Fig. 4. Schematics of electrical impedance in the extraneural and intraneural compartments. Upper left and right : needle placements. Lower left and right :simplified equivalent circuit diagrams. The extraneural tissue, with low electrical impedance, provides a path through which most of the stimulating current will conduct. As the needle tip punctures the nerve, the low-resistance path is no longer available and a substantial increase in the electrical impedance of the circuit occurs.