Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:When MgTx was applied to the rare cells subsequently shown to express only TTX-sensitive Na current (A-type neurons) (Stansfeld and Wallis 1985; Schild and Kunze 1997; Li and Schild 2007b), there was no change in the duration of the action potential measured at 0 mV (0.74 0.09 vs. 0.76 0.1 ms, n 3,…
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