1 Int roduct ion

AT MICROSCOPIC scale skeletal muscle tissue as an electrical conductor is characterised by the intra- and extracellular conductivities (tr i and ~re, respectively), the impedance of the muscle fibre membrane Z = and by parameters which describe the tissue structure. In model studies extracellular action potentials are often simulated with a macroscopic approximation i which skeletal muscle tissue is assumed to be a noncapacitive, homogeneous, anisotropic volume conductor (NANDEDKAR and ST/~LBERG, 1983; ROSENFALCK, 1969). In such an approach the tissue is elec-trically characterised by parameters observed at large, macroscopic scale, namely the conductivities normal and parallel to the fibre direction (EPSTEIN and FOSTER, 1983; ZrmNG et al., 1984). ALBERS et al. (1986) introduced a microscopic volume conductor model which accounts for the tissue structure and the microscopic electrical parameters. Conduction of current normal to the fibre direction occurs almost solely through the extracellular medium, owing to the high value of the membrane impedance. Parallel to the muscle fibre direction and close to the site of current injection a major part of current is conducted through the extracellular medium. However, at larger distances from the source