Electrodes

1. What is Ohm's Law?
2. Where is the active electrode placed?
3. Where is the reference electrode placed?
4. What is the disadvantage of using a needle electrode for NCS?
5. What are the advantages and disadvantages of using a monopolar needle electrode?
6. What are the advantages and disadvantages of using a standard concentric (coaxial) needle electrode?
7. What is the ground electrode and where is it placed?

Answers:
1. Intensity of current (I) = Electromotor source (E)/Resistance (R)
2. The active is placed over the endplate region of the muscle to measure CMAP or over the nerve to measure SNAP.
3. The reference is placed over the tendon for CMAP and over the nerve for SNAP.
4. The waveform's amplitude and CV can't be assessed because only a few fibers are being sampled.
5. A monopolar needle is a 22-30 gauge Teflon coated needle with an exposed tip of 0.15-0.2 mm. The advantages are that it's inexpensive, has omnidirectional recording, less painful due to reduced friction from Teflon, has a larger recording area, and records more PSWs. The disadvantages are that it requires a separate surface reference, has an unstandardized tip area, there's more interference, and the Teflon may fray.
6. A concentric needle is a 24-26 gauge needle with a bare inner wire. The advantage is that it has a standardized exposed area, a fixed location from the reference, less interference, and no separate reference. The disadvantages are that it has a beveled tip which allows only unidirectional recording, a smaller recording area, MUAPs have smaller amplitudes, and it is more painful.
7. The ground is the zero-voltage, neutral, surface reference point. It is placed between the recording electrode and the stimulating electrode.