A distributed-parameter model of the myelinated human motor nerve fibre: temporal and spatial distributions of action potentials and ionic currents

Abstract

A double cable model of the myelinated human motor nerve fibre is presented. The model is based on the nodal and internodal channels in a previous, two-component model of human motor axons (Bostock et al. 1991), added to a complex extended cable structure of nodal, paranodal and internodal segments. The model assumes a high-resistance myelin sheath and a leakage pathway to the internodal axolemma via the paranodal seal resistance and periaxonal space. The parameter values of the model were adjusted to match the recordings of threshold electrotonus in human motor fibres from Bostock et al. (1991). Kirchoff's current law was used to derive a system of partial differential equations for the electrical equivalent circuit, and numerical integration was performed with a fixed time increment and non-uniform spatial step sizes, in accordance with the complex structure of the fibre. The model calculations provide estimates of the spatial and temporal distributions of action potentials and their transaxonal and transmyelin components, both in different segments of the fibre and at different moments during action potential propagation. The distribution of transaxonal and transmyelin currents along the fibre and their contributions from different ionic channels are also explored.

Differences in Membrane Properties in Simulated Cases of Demyelinating Neuropathies: Internodal Focal Demyelinations with Conduction Block

“…The demyelinations are associated with a corresponding loosening of the myelin lamellae away from the axolemma in the given internodal segments. The other membrane parameter values for the human motor fibre are the same as described earlier [18,20] 3. We would like to remind the reader that the channel types are also in accordance with the directly investigated ion channels in human axons [22,23].…”

“…The electrogenesis of myelinated human motor nerve fibre can be studied successfully using the double cable model of the fibre [18][19][20]. The interested reader is referred to the first figure of [18] for an electric equivalent circuit representation of the human motor fibre.…”

“…The interested reader is referred to the first figure of [18] for an electric equivalent circuit representation of the human motor fibre. In the cited paper, Kirchoff's current law is used to derive a system of partial differential equations for the electric equivalent circuit.…”

“…This means that the membrane parameters such as R ax (axoplasmic resistance), R pa (periaxonal resistance) and R pn (paranodal seal resistance) are not incorporated. These new membrane parameters and their values involved in the double cable model have been thoroughly discussed previously [18][19][20]. The model assumes a high-resistance myelin sheath and a leakage pathway to the internodal axolemma via the paranodal seal resistance and periaxonal space.…”

Differences in Membrane Properties in Simulated Cases of Demyelinating Neuropathies: Internodal Focal Demyelinations without Conduction Block

“…The electrogenesis of myelinated human motor axons can be studied successfully using the double cable model of these fibres [14][15][16]. The interested reader is referred to the first figure of [14] for an electric equivalent circuit representation of the human motor fibre.…”