A modified bidomain model for extracellular potential of the active bundle of compound nerve fibres

The bidomain concept ignores the presence of different natured fibres within a bundle. A modified bidomain approximation is presented to formulate the extracellular potential generated by a bundle of compound nerve fibres with small diameters (grouped into N classes according to their nature). Fibres in each group are assumed to have a homogeneous distribution within the bundle and tightly packed with ones in other groups. It is possible to consider that the space of each group is continuous in the bundle and is separated with interstitial space everywhere by the membrane. Thus discrete bundle structure can be replaced with N+1 coexisting continuous domains (N tntracellular and one interstitial). Based on these representation, the mathematical descriptions of extracellular and interstitial potentials as functions of both spatial variable and temporal variable are given.