Finite Element Models of Transcutaneous Spinal Cord Stimulation

Transcutaneous spinal cord stimulation (SCS) is a noninvasive method to electrically stimulate afferent structures of the human lumbar spinal cord. These are the same neural targets as predominantly activated by epidural implants. Biophysical principles derived from computer simulations contributed to the identification of the directly activated neural structures. These simulations combine finite element (FE) models with nerve fiber models and the activating function concept. The transcutaneously generated electric field is inherently non-focal, and thus, nerve fiber activation relies on inhomogeneities of the volume conductor and the anatomical paths of the target fibers relative to the electric field. The anatomy, its electrical properties, and the neurogeometry cause localized low-threshold sites within the widespread field and allow for the observed selective stimulation. Computer simulations are crucial for understanding the neuromodulative effects of transcutaneous SCS resulting from the transsynaptic activation of the spinal cord neural circuits and for further improving the stimulation technique.