Interlimb Coordination After a Thoracic Hemisection in Rats

An important role for sensory input in the motor performance of spinal cord injured (SCI) humans and animals has been established. For example, loading of the hind limbs during assisted locomotion on a treadmill improves the stepping ability of SCI patients and of complete spinal cord transected cats and rats. In our laboratories we are now using robotic devices to control and modulate the amount and timing of loading of the hind limbs during treadmill locomotion of SCI humans and animals. In addition, these robotic devices are being used to provide a quantitative kinematic assessment of performance during test trials of stepping ability. Recently, we have been studying the effects of spinal cord hemisection on interlimb coordination using these procedures. Our initial studies include adult male rats hemisected at a mid-thoracic level and maintained for ~4 weeks. When the robots were used to provide a load level of 25% of body weight, there was an asymmetry in the hind limbs during bipedal stepping. The cycle periods were shorter for the limb on the nonhemisected vs. the hemisected side and the number of steps performed during 30 sec on the treadmill was greater for the limb on the non-hemisected vs. hemisected side. Furthermore, the cycle periods and number of steps taken by the limb on the non-hemisected side during the test period were similar to the pre-hemisection values for the same rats. When the rats were allowed to step bipedally with weight bearing, but without the robotic arm, the stepping was symmetrical. In a second experiment a smaller group of rats was tested for symmetry of the two hind limbs using the robotic device. These rats were able to step symmetrically even with the robotic arms attached. Combined, these results demonstrate that even when there is asymmetrical descending input to a normal spinal cord, symmetrical stepping can be achieved. Furthermore, these results suggest that the normality of the symmetry following a hemisection can be a function of the difficulty of the task imposed on the rat.