Instrumentation and Technical Factors

Section 1: Ohm’s Law, Electrodes, and Stimulating the Nerve

~2:17 – Ohm’s Law, Electrodes, and Stimulating the Nerve

Bottom line: every EDX study is governed by Ohm’s law (V = I × R), with impedance replacing resistance for AC biological signals; surface silver-silver chloride disc electrodes with target impedance under 5-10 kOhms are the workhorse, monopolar needles record larger and longer motor unit action potentials (MUAPs) than concentric needles studying the same fibers (different normal values required), single-fiber EMG (SFEMG) uses a 25-micrometer recording surface with a 500 Hz high-pass filter, and the cathode is the negative pole that depolarizes the nerve and is placed closer to the recording electrodes; anodal block is the trap when this is reversed.

Every electrodiagnostic study is governed by Ohm’s law: V = I × R. In the laboratory, voltage represents the bioelectric signals being recorded (millivolts for CMAPs and microvolts for SNAPs and MUAPs); current represents the charge delivered by the stimulator; and resistance, or more precisely impedance (Z) for AC biological signals, opposes current flow. Tissue impedance, electrode impedance, and amplifier input impedance all influence signal quality. For AC signals, impedance includes resistive and reactive components: Z = sqrt(R² + X²), where the reactance X arises from capacitive and inductive elements. Capacitance is relevant throughout the laboratory: the skin-electrode interface acts as a capacitor, stray capacitance between nearby cables introduces noise, and capacitive coupling between the stimulator and recording electrodes can prolong stimulus artifact. Power dissipation, P = V × I, underlies electrical safety, though modern EMG machines operate at power levels inherently safe for routine use.

Surface electrodes are placed on the skin for recording CMAPs and SNAPs during nerve conduction studies. The standard is the silver-silver chloride (Ag/AgCl) disc, which provides a stable potential and low noise. Ring electrodes (metal bands around a finger) are convenient for digital sensory studies. Bar electrodes (two metal contacts at a fixed interelectrode distance) provide reproducible spacing. Adhesive pre-gelled disposable electrodes are convenient for single use but may have higher impedance if the gel layer is insufficient. Skin preparation is essential: clean with alcohol or abrasive prep to reduce impedance. The target impedance is below 5-10 kOhms at each electrode. Dead skin cells, oils, and lotions all increase impedance. E1 placement: for motor studies, E1 is placed directly over the motor point (the muscle surface closest to the terminal zone of the motor nerve), producing the largest CMAP with an initial negative deflection. For sensory studies, E1 is placed over the nerve trunk. E2 placement: the reference electrode is placed over an electrically inactive area, typically 3-4 cm distal to E1 over the tendon for motor studies, or at a defined fixed distance for sensory studies. Impedance matching between E1 and E2 must be as close as possible; impedance mismatch is the single most common cause of degraded common mode rejection (Section 3).

Two needle types dominate EMG. Monopolar needles are solid, Teflon-coated stainless steel needles with only the bare tip exposed as the recording surface. A separate surface electrode serves as the reference. The recording area is typically 0.03-0.07 mm², and the effective recording radius extends approximately 1-2 mm. Because of the larger recording radius and distant reference, monopolar needles record MUAPs that appear slightly larger in amplitude and longer in duration than the same motor unit recorded with a concentric needle. They are generally less painful but produce slightly more electrical noise and require their own normal values. Concentric (coaxial) needles consist of a fine wire insulated from and running through the center of a hollow cannula. The central wire is the active electrode (E1), and the cannula serves as the reference (E2). The recording radius is smaller, approximately 0.5-1 mm, making recording more selective. MUAPs are smaller in amplitude and shorter in duration, with less noise but slightly more pain. The board consequence: monopolar needles produce MUAPs roughly twice the amplitude of concentric for the same motor unit, so reference values must match the needle type. Mixing needle types without adjusting reference values produces interpretive errors. The single-fiber EMG (SFEMG) electrode is a specialized concentric needle with a 25-micrometer recording surface exposed through a side port ~3 mm from the tip. It records from individual muscle fibers within a radius of ~300 micrometers, measures jitter and fiber density, and is the most sensitive test for NMJ disorders (sensitivity >95% for myasthenia gravis). SFEMG requires a 500 Hz high-pass filter to isolate single-fiber potentials.

Surface stimulating electrodes consist of a cathode (negative pole) and an anode (positive pole) separated by 2-3 cm. The cathode is placed closer to the recording electrodes. Under the cathode, current flows outward across the nerve membrane (depolarization), and this is where nerve activation occurs. Under the anode, current flows inward, hyperpolarizing the membrane. Anodal block is the trap: if the anode is too close to the nerve and the stimulus is high, hyperpolarization under the anode blocks propagation of action potentials generated under the cathode, producing a falsely reduced response. The fix is to orient the cathode toward the recording electrodes with the anode placed proximally. Constant-current stimulators are the clinical standard. They deliver a specified current regardless of tissue impedance variations. Constant-voltage stimulators deliver a fixed voltage; because tissue impedance varies (skin thickness, subcutaneous fat, hydration, temperature), the actual current reaching the nerve changes from site to site, making stimulation poorly reproducible. Total charge delivered equals current × duration (Q = I × t). Standard stimulus duration for NCS is 0.1-0.2 ms. For obesity, edema, or deep nerves, duration can be increased to 0.5-1.0 ms before pushing current to painful levels.

For motor NCS, the goal is to activate every axon in the nerve to obtain a maximal CMAP. The protocol: gradually increase stimulus intensity until CMAP amplitude no longer increases (maximal stimulation), then add an additional 20-25% above that level (supramaximal stimulation). Submaximal stimulation is the single most common technical error in motor NCS; it produces a falsely low CMAP amplitude that can be misread as axonal loss. Warning signs include incremental amplitude increases as stimulus intensity is raised, inconsistent waveform morphology between trials, and less-than-expected patient discomfort in an obese limb.