Morphological characterization of respiratory neurons in the pre-Bötzinger complex

Although the pre-Bötzinger complex (preBötC) was defined as the inspiratory rhythm generator long ago, the functional-anatomical characterization of its neuronal components is still being achieved. Recent advances have identified the expression of molecular markers in the preBötC neurons that, however, are not exclusive to specific respiratory neuron subtypes and have not always been related to specific cell morphologies. Here, we evaluated the morphology and the axonal projections of electrophysiologically defined respiratory neurons in the preBötC using whole-cell recordings and intracellular biocytin labeling. We found that respiratory pacemaker neurons are larger than expiratory neurons and that inspiratory neurons are smaller than pacemaker and expiratory neurons. Other morphological features such as somata shapes or dendritic branching patterns were not found to be significantly different among the preBötC neurons sampled. We also found that both pacemaker and inspiratory nonpacemaker neurons, but not expiratory neurons, show extensive axonal projections to the contralateral preBötC and show signs of electrical coupling. Overall, our data suggest that there are morphological differences between subtypes of preBötC respiratory neurons. It will be important to take such differences in consideration since morphological differences would influence synaptic responses and action potential propagation.