Modelling activity-dependent changes of velocity in C-fibers.

Aims When C-nociceptors are activated repeatedly using electrical stimulation at relatively low frequencies (0.125-2 Hz), their propagation velocity will decrease. This is referred to as activity-dependent slowing (ADS). The main reason why activity-dependent changes in velocity are of interest is that they can be recorded directly, using invasive methods (microneurography), in patients with chronic pain. Interestingly, in certain patients with neuropathic pain, reduced activity-dependent slowing of conduction has been observed, indicating that these axons have an increased excitability. Through a computational model, it is possible to link such velocity alterations with changes in active conductances, opening for an understanding the underlying excitability changes occurring in these patients. Methods We have developed a detailed multicompartment model of a C-nociceptor fiber. This model incorporates a wide range of voltage-gated ion channels (Nav1.7, Nav1.8, Nav1.9, Kdr, KA, KM, KNa and h) which were implemented across a detailed and realistic axon morphology. Results The model predicts that the small diameter of the axon can accumulate intracellular sodium when it is repeatedly activated in a similar fashion as during single fiber microneurography. This increase of intracellular sodium concentration can shift the balance between ion channel currents, shift the membrane potential and membrane input resistance, and thereby generate activity-dependent changes of velocity, such as ADS as well as recovery cycle supernormality. Conclusions Our results thus provide insight into how activity-dependent excitability changes can be generated in C-fibers. By identifying which ion channels are contributing to activity-dependent changes of velocity this could provide insight into ion channel alterations in neuropathic pain patients.