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

The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibres, incorporating three different myelin represen tations, are compared. The three myelin representations are a perfectly ins ulating single cable (model A), a finite impedance single cable (model B) a nd a finite impedance double cable (model C). Extracellular stimulation of the three models is used to study their strength-duration and current-dista nce (I-X) relationships, conduction velocity (CV) and action potential shap e. AII three models have a chronaxie time that is within the experimental r ange. Models B and C have increased threshold currents compared with model A, but each model has a slope to the I-X relationship that matches experime ntal 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, respective ly 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 (< sim ilar to 25 Hz) is used, owing to their efficiency and accurate excitation a nd conduction properties. For high frequency stimulation (similar to 25 Hz and greater), model C, with its ability to produce a DAP, is necessary accu rately to simulate excitation behaviour.