Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting.

The central pattern generator for the respiratory rhythm in adult lampreys was studied in isolated brain preparations. At least three different types of respiratory units were found in intra- and extracellular recordings near the trigeminal nucleus (Fig. 1), including: units that start firing before the motorneurons, recorded from the ventral surface (Figs. 2 and 3), follower cells near rostral nucleus V (Fig. 4), units bursting after the motorneurons, found in the sulcus limitans (Figs. 5 and 6). Transections of the medulla indicated that the rostral half of nucleus V could be removed without reducing the respiratory frequency (Fig. 8). The earliest respiratory events that could be recorded from the ependymal surface or the cranial nerves were observed near nuclei IX-VII-caudal V during quiet breathing. There was a rostrocaudal delay in the onset of bursts of more caudal motorneurons (Figs. 9, 10, and 12). The rostrocaudal delay became reversed, such that bursts started earlier in n.X than n.IX, during episodes of intense breathing that were accompanied by prolonged discharges in the medial reticular formation (Fig. 11). Stimulation of the medulla surface, near the base of nerve V, could trigger bursts prematurely and reset the timing of the respiratory rhythm, yielding a discontinuous phase response curve (Fig. 14). In sum, 6 types of respiratory interneurons can presently be distinguished (Fig. 15). The sulcus limitans near nucleus V is a candidate location for components of the pattern generator.