Unmyelinated Aplysia nerves exhibit a nonmonotonic blocking response to high-frequency stimulation

The application of high-frequency alternating current (HFAC) stimulation to reversibly block conduction in peripheral nerves has been under investigation for decades. Computational studies have produced ambiguous results since they have been based on axon models that are perhaps not valid for the nerves in which the phenomenon has been demonstrated. Though simulations based on the Hodgkin-Huxley unmyelinated nerve cable model have been used to understand the phenomena, the isolated response of an unmyelinated nerve to HFAC waveforms has not been experimentally investigated. To understand the effect of HFAC waveforms in homogenous nerves, experiments were conducted on purely unmyelinated nerves of the sea-slug Aplysia californica. Sinusoidal waveforms in the range of 5-50 kHz were used to block the propagation of action potentials through the nerves. The time for complete recovery from block was found to be dependent on the duration of application of the HFAC waveform but was independent of the frequency of the waveform tested. Unlike data from simulations and experiments on myelinated nerves, the minimum HFAC amplitude for blocking conduction in these unmyelinated nerves exhibited a unique nonmonotonic relationship with frequency, which may be advantageous in various neurophysiological applications.