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. c^node= capacitance of the nodal membrane; δ^axon= inner diameter of the nerve fiber; δ^fiber= outer diameter of the nerve fiber; FH = Frankenhaeuser-Huxley model of the nodal ionic currents; L^node= internode spacing; ρ^e= extraneural resistivity; ρⁱ= axoplasmic resistivity; V^e= extracellular potential at the node; Vⁱ= intracellular potential at the node; x^insert= x coordinate of the needle insertion point; x^node= length of the nodes of Ranvier; y^insert= y coordinate of the needle insertion point.