatomotor function or enhance physical fitness. Our approach will be to measure tendon tap reflexes, voluntary muscle activation, and blood flow of the knee (below injury) and elbow (above injury) before and after sympathetic stimuli consisting of cold pressor tests, mental math and an acute bout of exercise. These data will provide information about sympathetic control of blood flow during muscle activity. Plasticity of the sympatheticsomatomotor coupling will also be investigated by making measurements before and after a treadmill training exercise program. These experiments will enable us to address three aims. Aim 1 will be to characterize coupling of sympathetic and somatomotor systems below the level of spinal injury. This aim will examine spinal sympathetic and motor reflexes and their interactions. It will also examine how descending somatomotor coupling is disrupted by the spinal injury. In Aim 2, we will identify changes in the interactions of sympathetic and somatomotor systems above a spinal injury. Because of the injury and the changes that occur below the injury, the sympathetic-somatomotor coupling is also likely to be disrupted in the arm. Aim 3 will then demonstrate plasticity of sympathetic-somatomotor coupling after exercise training. Three different eight week exercise training programs will be tested including 1) treadmill training with exertion level matched to 70-80% of HRR 2)treadmill training with 30-40% HRR. The exercise training will be tested in a randomized crossover study design with one month between exercise training paradigms. We anticipate that there will be plasticity of sympathetic-somatomotor coupling and that the exercise training effects will normalize control of these systems. However, because of the injury, we anticipate that adaptations will differ from non-injured controls.

This study has implications for exercise training in human SCI. The coupling of sympathetic and somatomotor systems is expected to depend on whether exercise targets the upper or lower body. The recovery of function requires both the improvement in the control of movement as well as in the regulation of blood flow to active muscle groups. In addition, this study is important for understanding the potential impact of treadmill exercise training on cardiovascular fitness, a topic of increasing interest in people with limitations to physical activity.