Extraction of the EPP Component from the Surface EMG

Summary

The endplate potential (EPP) component can be extracted from the surface EMG using a digital filter. The extracted EPP shows oscillation with a frequency of about 30 Hz.

Abstract

A surface electromyogram (EMG), especially when recorded near the neuromuscular junction, is expected to contain the endplate potential (EPP) component which can be extracted with an appropriate signal filter. Two factors are important: the EMG must be recorded in monopolar fashion, and the recording must be done so the low frequency signal corresponding the EPP is not eliminated. This report explains how to extract the EPP component from the EMG of the masseter muscle in a human subject. The surface EMG is recorded from eight sites using traditional disc electrodes aligned along over the muscle, with equal inter-electrode distance from the zygomatic arch to the angle of mandible in response to quick gum clenching. A reference electrode is placed on the tip of the nose. The EPP component is extracted from the raw EMGs by applying a high-cut digital filter (2nd dimension Butterworth filter) with a range of 10-35 Hz. When the filter is set to 10 Hz, the extracted EPP wave deflects either negative or positive depending on the recording site. The difference in the polarity reflects the sink-source relation of the end plate current, with the site showing the most negative deflection corresponding to the neuromuscular junction. In the case of the masseter muscle, the neuromuscular junction is estimated to be located in the inferior portion close to the angle of mandible. The EPP component exhibits an interesting oscillation when the cut-off frequency of the high-cut digital filter is set to 30 Hz. The EPP oscillation indicates that muscle contraction is adjusted in an intermittent manner. Abnormal tremors accompanying various sorts of diseases may be substantially due to this EPP oscillation, which becomes slower and is difficult to cease.

Protocol

1. Preparation of EMG electrodes

1. Begin by preparing nine electrodes. Eight of them are used for recording signals from sites over the muscle, and one is a reference electrode.
2. After connecting the recording and reference electrodes to the amplifier, fill the discs with electric-conductance paste. Every type is OK, but a low fluid type is better.
3. Before attaching the electrodes to the skin, have the subject down firmly, and determine the approximate location of the masseter muscle on the face.
4. The superior tendon of the masseter muscle attaches to the zygomatic arch, and its inferior tendon to the angle of ....

Discussion

1. As the negative potential is considered to be formed by a flow-in of the endplate current, and the positive deflection, by its flow-out [1,2,3,4], the trace showing the most negative deflection must correspond to the site locating the neuromuscular junction [5]. Figure 1b indicates the neuromuscular junction of the masseter muscle locate in its inferior portion close to the angle of mandible, which is approximately the same as results obtained by another method utilizing the conduction manner of the motor unit action potential [6,7].

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Analogue filter	Nihon Kohden	MEG6100	pre-,main-amplifier
A/D converter	National Instruments	PCI-MIO-16E-4	PCI board for computer
Connection interface	National Instruments	BNC-2090	8ch BNC adaptor
Disc electrodes	Nihon Kohden	NS-11	Ag/AgCl (Φ8mm)
Electrode past	Sanshin	SA-5	Semi-fluid carbon past
LabVIEW (Digital filter)	National Instruments	V. 8.5	Programming language
Chewing gum	Lotte	Green gum	Test food in clenching