Microscopic and macroscopic volume conduction in skeletal muscle tissue, applied to simulation of single-fibre action potentials

Extracellular action potentials of a single active muscle fibre in a surrounding of passive muscle tissue were calculated, using a microscopic volume conductor model which accounts for the travelling aspect of the source, the structure of skeletal muscle tissue and the electrical properties at the level of single muscle fibres. Owing to the capacitive properties of the muscle fibre membranes this inhomogeneous model is frequency dependent. The results of these calculations were compared with results obtained with a macroscopic homogeneous, frequency-independent version of the model. Close to the excited fibre the extracellular action potentials of both descriptions differ in their sensitivity to variations in the intra- and extracellular conductivities. It is demonstrated that within about 300 μm the muscle fibre membrane influences the amplitude as well as the timing of the peaks of the extracellular action potential. Both volume conduction descriptions result in almost the same single-fibre action potential at large radial distances from the excited fibre. These model results are of primary interest for the quantitative interpretation of single-fibre electromyograms recorded close to an active fibre, as for example in clinical fibre-density measurements