The toggle switch responsible for the change from slow to fast desensitization observed in P2X2aR and P2X2bR during repetitive ATP stimulation. The nullclines (A) and the steady states (bifurcation diagram) (B) of subsystem 13–14, obtained by making [Ca²⁺]_i a parameter, are shown. The X nullcline (gray line) in A intersects the L₂ nullcline initially at one stable steady state when [Ca²⁺]_i (=0.1 µM) is low (solid line), and then intersects it at three steady states, two of which are stable, when [Ca²⁺]_i (=1 µM) is intermediate (dashed line), and finally at one stable steady state when [Ca²⁺]_i (=32.6 µM) is high (dotted line). The changes in the number of steady states are summarized by the S-shaped curve in B, which shows the hysteresis. Stable steady states are shown as solid lines, and unstable steady states are shown as dashed lines; the default values in Table 1 are used. The arrows in B indicate the values of [Ca²⁺]_i used to plot the L₂ nullclines in A.