Blocking Aδ- and C-fiber neural transmission by sub-kilohertz peripheral nerve stimulation

Abstract

Introduction: We recently showed that sub-kilohertz electrical stimulation of the afferent somata in the dorsal root ganglia (DRG) reversibly blocks afferent transmission. Here, we further investigated whether similar conduction block can be achieved by stimulating the nerve trunk with electrical peripheral nerve stimulation (ePNS).

Methods: We explored the mechanisms and parameters of conduction block by ePNS via ex vivo single-fiber recordings from two somatic (sciatic and saphenous) and one autonomic (vagal) nerves harvested from mice. Action potentials were evoked on one end of the nerve and recorded on the other end from teased nerve filaments, i.e., single-fiber recordings. ePNS was delivered in the middle of the nerve trunk using a glass suction electrode at frequencies of 5, 10, 50, 100, 500, and 1000 Hz.

Results: Suprathreshold ePNS reversibly blocks axonal neural transmission of both thinly myelinated Aδ-fiber axons and unmyelinated C-fiber axons. ePNS leads to a progressive decrease in conduction velocity (CV) until transmission blockage, suggesting activity-dependent conduction slowing. The blocking efficiency is dependent on the axonal conduction velocity, with Aδ-fibers efficiently blocked by 50-1000 Hz stimulation and C-fibers blocked by 10-50 Hz. The corresponding NEURON simulation of action potential transmission indicates that the disrupted transmembrane sodium and potassium concentration gradients underly the transmission block by the ePNS.

Discussion: The current study provides direct evidence of reversible Aδ- and C-fiber transmission blockage by low-frequency (<100 Hz) electrical stimulation of the nerve trunk, a previously overlooked mechanism that can be harnessed to enhance the therapeutic effect of ePNS in treating neurological disorders.

Keywords: action potential; nerve block; neuromodulation; peripheral nerve stimulation; sub-kilohertz.

Figure 1

Schematic of multichannel single-fiber recordings to assess neural transmission block by electrical peripheral nerve stimulation (ePNS). (A) The diagram and photo showing the ex vivo single-fiber recordings from a peripheral nerve harvested from mice. (B) The synchronized stimulation protocol to study the effect of ePNS on axonal neural transmission.

Figure 2

Schematic of the neural membrane models that simulate the AP transmission in an unmyelinated C-fiber (A) and a thinly myelinated Aδ-fiber axon (B).

Figure 3

Single-fiber recordings of APs from the same axon when stimulated at both the initiation (E-Stim 1) and neuromodulation (E-Stim 2) sites. (A) Representative recordings from an Aδ-fiber and a C-fiber axon. The extended view indicated comparable AP waveform from both stimulation sites. (B) The conduction delays (CD) of APs in (A) from 10 consecutive stimulation (0.5 Hz) at both sites. (C) Normalized standard deviations (STD) of 10 consecutive CD as calculated by dividing the STD with the mean CD.

Figure 4

Reversible transmission block of Aδ- and C-fiber axons by suprathreshold ePNS at 50 and 10 Hz, respectively. (A) Representative single-fiber recordings from Aδ- and C-fiber axons before, during, immediately after, and 15–30 min after the neuromodulation delivered at “E-Stim 2.” Displayed in (B,C) are representative CD recorded once every 2 s during the suprathreshold and subthreshold ePNS, respectively.

Figure 5

Frequency-dependent axonal transmission block by suprathreshold ePNS assessed at six different frequencies from 5 to 1,000 Hz. (A) The blocking probability of Aδ- and C-fiber axons by at least one of the six stimulus frequencies in vagal, saphenous, and sciatic nerves. (B) The blocking probability of Aδ- and C-fiber axons by ePNS at six different stimulus frequencies. (C) The CV of axons that are blocked (solid dots) or unblocked (open dots) by ePNS at different frequencies.

Figure 6

The conduction delay increase (CDI) in blocked and unblocked axons. (A) representative CD recorded from one C-fiber axon undergoing ePNS at four different frequencies. (B) Summary of maximum CDI (CDI_max) in blocked and unblocked axons. * indicates p < 0.05 between blocked and unblocked CDI_max.

Figure 7

The NEURON simulation of axonal transmission block by ePNS. The model-simulated transmembrane potential voltages from the onset of ePNS till conduction block was plotted in (A) for the Aδ-fiber model and in (B) for the C-fiber model. (C) The model-simulated conduction delay (CD) following the ePNS protocol. The CD was calculated once every 2.5 s. (D) The model-simulated change in intracellular Na⁺ and K⁺ concentrations during the ePNS protocol.

Figure 8

Sub-kilohertz stimulation failed to block the AP transmission in the absence of disrupted transmembrane ionic gradients. (A) Stimulation at 20 Hz did not block the C-fiber model. (B) The Aδ-fiber model was not blocked by 50 Hz stimulation.