(A) The model switches to high-frequency oscillations at the onset of NMDA receptor stimulation at 500 msec. Dashed is the Ca2+ concentration; Solid is the voltage. The increase in the frequency is based on the reduction in the amplitude of Ca2+ oscillations. (B) The structure of the model. The SK-type Ca2+-dependent potassium current and the putative voltage-dependent potassium current create two negative feedback loops. The loops are interlocked by the voltage variable.