(A1) Sample traces of simulated activity by oscillator heart interneurons functioning as a half center oscillator in normal saline, which were observed when parameters = 4.9 nS and = 0 s−1. (A2) Simulated activity of oscillator heart interneurons with a Na+/K+ pump stimulated by monensin (monensin saline), observed when = 4.9 nS and = 2.2 × 10−3 s−1. (A3) Simulated activity of a half-center oscillator with blocked h-current and pump stimulated by monensin (monensin plus Cs+ saline), observed when = 0.1 nS and = 1.9 × 10−4 s−1. Sample traces representing membrane potentials (Vm) of both left (L, blue) and right (R, vermilion) oscillator heart interneurons as well as h-current (Ih, yellow), pump current (Ipump, reddish purple), and intracellular Na+ concentration [Na]i (black) belonging to the right oscillator heart interneuron. (B1) A scatterplot depicting incremental shortening of the period as the monensin rate constant increases towards 2.2 × 10−3 s−1 in a model of a half-center oscillator with the h-current present. Spiking activity was suppressed at rate constant values larger than 2.2 × 10−3 s−1. (B2) In a simulation of a half-center oscillator, the average pump current over the entire burst cycle was fixed at 155.5 pA, which resulted in a longer period, burst duration, and interburst interval. Intracellular Na+ concentration [Na]i appeared to increase and decrease more slowly relative to a (A1) normal half-center oscillator with a dynamic pump current. (B3) Fixing the average pump current over the entire burst cycle to 144.4 pA (a value lower than 155.5 pA) produced irregular bouts of bursting.