Four-level sensitivity tests for model 2a revealed relative dependencies of the spike number response on each ionic conductance that were similar to those seen for models 1a and 1b, with the exception of I-KCa (figures 19a, 19b). Whereas varying the maximum conductance of I-KCa between values of 50% and 150% with respect to the model value in the extrinsically controlled models produced negligible effects on the spike number response, model 2a displayed tuning sensitivity of the spike response to this range of I-KCa variation (figure 19b-ii). This is consistent with the classification of I-CaHVA as a tuning conductance for models 1a and 1b, since I-CaHVA and I-KCa act together in a synergistic manner during the action potential upstroke and hyperpolarizing step. Furthermore, such a finding suggests that intrinsically controlled bursting in the granular cell is controlled by both the I-NaP / I-KM and I-CaHVA / I-KCa conductance ratios rather than the I-NaP / I-KM ratio alone. This could be tested directly in vitro by patch clamping of the granular cell soma and application of chemical antagonists specific for each type of ion channel.

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