Phase space representation for the QIF neuron. The relationship of membrane current, proportional to the derivative of , is plotted against membrane potential, . Due to the quadratic relationship, there are two fixed points: one stable (the resting potential) and one unstable one, the threshold, at the border between the subthreshold (blue) and superthreshold (red) regions. The model generates the upstroke of the action potential when exceeds the threshold. To avoid instability, an ad hoc reset is provided when the arbitrary reset value is reached.
