Determination of the extracellular tortuosity in nuclear layers of the central nervous system by resistance measurements on a geometrical model.
The electrical resistivity of nuclear layers within CNS tissues is simulated by a densely packed array of glass spherules with a thin silver layer on their surfaces. Various mean spherule diameters and different thicknesses of the silver layer are tested; the relations between spherule diameter and thickness of silver mantle are in the same range as those between cell soma diameters and width of the extracellular clefts. Measuring the contribution of the silver layers to the total volume--corresponding to the extracellular volume fraction alpha within CNS tissues--and the resistance of columns of silvered spherules, tortuosity factors lambda 2 are calculated. Means of lambda 2 are found to vary within 5.6 and 9.2, independent of both spherule diameters and thickness of the silver layer, but strongly dependent on the packing density of the spherules. This latter dependence is described by a simple formula. These results are used to calculate the resistivities of the nuclear layers of the retina and of the granular layer in the hippocampal area dentata, based on morphometric data gained from own studies and literature reports, respectively. It is shown that such layers with densely packed cells express very high resistivity because of both low extracellular volume fraction and high tortuosity. Implications for current source density analyses, and for pathological events like epileptogenesis, are discussed. In an appendix, an analytical solution of the problem is given for the case of a cubic array of surface-conducting spheres.
Eberhardt
W
W
Carl Ludwig Institute of Physiology, Karl Marx University, Leipzig, German Democratic Republic.
Woidich
D
D
Reichenbach
A
A
D000818:Animals; D008959:Models, Neurological; D008962:Models, Theoretical; D011817:Rabbits; D012160:Retina / Q000166:cytology / Q000502:physiology*; D012834:Silver
D012834:Silver