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 current0distance (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 a 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 (<∼25Hz) is used, owing to their efficiency and accurate excitation and conduction properties. For high frequency stimulation (∼25Hz and greater), model C, with its ability to produce a DAP, is necessary accurately to simulate excitation behaviour.