Modelling the effects of electric fields on nerve fibres: influence of the myelin sheath.

The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibres, incorporating three different myelin representations, are compared. The three myelin representations are a perfectly insulating single cable (model A), a finite impedance single cable (model B) and a finite impedance double cable (model C). Extracellular stimulation of the three models is used to study their strength-duration and current-distance (I-X) relationships, conduction velocity (CV) and action potential shape. All three models have a chronaxie time that is within the experimental range. Models B and C have increased threshold currents compared with model A, but each model has slope to the I-X relationship that matches experimental results. Model B has a CV that matches experimental data, whereas the CV of models A and C are above and below the experimental range, respectively. Model C is able to produce a depolarising afterpotential (DAP), whereas models A and B exhibit hyperpolarising afterpotentials. Models A and B are determined to be the preferred models when low-frequency stimulation (< approximately 25 Hz) is used, owing to their efficiency and accurate excitation and conduction properties. For high frequency stimulation (approximately 25 Hz and greater), model C, with its ability to produce a DAP, is necessary accurately to simulate excitation behaviour.