Figure 6.

Kinetics of g_NaP affect traversal of the saddle point. (A) When the kinetics of g_NaP are relatively fast, z increases rapidly during each spike but so too will it decrease rapidly after each spike. As the saddle point creeps rightward as E_Na decreases, large post-spike dips in z increase the likelihood of the V–z trajectory crossing back over the saddle point. Compare with B. (B) When the kinetics of g_NaP activation are slowed down, z changes more slowly. Consequently, initiation of afterdischarge required three spikes evoked in quick succession (5 ms intervals) to drive z across the saddle point. However, once initiated, afterdischarge did not terminate (despite an even larger reduction in E_Na than in A), consistent with the smaller dips in z. For both A and B, g_Na = 30 mS cm⁻² and g_NaP = 0.8 mS cm⁻². Note that peak g_NaP activation was equivalent in both cases (dotted orange line). Bifurcation diagrams on the right show that [Na⁺]_i can rise higher without terminating afterdischarge for conditions in B. Note the spacing between the maximum and minimum of the limit cycle, as shown on these bifurcation diagrams.