Reciprocal excitatory synapses convert pacemaker-like firing into burst firing in a simple model of coupled neurons

Model neurons that are intrinsically capable of only repetitive single-spike firing can fire in bursts when reciprocally coupled via excitatory chemical synapses. The bursting mechanism is analyzed using a phase portrait consisting of nullclines both in the presence and in the absence of a fully activated synaptic current. The system of coupled relaxation oscillators is symmetric, and the trajectory can be described by simultaneous jumps between the free and excited potential nullclines for each oscillator. A new variant of the synaptic release mechanism for both excitation and inhibition emerges in which spike frequency adaptation terminates the synaptic coupling. (C) 2000 Elsevier Science B.V. All rights reserved.