We will explore the effects that tissue conductivities have on the populations of nerve fibers activated. Two populations are considered for this discussion, a 10 µm myelinated, mid-range, axon, represented by black text, and a 20 µm, large diameter axon, represented by red text. The electrode configuration is a monopolar cuff, shown (here) in the upper left. We’ll start with an isotropic nerve, meaning the conductivity in the longitudinal direction and the transverse directions are the same. Add a stimulating electrode providing a cathodic pulse, embedded in an insulating cuff, with a thin layer of saline between the nerve and cuff. From the center of the nerve we measure 1.25 mm and define a point we call Point “A”. This will be a reference point in the nerve for all simulations and it will represent the location of a 10 µm nerve fiber at a stimulus threshold for creating a propagating action potential, which, in this configuration required a cathodic stimulus of -0.104V. A voltage source was used, instead of a current source, to achieve a nonuniform current density from the metal electrode. At this same stimulus level all 20µm axons are activated in this region. When the stimulus current density was doubled, more 10µm axons in the nerve were activated and all 20 µm axons in the nerve were activated.
I’m going to repeat the same exploration process taking into account the conductivity of the nerve trunk, which is greater in the longitudinal direction than in the transverse direction. First, the region of the nerve containing 10µm fibers activated, then the 20µm population activated at the same stimulus magnitude. Doubling the Jth value a larger number of 10µm axons are activated and a larger number of 20µm fibers are recruited. Note, that taking into account the anisotropic conductivity produces limit lines, solid and dashed, that switch from concave upward to concave downward, compared to the previous case.
Next I will add the perineurial membrane. When a stimulus of -1.136 volts is applied, 10 µm axons in the shaded area are activated. Note the increased downward concavity of the limit line. At this same stimulus amplitude, all 20 µm axons in the fascicule are activated. Doubling the current density extends the region where all 10 µm axons are activated; take note to the downward concavity of the activation limit.
RR Chintalacharuvu, DA Ksienski, JT Mortimer, Annual Int Conf of the IEEE Engr in Med and Biol Vol 13, No 2 1991, pp 0912-13
Rekha R. Chintalacharuvu, A Numerical Analysis of the Electric Field Generated by the Nerve Cuff Electrode, MS thesis, 1991 Case Western Reserve University