Functional Responses in the Human Spinal Cord during Willed Motor Actions: Evidence for Side- and Rate-Dependent Activity

Although the spinal cord is the output station of the central motor system, little is known about the relationships between its functionalactivity and willed movement parameters in humans. We investigated here blood oxygenation level-dependent functional magneticresonance imaging (fMRI) signal changes in the cervical spinal cord during a simple finger-to-thumb opposition task in 13 right-handedvolunteers, using a dedicated array of 16 receive-only surface coils on a 3 Tesla MRI system. In a first experiment, we found significantfMRI signal increases on both sides of the lower cervical spinal cord while subjects performed the motor task at a comfortable pace (0.5Hz) using either hand. Both the spatial extent of movement-related clusters and peak signal increases were significantly higher on the sideof the cord ipsilateral to the moving hand than on the contralateral side. Movement-related activity was consistently larger than signalfluctuations during rest. In a second experiment, we recorded spinal cord responses while the same motor sequence was performed usingthe dominant hand at two different rates (0.5 or 1 Hz). The intensity but not the spatial extent of the response was larger during higherrates, and it was higher on the ipsilateral side of the cord. Notwithstanding the limited spatial resolving power of the adopted technique,the present results clearly indicate that the finger movement-related fMRI signals recorded from the spinal cord have a neural origin andthat as a result of recent technological advances, fMRI can be used to obtain novel and quantitative physiological information on theactivity of spinal circuits