Altered excitability of small cutaneous nerve fibers during cooling assessed with the perception threshold tracking technique

Abstract

Background: There is a need for new approaches to increase the knowledge of the membrane excitability of small nerve fibers both in healthy subjects, as well as during pathological conditions. Our research group has previously developed the perception threshold tracking technique to indirectly assess the membrane properties of peripheral small nerve fibers. In the current study, a new approach for studying membrane excitability by cooling small fibers, simultaneously with applying a slowly increasing electrical stimulation current, is evaluated. The first objective was to examine whether altered excitability during cooling could be detected by the perception threshold tracking technique. The second objective was to computationally model the underlying ionic current that could be responsible for cold induced alteration of small fiber excitability. The third objective was to evaluate whether computational modelling of cooling and electrical simulation can be used to generate hypotheses of ionic current changes in small fiber neuropathy.

Results: The excitability of the small fibers was assessed by the perception threshold tracking technique for the two temperature conditions, 20 °C and 32 °C. A detailed multi-compartment model was developed, including the ionic currents: Na_TTXs, Na_TTXr, Na_P, K_Dr, K_M, K_Leak, K_A, and Na/K-ATPase. The perception thresholds for the two long duration pulses (50 and 100 ms) were reduced when the skin temperature was lowered from 32 to 20 °C (p < 0.001). However, no significant effects were observed for the shorter durations (1 ms, p = 0.116; 5 ms p = 0.079, rmANOVA, Sidak). The computational model predicted that the reduction in the perception thresholds related to long duration pulses may originate from a reduction of the K_Leak channel and the Na/K-ATPase. For short durations, the effect cancels out due to a reduction of the transient TTX resistant sodium current (Na_v1.8). Additionally, the result from the computational model indicated that cooling simultaneously with electrical stimulation, may increase the knowledge regarding pathological alterations of ionic currents.

Conclusion: Cooling may alter the ionic current during electrical stimulation and thereby provide additional information regarding membrane excitability of small fibers in healthy subjects and potentially also during pathological conditions.

Keywords: Electrical stimulation; Multicompartment model; Na/K-ATPase; Perception threshold tracking; Temperature; Voltage-gated ion channels.

Fig. 1

The experimental setup and the computational model design. A computer (a) was used to control the pulses delivered by the Digitimer DS5 stimulator (b) which electrically stimulated through the custom-made planer array electrode (c), that was placed on the volar forearm of the subject. A thermode (d) from the Pathway system, used for controlling the temperature, was placed on top of the electrode (e). In the other hand the subject held a push button (f) which was used to indicate the perception threshold. Computational model design (g)

Fig. 2

The shape of a 50 ms bounded exponential pulse. For the three other pulses, the x-axis was scaled to 1, 5 and 100 ms respectively

Fig. 3

The effect of cooling the skin on the perception threshold. The experimental result of perception thresholds to electrical currents delivered to the skin with temperature regulated to 20 °C or 32 °C. Data is shown as mean and standard error. Statistically significant differences between the two temperature conditions are indicated by asterisks, ***p < 0.001, Sidak multiple comparisons

Fig. 4

Depolarization of the resting membrane potential by reduced temperature. a The axon model design. b The temperature along the axon model for the two temperature conditions. The red and blue lines represent the temperature conditions 32 °C and 20 °C respectively. c The steady state solution of the membrane potential along the axon model. Note the large depolarization occurring at the distal end of the axon model (x axis = 0)

Fig. 5

The experimental results could be reproduced in the computational model. a Extracellular field alteration potential at the distal end of the axon model. b The membrane potential at the distal end of the axon model induced when the alteration in extracellular field according to A was applied. c The activation threshold for different durations of the extracellular field alteration for the two temperature conditions. The activation threshold was defined as the extracellular field alteration required to generate and action potential propagating to the end of the axon model

Fig. 6

Ionic currents during an action potential generation. The extracellular field was altered for 1 ms with a bounded exponential shape. The figures to the left represent the simulation performed at 32 °C and to the right when the temperature is slowly decreased to 20 °C. a Membrane potential recorded at the distal end of the axon model. b The slow ionic currents. c The fast ionic currents

Fig. 7

The influence of temperature on subtypes of ionic currents. a The activation threshold was estimated for each ionic current when temperature change was set to only influence that specific ionic current and no other currents. b The relative change of the activation threshold due to cooling when temperature change was set to only influence that specific ionic current and no other currents

Fig. 8

Cooling may increase understanding of abnormal ionic current during neuropathy. The Na/K-ATPase (0%), Na_TTXr (130%), Na_TTXs (500%) and Na_P (200%) maximal conductances were altered to generate the hyperexcitability models. a The activation threshold estimated in the hyperexcitability models and the control model (no alteration of the ionic conductances) for the temperature condition 32 °C. b The activation threshold estimated in the hyperexcitability models and the control model (no alteration of the ionic conductances) for the temperature condition 20 °C. c The relative difference between the activation threshold for the two temperature conditions (20 °C and 32 °C)