Synaptic control of an endogenous pacemaker network.

1. The present study consists of an analysis of the coordinating effects of monosynaptic EPSPs and IPSPs on the discharge of the endogenous pacemaker neurons which drive the pyloric motor system of the spiny lobster. The experiments were performed on isolated nervous systems. 2. An analysis of the characteristic phase response curves to both classes of input (fig. 1) shows that the pyloric oscillator possesses the necessary characteristic for entrainment: i.e. a periodically varying sensitivity to synaptic drive. 3. By repetitive stimulation of either input at frequencies near the endogenous frequency of the PD slow wave, it was possible to entrain the discharge of the pacemaker system to the cyclic stimulus (figs. 2b and 3b). The pyloric discharge tends to occur at different characteristic phase relations in response to the two inputs (figs. 2c and 3c), which reflect features of the corresponding phase response curves (fig. 1). 4. It is argued that the periodic sensitivity of these neurons to synaptic input reflects interactions between the synaptically induced currents and the endogenous currents which underlie the slow wave.