The equivalent source description representing the extinction of an action potential at a muscle fiber ending

Extracellular potential waveforms resulting from the termination of the propagation of the transmembrane potential at the end of an active muscle fiber are studied. The transmembrane potential along the muscle fiber is calculated from Hodgkin-Huxley-like equations with axially varying parameters under the condition of increasing axial resistance, decreasing ionic conductances, or decreasing radius at the muscle fiber end. The extracellular potential distribution is calculated using the derived transmembrane current as a line source in an unbounded volume conductor. Potential waveforms with constant latency at extracellular points far from the fiber end are presented and are compared with measurements and simulations from literature. Monophasic waveforms with constant latency are found. The polarity and the amplitude as a function of axial and radial position relative to the fiber end indicate that an equivalent dipole source adequately represents the propagation stop at the fiber end. An expression for the dipole moment in terms of fiber radius, intracellular conductivity, and transmembrane potential is derived.