The left upper quadrant bursting neurons in the abdominal ganglion of Aplysia are isochronous, nonlinear oscillators. Transmembrane current and temperature are parameters of the bursting oscillator. 2. The phase-response curve (PRC) for pulsed inhibitory synaptic input from an interneuron describes the phase shift produced by synaptic input at different phases of the burst cycle. 3. The characteristic shape of the PRC consists of two linear functions that intersect at the point in the cycle where the burst of spikes ends. Whether the net effect of the synaptic input at a given phase is phase advance or phase delay depends on 1) the number of spikes inhibited, and 2) the duration of the inhibition relative to the duration of the free-run period. 4. The shape of the PRC remains constant when a stepwise change in a parameter is introduced, when the duration of the synaptic input is increased, when the fast component of the IPSP is blocked, and when a long hyperpolarizing pulse is used to mimic the slow IPSP. 5. The shape of the PRC is changed when short hyperpolarizing pulses or antidromic action potentials are used and when only the pacemaker oscillation is present in the bursting neuron. 6. Therefore, the synaptic modulation of the bursting rhythm is determined by the voltage change produced by the IPSP and its inhibition of spikes in the bursting neuron.
