CHARACTERIZATION OF LONGITUDINAL-FIELD GRADIENTS FROM ELECTRICAL-STIMULATION IN THE NORMAL AND INJURED RODENT SPINAL-CORD

THE PURPOSE OF this experiment was to characterize the longitudinal fi eld gradients from exogenously applied electrical stimulation in the n ormal and injured rodent spinal cord. In addition, we compared the fie ld gradients arising from stimulation with two different types of stim ulating electrodes. Twenty normal rats underwent the surgical implanta tion of either extradural disc (n = 10) or cuff (n = 10) electrodes in the lower cervical and upper thoracic spinal cord. Sine waves of 1.5 to 50 muA and 0.5 to 50 Hz were used for stimulation. Field gradients were measured differentially from two extracellular glass microelectro des, positioned stereotactically in the spinal cord at different locat ions between the stimulating electrodes. The effect of acute spinal co rd injury on local field strength was studied in five animals from eac h group. The field gradients from stimulation with disc electrodes wer e greatest in close proximity to the discs and decreased markedly towa rd the point equidistant between the electrodes. In contrast, the grad ients produced by cuff electrodes were much more evenly distributed al ong the spinal cord, increasing only slightly in proximity to the elec trodes. These fields were also more evenly distributed throughout the spinal cord in cross-section but were generally weaker than those aris ing from disc electrodes. Acute spinal cord injury significantly incre ased the field gradients arising from both disc and cuff electrodes. H owever, the observed gradients remained substantially lower than those reported to enhance neurite growth in vitro. We conclude that the pos ition and design of stimulating electrodes has a profound effect on lo ngitudinal field gradients within the mammalian spinal cord, as does t he presence of an acute spinal cord injury.