Monosynaptic entrainment of an endogenous pacemaker network: A cellular mechanism for von Holst's magnet effect

Summary1.The pyloric rhythm of the lobster (Panulirus interruptus, Palinurus vulgaris) stomatogastric ganglion is generated by a set of 3 electrically coupled endogenously bursting neurons. The phasic coordinating effects of monosynaptic excitatory (EPSP) and inhibitory (IPSP) inputs to these neurons were examined in isolated nervous systems.2.Periodic stimulation of both inputs at frequencies near the endogenous frequency of the pacemakers can produce large cycle by cycle variations in the burst period of the system (Fig. 4). Similar variation in PD burst period is also observed in intact freely behaving animals (Fig. 2).3.The effect of both inputs depends on the phase at which they occur in the endogenous pacemaker cycle. Both EPSP's and IPSP's can advance or delay subsequent bursts. The two inputs exhibit qualitatively different characteristic phase response curves (Figs. 5, 6).4.Both EPSP's and IPSP's can entrain the endogenous rhythm (Figs. 7, 8). The coordinating effects of EPSP's are stronger when their repetition frequency is slightly higher than the endogenous pacemaker frequency, while those produced by IPSP's are stronger when the repetition frequency of the input is slightly slower than the repetition frequency of the oscillator.5.The phase relationships of the discharge of the pacemakers in the stimulus cycle are qualitatively different when the cyclic stimulus is slower or faster than the endogenous rhythm for both types of inputs (Fig. 9). The entrainment of the oscillator by EPSP inputs which repeat at a frequency which is greater than the endogenous frequency can be attributed to the input triggering the burst of spikes at a fixed latency (Fig. 10).