A multidimensional model of cellular effects on the spread of electrotonic currents and on propagating action potentials.

This study was designed to develop a two-dimensional cellular model of uniform anisotropic muscle and to determine how irregularities of shape and variations in size of cardiomyocytes influence the passive (electrotonic) spread of currents at a microscopic level. A secondary purpose was to determine how the passive transfer of impressed currents across the gap junctions is related to the charge flow across the gap junctions during active propagation of depolarization. The decrease in electrotonic Vm with distance at a large size scale was described by a single exponential in both the longitudinal and transverse directions, as occurs in a continuous anisotropic medium. At a microscopic level, however, the falloff of Vm with distance was directionally different. Longitudinally, Vm decreased primarily along the length of cells, with small step-like decreases at the intercalated disks. Transversely, Vm was more nearly isopotential throughout each cell, and most of the decay in Vm occurred as large step-like decreases across the borders of the cells. Different gap junctions were used for charge flow for longitudinal versus transverse electrotonus. Remarkably similar results were obtained for propagating action potentials, i.e., different gap junctions were used for longitudinal versus transverse conduction. A major implication of the results is that it may be possible to gain information about the different longitudinal and transverse effects of the nonhomogeneous distribution of the cellular connections by improved measurements of propagation at a microscopic level.