Influence of the myelin sheath on excitation properties of nerve fibers

The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibers incorporating three different myelin representations were compared. The three myelin representations were 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 was used to study strength-duration, current-distance (I-X), conduction velocity (CV) properties. All three models had a chronaxie time (Tch) that was within the experimental range. Models B and C had increased threshold current as compared to model A, but each model had a slope to the I-X relationship that matched experimental results. Model B had a CV that matched experimental data while the CV of models A and C were above and below the experimental range, respectively. These results indicate that the presence of a finite impedance myelin sheath does influence the excitation properties of nerve fiber models and must be considered when using models for design of neural stimulation paradigms.