system rests in a stable steady state, but after it is excited beyond a threshold, it emits a pulse. In the following, we consider electrically coupled neurons. Such electrical synapses are less common than chemical synapses, but it has been shown in hippocampal slices that high-frequency synchronized oscillations are independent of chemical synaptic transmission [@JEF82], and physiological, pharmacological, and structural evidence was provided [@SCH01e] that axons of hippocampal pyramidal cells are electrically coupled (Fig. \[fig:scheme\] (a)). Time delays in the coupling must be considered particularly in the case of high-frequency oscillations.

The simplest model that displays features of neural interaction consists of two coupled neural systems. A single electrical synapse can lead to synchronized co-operative behaviour between two axo-axonally coupled hippocampal pyramidal cells when only one of them is stimulated antidromically at high frequency [@TRA94; @TRA99]. Moreover, such neurons have been shown to reflect antidromic spikes (propagating opposite to the normal direction) at the soma, but a continuous reverberating activity is not generated by axonal reflections in two axo-axonally coupled cells alone [@SCH01e]. In order to describe the complicated interaction between billions of neurons in large neural networks, the neurons are often lumped into highly connected sub-networks or synchronized sub-ensembles. 