Intracellular contribution to extracellularly recorded waveforms: The 'membrane rent' hypothesis

Objective: This investigation uses simulation studies to account for single muscle fiber waveforms with complex configurations as arising from the simultaneous recording of a partial intracellular discharge and its associated extracellular manifestation by way of an electrode-induced 'rent' or tear in the sarcolemma. Methods: Published material on intracellular action potentials from healthy and 7 day denervated rat skeletal muscle was used as the basis for calculations. A single muscle fiber computer simulation capable of formulating a cut or crush termination effect used the modeled action potentials to generate extracellular waveforms at different locations along the muscle fiber. These extracellular waveforms were then summated with a varied fraction of the intracellular action potential to yield a combined potential. These simulated waveforms were then compared to previously recorded single muscle fiber discharges in order to establish if a combined waveform could reproduce the clinically recorded potentials' configuration. Results: It was not possible to simulate any of the previously detected innervated single muscle fiber discharges by combining the action potential's intracellular and extracellular configurations. However, 12 of the 14 clinically observed complex waveforms documented in denervated tissue could be simulated with morphologies similar to the clinical potentials. Conclusions: Presumed single muscle fiber discharges with complex configurations may result from the needle electrode simultaneously recording the action potential's intracellular and extracellular waveforms secondary to a 'rent' in the sarcolemma. This explanation may in part account for some of the complex appearing potentials detected in denervated tissue, but this 'rent' hypothesis is likely not the explanation for potentials observed in innervated muscle tissue. The apparent success of the 'rent' hypothesis in denervated tissue may be a result of the denervated action potential's unique morphology rather than an actual tear in the sarcolemma. Further investigations are necessary to determine if it is possible for a needle electromyographic electrode to actually record in part the intracellular action potential without disrupting the fiber.