請問phrenic nerve conduction study - 醫療資訊,議題討論

請問在一篇paper的內容提到phrenic nerve conduction study中
一有提到如下面一段:
The active and reference surface Ag/AgCl disc electrodes
were used to record the phrenic nerve compound action
potential (CMAP) using the modified MACLEAN and MATTIONI
[12] technique. The active electrode was placed on the xiphoid
process, the reference electrode was placed on the ipsilateral
eighth intercostal space at the costocondral junction and the
ground electrode was placed on the sternum (Medelec, Oxford,
UK). The phrenic nerve was stimulated at a supramaximal
voltage by pressing deeply along the posterior margin of the
sternocleidomastoid muscle at the level of the cricoid cartilage.
The supramaximal stimulus consisted of a single square pulse
(intensity 10 V; duration 0.5 ms). At regular intervals it was
ensured that stimulation voltage remained supramaximal. The
recorded signals were then rectified and electrically integrated.
The sampling rate was chosen as 20,000 samples?s-1. The
amplitude from the baseline to the negative peak, the duration
and the latency of CMAP were measured.

我主要想問的是為什麼要有active electrode, reference electrode,
ground electrode?他們的作用是什麼?而且為什麼內容中The phrenic nerve was stimulated at a supramaximal
voltage by pressing deeply along the posterior margin of the
sternocleidomastoid muscle at the level of the cricoid cartilage
是什麼意思??

請各位大大幫我解答~~感激不盡!!

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