Verification of the effect of the axon fluid as a highly dielectric medium in the high-speed conduction of action potentials using a novel axon equivalent circuit.

Both sensory neurons and motor neurons transfer signals rapidly through long pathways. Such signals propagate as action potentials through neurons. In myelinated neurons, high conduction velocities of 120 m/s have been reported, even for axons of just 20 μm in diameter. Such a high conduction velocity is enabled by the characteristic morphology of a myelinated axon: repeated regions encased by long uniform myelin sheaths alternating with extremely short exposed regions of the axon called nodes of Ranvier, which generate extremely sharp action potentials. Although the need for the action potential to cross many nodes increases the relay time, it is still able to propagate rapidly. This phenomenon motivated us to derive a new mechanism of the action potential propagation. First, the dielectric effect of the axonal fluid was considered, and it was investigated whether the combination of the characteristic axonal morphology and the dielectric constant of the axonal fluid contributes significantly to the realization of high conduction velocities even with the inclusion of a large loss in the relay time. To this end, we propose a new axon equivalent circuit that incorporates the effect of the dielectric characteristics of the axonal fluid. It was confirmed that a realistically high conduction velocity could be calculated using the proposed circuit and that the dielectric constant calculated using the proposed circuit was in agreement with that of an ionic fluid similar to axonal fluid. Moreover, the contribution of the combination of the axonal morphology and axonal fluid to the conduction velocity was confirmed.