Extracellular potentials produced by a transition between an inactive and active regions of an excitable fibre

A mathematical modelling was used to study effects of the length, duration, and propagation velocity of a transition formed between the inactive and active (depolarized at most) regions of an infinite homogeneous fibre, on the extracellular potentials (EPs) produced by the transition. In this case the sources generating the extracellular potentials may be thought of as a distributed sequence of unidirectional dipoles lying along the fibre axis. The results have shown that the EPs have the dipole nature. In the proximity of the fibre, an increase in the transition length leads to a strong reduction of the EP amplitude; the inter-peak time interval is proportional to the transition duration and is independent of the propagation velocity. At small and middle radial distances lengthening of the transition reduces the radial decline of the EP amplitude. At great radial distances the EP field produced by a distributed source coincides with that produced by a lumped dipole having an intensity equal to sum of the intensities of all the dipoles of the distributed sequence. In isotropic volume conductor, at radial distances greater than the transition semilength, the radial decline of the potential amplitude is independent of the transition length; the inter-peak time interval is equal to y x square root of 2/v. Some problems of analysis of extracellular single muscle fibre potentials are discussed.