Contribution of reference electrode to the compound muscle action potential

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Muscle & Nerve (Impact Factor: 2.28). 07/2007; 36(1):87-92. DOI: 10.1002/mus.20798
Source: PubMed


In compound muscle action potential (CMAP) recording, the contribution by the reference electrode is considered to be much smaller than that of the active electrode. We tested this assumption by making quantitative measurements of the signals recorded individually by the active and reference electrodes. In the thenar (median nerve) and extensor digitorum brevis (peroneal nerve) muscles, the reference electrode did contribute less. In the hypothenar muscle (ulnar nerve), however, the signals recorded by active and reference electrodes were of similar amplitude. In tibial nerve conduction studies (NCS), the CMAP from the abductor hallucis (AH) muscle was recorded mainly by the reference electrode; the large-amplitude signal recorded by the reference electrode is attributed to volume-conducted activity from other muscles stimulated during the study. The onset latency of the potential recorded by the active and reference electrodes was similar despite significantly different distances from the stimulating site. Hence, the merits of using anatomic landmarks for defining the distal stimulation site are assessed. When the reference electrode makes a large contribution, the CMAP amplitude may not decrease commensurate with any wasting of the muscle under the active recording electrode, and the need to use another muscle for recording the CMAP for that nerve should be considered.

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    • "However, it has been shown that E2 can be electrically active.16,17 A recent study18 measured the signals recorded individually by E1 and E2 in order to assess the contribution of E2 to the signal. Significant contributions from E2 were observed, especially in the hypothenar and abductor hallucis muscles. "
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    • "The CMAP may be generated by many muscles (e.g. AH, see above) with significant contribution from the reference recording electrode (Nandedkar and Barkhaus, 2007). The SIP only comes from motor units within a depth of less than 20 mm directly under the recording surface electrode (Barkhaus and Nandedkar, 1994) In large muscles like the biceps the uptake area of the surface electrode is smaller than the size of muscle. "
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