Fig. 1. Model geometry. The three-dimensional model space with typical rectangular coordinates. Positions and vectors are represented by ( x, y, z ) sets. The outer skin surface is the x – y plane ( z = 0). A needle penetrates the skin at the marked insertion point and extends toward the nerve plane. The bottom sketch shows the structure and electrical properties of the myelinated nerve fiber. cnode= capacitance of the nodal membrane; δaxon= inner diameter of the nerve fiber; δfiber= outer diameter of the nerve fiber; FH = Frankenhaeuser-Huxley model of the nodal ionic currents; Lnode= internode spacing; ρe= extraneural resistivity; ρi= axoplasmic resistivity; Ve= extracellular potential at the node; Vi= intracellular potential at the node; xinsert= x coordinate of the needle insertion point; xnode= length of the nodes of Ranvier; yinsert= y coordinate of the needle insertion point.