Fig. 1. Experimental set-up and protocol for comparison of PMN firing behavior during inspiratory and expiratory phases.A, Schematic of the rhythmically active brainstem–spinal cord preparation showing the configuration of the suction electrode for recording first cervical nerve (C1) activity, the whole-cell recording electrode at the level of the fourth cervical nerve for recording PMN activity, and the triple-barrel drug ejection pipette. B, Rectified, integrated recording of C1 output (∫C1, top trace) showing population inspiratory activity and whole-cell current-clamp record of a PMN (VM, middle trace) (i) under control conditions with inspiratory synaptic potential present, (ii) after complete blockade of the PMN excitatory inspiratory input with NBQX and MK801, and (iii) during the repetitive firing protocol in the continued presence of NBQX and MK801. After block of excitatory inspiratory drive to the PMN (Bii), population inspiratory activity on ∫C1 was used to trigger injection of square-wave current pulses during inspiration (Biii). Responses of PMNs to current pulses injected during inspiratory and expiratory periods were then compared. C, Expanded versions of voltage traces (VM) inB, showing the firing responses of the PMN to (i) endogenous inspiratory synaptic input, (ii) a 380 pA pulse delivered during inspiration, and (iii) the same amplitude current pulse delivered during expiration.