请问在一篇paper的内容提到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. 我主要想问的是为什麽要有active electrode, reference electrode, ground electrode?他们的作用是什麽?而且为什麽内容中The phrenic nerve was stimul\ ated at a supramaximal voltage by pressing deeply along the posterior margin of the sternocleidomastoid muscle at the level of the cricoid cartilage 是什麽意思?? 请各位大大帮我解答~~感激不尽!! --

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