Spatiotemporal organization of frog respiratory neurons visualized on the ventral medullary surface

We visualized the spatiotemporal activity of respiratory-related neurons in the frog using the isolated brainstem spinal cord preparation. We recorded optical signals from the ventral surface of the medulla using a voltage-sensitive dye, and calculated cross-correlations with the integrated respiratory activity of the trigeminal nerve. Lung burst-related depolarizing optical signals were observed bilaterally as longitudinal columns in the ventrolateral medulla between the levels of trigeminal and hypoglossal rootlets, mostly caudal to the vagal rootlet. However, we could not differentiate between neurons involved in rhythm generation and motoneurons. The dye weakened the buccal rhythm and slowed the lung rhythm, which might have influenced the results. Extracellular recording of respiratory neurons verified the optically identified area. Strychnine disrupted the spatiotemporal organization of optical signals, although trigeminal periodic bursts persisted. We conclude that the pattern generator but not the rhythm generator of lung burst in the frog involves glycinergic mechanisms and lies as longitudinal columns in the reticular formation of the ventrolateral medulla.