Interactions between adjacent fibers in a cardiac muscle bundle

Abstract

A strand of cardiac muscle was modeled as a small bundle of individual fibers surrounded by a large volume conductor. The bundle is a uniform assembly of small identical cylindrical fibers, arranged as a series of concentric layers, and its behavior is examined in the presence (coupled bundle) or absence (uncoupled bundle) of transverse resistive coupling between adjacent fibers. Individual fibers are continuous cables of excitable membrane, with circumferential segmentation into 12 equal patches to make the membrane potential changes dependent upon the local interstitial potential. The minimum spacing (d) between adjacent fibers is used to modify the interstitial microstructural organization and the intracellular volume fraction (f _i). Whend is small enough (d<0.01 μm),f _i remains unchanged at its maximum of about 90%, the interstitial potential is large, the transveerse interstitial resistance is high, and the proximity effect arising from the close juxtaposition of adjacent fibers is important. A surface fiber of the uncoupled bundle exhibits little sensitivity to changes in the intestitial microstructure, owing to the dominant influence of the external volume conductor, whereas the central fiber shows a large decrease in velocity, substantial waveshape modifications, and a large increase in interstitial potential asd is reduced. In the coupled bundle, all fibers adopt the same velocity during uniformo propagation owing to the strong transverse resistive coupling; whend is reduced in the range ofd<0.01 μm, the velocity and interstitial potential changes are less pronounced than in the uncoupled bundle. Whend is large enough (d>0.01 μm), the bundle behavior (coupled and uncoupled) approaches that obtained with a bidomain formulation.