Fig. 4. Descending brain interneuron Tr2 receives rhythmic inhibition in phase with the elongation phase of the fictive crawling motor rhythm. The CV neuron shows bursting during the elongation phase of crawling and the DE-3 motor neuron (DP nerve) bursts during the contraction phase. (A) At a resting membrane potential of –68 mV, inhibitory post-synaptic potentials (IPSPs) ranging from 3–5 mV in amplitude are apparent during the peak of CV burst activity during each crawl cycle. (B) When the membrane potential of Tr2 is hyperpolarized to –98 mV, by an injection of –0.2 nA of constant current, the IPSPs are evident as positive deflections of the membrane potential, ranging in amplitude from 2-4 mV. (C) Between crawl episodes, synchronous activity is observed between DE-3 (the largest unit in the DP nerve), circular contractor motor neuron CV, and Tr2, suggesting a shared synaptic input between these three neurons. During this recording, Tr2 is once again hyperpolarized to –98 mV with a constant current injection of –0.2 nA. (D) Portions of the CV and Tr2 traces shown in C (see rectangle in C) are expanded in time to resolve whether individual PSPs are 1:1. Although the summed potentials are clearly phase locked, unitary PSPs are not.