Small and large cutaneous fibers display different excitability properties to slowly increasing ramp pulses

The excitability of large nerve fibers is reduced when their membrane potential is slowly depolarizing, i.e., the fibers display accommodation. The aim of this study was to assess accommodation in small (mainly Aδ) and large (Aβ) cutaneous sensory nerve fibers using the perception threshold tracking (PTT) technique. Linearly increasing ramp currents (1 ms-200 ms) were used to assess the excitability of the nerve fibers by cutaneous electrical stimulation. To investigate the PPT technique's ability to preferentially activate different fiber types, topical application of lidocaine/prilocaine (EMLA) or a placebo cream was applied. By means of computational modeling, the underlying mechanisms governing the perception threshold in the two fiber types was studied. The axon models included the voltage-gated ion channels: transient TTX-sensitive sodium current, transient TTX-resistant sodium current (Na_TTXr), persistent sodium current, delayed rectifier potassium channel (K_Dr), slow potassium channel, and hyperpolarization-activated current. Large fibers displayed accommodation, whereas small fibers did not display accommodation (P < 0.05). For the pin electrode, a significant interaction was observed between cream (EMLA or placebo) and pulse duration (P < 0.05); for the patch electrode, there was no significant interaction between cream and duration, which supports the pin electrode's preferential activation of small fibers. The results from the computational model suggested that differences in accommodation between the two fiber types may originate from selective expression of voltage-gated ion channels, particularly the transient Na_TTXr and/or K_Dr. The PTT technique could assess the excitability changes during accommodation in different nerve fibers. Therefore, the PTT technique may be a useful tool for studying excitability in nerve fibers in both healthy and pathological conditions.NEW & NOTEWORTHY When large nerve fibers are stimulated by long, slowly increasing electrical pulses, interactive mechanisms counteract the stimulation, which is called accommodation. The perception threshold tracking technique was able to assess accommodation in both small and large fibers. The novelty of this study is that large fibers displayed accommodation, whereas small fibers did not. Additionally, the difference in accommodation between the fiber could be linked to expression of voltage-gated ion channels by means of computational modeling.