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:Finally, the third group (C-type neurons) has the longest duration action potentials, and their voltage-gated Na currents are poorly blocked by TTX (Stansfeld and Wallis 1985; Bossu and Feltz 1984; Schild and Kunze 1997).
Category: