Extracellular sinusoidal low-frequency alternating current (LFAC) stimulation of peripheral motor nerves has been experimentally observed to induce an orderly, size-wise activation of nerve fibers. This pattern contrasts with the typical inverse recruitment order produced by extracellular pulsed stimulation. The study aimed to explore potential biophysical mechanisms that could explain LFAC-driven orderly recruitment using computational modeling rather than experimental manipulation.
The authors implemented a volume conductor model of a bipolar cuff electrode encasing a single rat-sized fascicle to compute extracellular potentials. These extracellular fields generated by either LFAC or pulsed stimulation were projected onto established multicompartment models of myelinated motor axons (McIntyre–Richardson–Grill models). The modeled fiber population spanned diameters from 5.7 to 16 μm to represent a physiologically relevant range of motor fiber sizes. Intracellular and extracellular stimulation modalities were directly compared within the modeling framework.
Two classes of stimulation were evaluated. Sinusoidal LFAC stimuli were tested across a frequency range of 1–20 Hz, and their strength–frequency relationships were characterized. Conventional extracellular pulsed stimuli were assessed using strength–duration curves. By applying the same extracellular potentials to the axon models, the authors compared thresholds and dynamic membrane responses across waveforms and fiber sizes.
To probe how axonal membranes respond to sustained and slowly varying fields, the simulations used a threshold-tracking technique to quantify membrane electrotonus and threshold electrotonus. These measures allow assessment of subthreshold behavior and accommodation phenomena — that is, how membrane ionic-state changes during slow or prolonged depolarization alter excitability. The analysis focused on how such subthreshold dynamics differ between small and large myelinated fibers under LFAC and prolonged pulse stimulation.
Simulations revealed that the inverse order of fiber recruitment classically reported for extracellular stimulation is an inherent feature of extracellular activation and, in theory, does not depend on the specific stimulation waveform. Nonetheless, LFAC produced an observable orderly, size-wise activation pattern in the modeled motor fibers. The modeling suggests that waveform shape alone does not fully determine recruitment order; instead, interactions with intrinsic membrane dynamics play a central role.
LFAC stimulation demonstrated an inverse strength–frequency relationship: as LFAC frequency increased (within the tested 1–20 Hz range), the threshold current required for activation decreased. This inverse relationship mirrors the inverse strength–duration relationship typically seen with pulsed stimulation, linking stimulus temporal features to recruitment thresholds through comparable biophysical principles.
Detailed analysis of subthreshold accommodation implicated ionic-state dynamics as a key contributor to orderly recruitment. Larger fibers exhibited greater accommodation than smaller fibers during slowly varying or prolonged stimulation. Specifically, larger fibers showed increased activation thresholds that correlated with a decrease in the fast sodium activation factor (referred to as m3h in the study) and an increase in slow potassium activation. These changes reduce excitability during sustained depolarization, effectively raising thresholds for larger fibers and favoring activation of smaller fibers at given stimulus parameters. The authors interpret this accommodation differential as supporting the observed size-wise activation under LFAC.
Increasing LFAC frequency up to 20 Hz reduced the accommodation-related differences between fiber sizes. With higher LFAC frequency, the state dynamics of large fibers shifted toward those characteristic of smaller fibers, lowering the relative accommodation penalty for large axons. This frequency dependence provides a mechanism by which LFAC parameters can tune recruitment patterns: frequency modulates the balance between accommodation and spike initiation across the fiber population.
The simulations also showed that LFAC can induce subthreshold membrane oscillations. These oscillations facilitated spike initiation during phases of slow depolarization, providing an additional mechanism for activating fibers without invoking complex selective block protocols. In other words, LFAC-promoted oscillatory dynamics can help overcome accommodation and promote action potential generation in a controlled manner.
Taken together, the modeling results indicate that LFAC leverages intrinsic membrane properties — notably subthreshold accommodation and oscillatory responses — to produce orderly recruitment of motor fibers. Because this recruitment can be achieved without specialized blocking techniques, LFAC may offer a controlled approach for selective nerve stimulation that preserves physiological recruitment order. The authors propose that these properties of LFAC have direct implications for clinical and neuroprosthetic applications where preserving or approximating natural motor unit recruitment is desirable.
Note on source and scope
All descriptions and interpretations above are drawn from the preprint abstract and reported simulation results. The article is a preprint and has not been peer reviewed; full methodological details, parameter values, and quantitative results beyond what is summarized in the abstract were not reported in the provided source text.