One lead of the low-output nerve stimulator is attached to a needle and the other lead is grounded somewhere on the patient. Lower current is required when a negative lead is attached to the needle. The reason for this phenomenon is that when the stimulating electrode is negative, the current flow alters the resting membrane potential adjacent to the needle, producing an area of depolarization which then spreads across the nerve. When the electrode adjacent to the nerve is an anode(positive), the current causes a hyperpolarization adjacent to the needle and a ring of depolarization distal to the needle tip. This arrangement is less efficient in propagating the stimulus. The needles used are insulated and only allow current flow at the tip so nerve localization is precise. Muscle contraction occurs and is of increased intensity as the needle approaches the nerve and diminishes as the needle moves away from the nerve. Optimal needle positioning produces evoked contrations with 0.5 mA or less. The evoked contraction rapidly fades after injection of 1-2 cc of local anesthetic.
Ability to electrically stimulate a peripheral nerve depends upon many variables: 1) conductive area at the electrode, 2) electrical impedance, 3) electrode-to-nerve distance, 4) current flow (amperage), and 5) pulse duration. Electrode conductive area follows the equation R=pL/A, where R=electrical resistance, p=tissue resistivity, L=electrode-to-nerve distance, and A=electrode conductive area. Therefore resistance varies to the inverse of the electrode’s conductive area. Tissue electrical impedance varies as a function of the tissue composition. In general, tissues with higher lipid content have higher impedances. Modern electrical nerve stimulators are designed to keep current constant, in spite of varying impedance. The electrode-to-nerve distance has the most influence on the ability to elicit a motor response to electrical stimulation. This is governed by Coulomb’s law: E=K(Q/r2) where E=required stimulating charge, K=constant, Q=minimal required stimulating current, and r-electrode-to-nerve distance. Therefore, ability to stimulate the nerve at low amperage (e.g. <0.5 mA), indicates an extremely close position to the nerve. Similarly, increasing current flow (amperage) increases the ability to stimulate the nerve at a distance. Increasing pulse duration increases the flow of electrons during a current pulse at any given amperage. Therefore, reducing pulse duration to very short times (e.g. 0.1 or 0.05 ms) diminishes current dispersion, requiring the needle tip to be extremely close to the nerve to elicit a motor response.