On the volume conduction in human skeletal muscle: in situ measurements

Volume conduction was studied in situ in human skeletal muscle, using a "single fibre multielectrode". The multielectrode was inserted perpendicular to the fibre axis, and the same single fibre action potential was recorded consecutively from 4 neighbouring leading-off surfaces. The transfer functions of 10 sections of muscle tissue lying between pairs of neighbouring electrodes were calculated from the Fourier transform of the input and output, i.e. the action potential recorded by the two neighbouring leading-off surfaces. The transfer function had one dominant pole, and was therefore modelled by a section of lumped RC elements The average gain factor and average time constant calculated from the transfer function for a muscle tissue section of 76 mu length were 0.55 and 69 musec, respectively. The radial attenuation of the different sinusoidal components of the extracellular action potential was calculated, and was found to be greater than previously reported in the literature, particularly for the slow frequency components. As an explanation of these findings, the possibility of a greater anisotropy of the muscle tissue than previously reported is discussed.