Electrical Characterisation of Aδ-Fibres Based on Human in vivo Electrostimulation Threshold

Abstract

Electrical stimulation of small fibres is gaining attention in the diagnosis of peripheral neuropathies, such as diabetes mellitus, and pain research. However, it is still challenging to characterise the electrical characteristics of axons in small fibres (Aδ and C fibres). In particular, in vitro measurement for human Aδ-fibre is difficult due to the presence of myelin and ethical reason. In this study, we investigate the in vivo electrical characteristics of the human Aδ-fibre to derive strength–duration (S–D) curves from the measurement. The Aδ-fibres are stimulated using coaxial planar electrodes with intraepidermal needle tip. For human volunteer experiments, the S–D curve of Aδ-fibre is obtained in terms of injected electrical current. With the computational analysis, the standard deviation of the S–D curve is mostly attributed to the thickness of the stratum corneum and depth of the needle tip, in addition to the fibre thickness. Then, we derive electrical parameters of the axon in the Aδ-fibre based on a conventional fibre model. The parameters derived here would be important in exploring the optimal stimulation condition of Aδ-fibres.

Synaptic Effect of Aδ-Fibers by Pulse-Train Electrical Stimulation

“…We then used the relationship between the current threshold and the number of presynaptic neurons to obtain the injection current threshold to The electric field distribution on the transversal plane of the skin model is illustrated, as an example, for an injection current of 0.02 mA. The response of the Aδ-fiber matched the experimental strength-duration curves for single-pulse stimulation in our previous study (Tanaka et al, 2021). In this work, the afferent spikes of the Aδ-fiber originated from the pulse-train are computed and integrated into a synaptic model.…”

“…The experimental protocol to measure the perception threshold (the lowest current to generate sensation in participants) was presented in our previous study (Tanaka et al, 2021). In brief, the participants were instructed to press a button as soon as they felt the sensation.…”

“…A computational multiscale electromagnetic model of Aδ-fibers during IES was developed in our previous studies (Motogi et al, 2016;Tanaka et al, 2021). This was based on strength-duration measurements using single-pulse stimulation.…”

“…The skin was modeled as a layered structure for hairy skin (Alekseev and Ziskin, 2007;Schmid et al, 2013;De Santis et al, 2015;Motogi et al, 2016). The thickness of the tissues and their conductivity values were identical to those observed in our previous studies and are summarized in Figure 2A (Tanaka et al, 2021). The dimensions of the skin model were 1.54 mm (depth) × 1.65 mm × 1.65 mm discretized by voxels of 5 µ m in length.…”

“…For simplicity, the computed thresholds of a single fiber model were assumed to be equal to the perception threshold based on in vivo experiments. In our previous study (Tanaka et al, 2021), the Aδ-fiber model was characterized based on S-D measurements considering the region where multiple fibers could be stimulated. These multiscale models (skin volume conductor and neural models) described local responses at fiber terminals and helped us to better understand the mechanism of IES for single-pulse stimulation.…”