Sensory Effects of Transient Electrical Stimulation - Evaluation with a Neuroelectric Model

A model of myelinated nerve axon was used to study the initiation and propagation of action potentials for a variety of extracellular electrical stimuli. Frankenhaeuser-Huxley nonlinearities were incorporated at each of several nodes in a longitudinal array, and the extracellular current pulse was modeled as a spatial distribution of voltage disturbance along the membrane. Results from the model were compared to data from human sensory experiments and from animal electrophysiological experiments. Effects of polarity, electrode position, pulse duration, and biphasic oscillation frequency were examined. Biphasic pulses have higher excitation thresholds than monophasic pulses, provided the duration of a single phase is short relative to the time constant of the membrane depolarization process. The shapes of strength/duration curves from sensory experiments conform well to model predictions for monophasic stimuli. Strength/frequency curves derived from the model are similar to those from sensory stimulation with sinusoidal currents. The shapes of strength/frequency curves can be explained by membrane integration effects at high frequencies and membrane leakage effects at low frequencies. The model predicts lower thresholds for cathodal than for anodal stimulation: the predicted degree of polarity selectivity is confirmed by direct stimulation of axons in animal experiments, but is at variance with the selectivity found in human transcutaneous stimulation.