1. Intracellular recordings were made from 50 dorsal respiratory group (DRG) neurons in the region of the ventrolateral nucleus of the solitary tract in anesthetized, paralyzed cats ventilated with a cycle-triggered pump whose inflation stroke was triggered by the onset of phrenic nerve inspiratory (I) discharge. Activity was recorded simultaneously in the ipsilateral nodose ganglion from sensory cell bodies of slowly adapting pulmonary stretch receptors (PSRs). 2. Respiratory cycle-related membrane potential changes of DRG neurons were recorded. Twenty-six neurons that did not exhibit spikes were classified as I alpha, I beta or pump (P)-cells by comparing their membrane potential trajectories during I in the presence of lung inflation with that observed during I, but with lung inflation withheld. The remaining 24 neurons were classified similarly, but the classification was based upon a comparison of their I-phase spike activity responses with and without lung inflation. I phase-related histograms of either membrane potential or spike activity were constructed to facilitate DRG neuronal classification. Additionally, steady lung inflation of varying magnitudes was applied during the expiratory phase. This prolonged expiration and produced different responses in the neurons. Generally, I beta and P-cells were depolarized, whereas I alpha cells were hyperpolarized. 3. Low-intensity electrical stimulation of the ipsilateral vagus nerve evoked excitatory postsynaptic potentials (EPSPs) in all three DRG neuronal types. P-cells and I beta cells exhibited EPSPs in response to the lowest intensity; generally this intensity was below threshold for the simultaneously recorded PSR. Overall, EPSPs in I alpha cells had the highest thresholds, but some EPSPs could be evoked at thresholds similar to those of the I beta cells. The distributions of the average onset latency of the evoked EPSP overlapped considerably. Thus vagal electrical stimulation cannot be used for unequivocal classification of DRG neurons into I alpha, I beta, and P-cell subpopulations. 4. Using intracellular spike-triggered averaging, single PSRs were shown to generate monosynaptic EPSPs in I beta neurons and P-cells but not I alpha cells. Divergence of single PSR afferents also was observed. Relationships between EPSP shape factors, amplitudes, and PSR afferent conduction velocity are similar to those previously observed for monosynaptic EPSPs in hindlimb motoneurons generated by spinal afferents.
