After introducing the notion of ”slow current” notion, no relative and absolute refractoriness exists, only refractoriness. The period, called ”relative refractoriness” in the time-independent discussion, is an illusion. The slow currents, received through the synapses, need time to reach AIS, so they appear dozens of microseconds later at the AIS. In that period, the output voltage on AIS is already below the resting potential, which is the extension of the absolute refractory period. Given that the AP is already in its hyperpolarized state in that period, its exciting contribution is much harder to observe than at the beginning of an AP, starting from the resting potential. However, the physical background is the same.
The causality is reversed: not ”the minimal distance between two spikes defines the absolute refractory period of the neuron” [1]. Instead, as we discuss, until the membrane’s potential is above the threshold (which period is defined by physiological parameters), the synaptic inputs are closed, so if another spike arrives until the synapses are re-opened, it is neglected.