Magnetic coil stimulation of the spinal cord in the dog. Effect of removal of bony structure on eddy current.

Magnetic coil stimulation applied to the spine excites the lumbosacral roots rather than the spinal cord itself. To evaluate the mechanism responsible for this phenomenon, the effects of volume conduction on the activation of neural elements were studied. In ten intact dogs compound muscle action potentials could be recorded in the soleus muscle only when the magnetic coil was placed over the cauda equina. After laminectomy and pediculotomy, compound muscle action potentials also could be elicited when the magnetic coil was placed over the spinal cord. These findings suggest that the bony structure surrounding the spinal cord interferes with the spread of magnetically induced eddy currents to the spinal cord.