Analysis of the longitudinal and radial resistivity measurements of the nerve trunk.

Of the many models of peripheral nerve in the literature, essentially all have relied on experimentally derived values for the anisotropic resistivity of the nerve bundle interstitial space. This paper is a comprehensive mathematical analysis of the most referenced experiment for the longitudinal and radial resistivity measurements of the nerve trunk, namely that of Tasaki. For the analysis of the longitudinal measurements we introduce intracellular as well as interstitial current pathways (not considered by Tasaki). For the radial measurement the resistivity is expected to depend on the geometric packing of fibers within the trunk. Since Tasaki's paper did not include a histological examination of the nerve trunk these measurements are difficult to evaluate. However, our analysis indicates the importance of including the above factors, in addition to the epineurial (and perineurial) sheath as a resistive pathway. Our mathematical analysis supports the experimental measurements and confirms an assumed nerve trunk composition with theoretically derived values for the interstitial resistivities. It is therefore concluded that, at present, an appropriate procedure in determining resistivity values for use in modeling is to derive these values for an idealized nerve bundle based on the microscopic (electrolytic) resistivity of the interstitial medium.