Threshold estimation with field stimulation [transmembrane potentials]
The central focus of this report is on the evolution of transmembrane potentials following initiation of a point-source field stimulus, particularly when the stimulus is short and the stimulating electrode is close to the responding membrane. A central question is whether it is possible to know in advance if a stimulus of specific magnitude, duration and location will result in a subsequent action potential. Such determination can be based on the membrane's “voltage threshold”, which unfortunately was found to vary markedly depending on the duration and location of the point current stimulus. A related question is whether all active membrane can be assumed to function initially as a passive membrane would; if so, more simple procedures (than would be required with accurate non-linear membrane properties) may be available to determine if an action potential will be elicited. Results from the numerical examples show that the passive response tracks active responses long enough to be a good estimator of subsequent action potential development. Examples show that the evaluation of the transmembrane potential, Vm, about 0.30 msec after stimulus initiation was a better predictor of subsequent excitation than was either initial transmembrane current or Vm at the time when the stimulus ends. The delay time of 0.30 msec was chosen on the basis of membrane (rather than stimulus) characteristics. Most of the circumstances analyzed here with electric field stimulation also appear likely to be valid with magnetic field stimulation
