A quantitative study of electrical stimulation of central myelinated fibers.

Quantitative extracellular electrical stimulation data were obtained from single fibers in the dorsal columns of anesthetized and paralyzed cats. Single-fiber responses were recorded with glass microelectrodes. The threshold current was found for fibers stimulated with a 100-p diameter monopolar electrode at the surface of the dorsal columns 0.5-2.0 cm from the recording electrode. Shock artifact was greatly reduced because of the geometry of the stimulating and recording scheme. The distance of the stimulating electrode from a recorded fiber could be determined since fibers in the medial dorsal columns maintain a relatively constant relation to the midline and the surface for several segments. The stimulus was usually a single cathodal current pulse of SO-psec duration. The conduction velocity was also determined for each fiber. Current-distance relations for threshold stimulation were determined for these fibers. In general, the further away the stimulating electrode, the greater the current required for stimulation. Moreover, fibers with larger conduction velocities required less current for stimulation. The fibers were also shown to be periodic in their longitudinal stimulation properties which probably was related to the length of their internodes. The time constant of the nodal membrane was determined from strength-duration data to be about 100120 @sec. The mean of the ratio of anodal to cathodal stimulating current was 4.57 + 1.00 (SD) and ranged from 3.19 to 7.70 for 50-,usec pulse durations.