The specific impedance of the dorsal columns of cat: An anisotropic medium

Abstract

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.

Computer modelling of spinal cord stimulation and its contribution to therapeutic efficacy

“…21 The worst case was a change in threshold stimulus of only 19% for DC ®bres and 27% for dorsal root ®bres (EG Olde Engberink, WA Wesselink, J Holsheimer, unpublished results).…”

“…In this frequency range the transverse and longitudinal conductivities of the DCs vary approximately 23% and 3.5%, respectively. 21 Electrode de®nition In all models electrode contacts are de®ned in the dorsal epidural space, generally next to the dura mater or CSF. Coburn and Sin 11,16,17 represented each cathode or anode by a constant current source (+1 mA) con®ned to a single node of the mesh.…”

Electric field distribution in a finite-volume head model of deep brain stimulation

“…Encapsulation tissue dimensions and material properties also vary among different studies [13], and there is wide variability in reported conductivity data for different biological tissues. Furthermore, many biological tissues, including brain tissue, are anisotropic [29]. Additionally, this model does not consider local tissue variations, nor does it include alternative return current paths caused by areas in the brain filled by cerebrospinal fluid.…”

“…The brain was assumed to consist of isotropic tissue of conductivity 0.27 S/m [29]. The conductivity of cerebrospinal fluid was assumed to be 1.6 S/m assuming human body temperature of 37 • C [30].…”

Determination of Optimal Parameters and Feasibility for Imaging of Epileptic Seizures by Electrical Impedance Tomography: A Modelling Study Using a Realistic Finite Element Model of the Head

“…Since the nerve fibres are randomly oriented, the white matter has been considered to have an equivalent volume of 1/3 of fibres longitudinal to the current path and 2/3 transverse (Ranck, Jr., 1963). The conductivity of a bundle of parallel axons was measured in the two directions in-vivo in the dorsal column of cats between 5 Hz and 50 kHz (Ranck, Jr. and BEMENT, 1965). An averaged conductivity (1/3 longitudinal + 2/3 transverse) was calculated as representative of the white matter.…”