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Low frequency, nonstimulating current was passed from a small electrode on the surface of the dorsal columns in the cervical cord of cats. A glass microelectrode was used to record the voltage at distances of 0.5 to 2 mm from the current electrode. The voltage fell off more rapidly in depth and across the dorsal columns than it did longitudinally-the resistance was lower in the longitudinal direction. Accordingly, the dorsal columns are anisotropic. An approximate equation is presented which describes the data fairly well and which is consistent with the anatomy.From this equation, the resistivity in the longitudinal direction was 138 to 212 ohm-cm and in the transverse direction, 1,211 ohm-cm. These values are shown to be consistent with the view that the anisotropy is primarily due to current flowing longitudinally in axons. The frequency dependence of the specific impedance was also measured. Some features of this frequency dependence have no clear explanation, but some of them are consistent with a nodal membrane having a time constant of roughly 50 psec.

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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.

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