The generation of antiphase oscillations and synchrony by a rebound-based vertebrate central pattern generator

Many neural circuits are capable of generating multiple stereotyped outputs after different sensory inputs or neuromodulation.We have previously identified the central pattern generator (CPG) forXenopustadpole swimming that involves antiphase oscillations of activitybetween the left and right sides. Here we analyze the cellular basis for spontaneous left–right motor synchrony characterized by simul-taneous bursting on both sides at twice the swimming frequency. Spontaneous synchrony bouts are rare in most tadpoles, and theyinstantly emerge from and switch back to swimming, most frequently within the first second after skin stimulation. Analyses show thatonly neurons that are active during swimming fire action potentials in synchrony, suggesting both output patterns derive from the sameneural circuit. The firing of excitatory descending interneurons (dINs) leads that of other types of neurons in synchrony as it does inswimming. During synchrony, the time window between phasic excitation and inhibition is 7.91 ms, shorter than that in swimming (412.3 ms). The occasional, extra midcycle firing of dINs during swimming may initiate synchrony, and mismatches of timing in theleft and right activity can switch synchrony back to swimming. Computer modeling supports these findings by showing that the sameneural network, in which reciprocal inhibition mediates rebound firing, can generate both swimming and synchrony without circuitreconfiguration. Modeling also shows that lengthening the time window between phasic excitation and inhibition by increasing dINsynaptic/conduction delay can improve the stability of synchrony.