Figure 3. Bursts are not essential for rhythmogenesis in transverse medullary slices in vitro.

A, rhythmic burstlets are distinct from bursts in medullary slices in vitro. In 9 mm K⁺/1.5 mm Ca²⁺, XIIn and preBötC bursting are synchronous and regular. In 6 mm K⁺/1.5 mm Ca²⁺, XIIn bursting is highly variable, while preBötC rhythm with both bursts and burstlets (*), which do not produce XIIn bursts, is more regular. Integrated preBötC and XII activity is shown, which represents suprathreshold, i.e. action potential, activity. Scale bar, 10 s. B, traces showing interburst intervals (IBIs) with different numbers of burstlets. Scale bar, 2 s. C, superimposed average waveforms of XIIn bursts (black) and preBötC burstlets (green) and bursts (red) aligned by the start of burstlet/pre-inspiratory preBötC activity (arrowhead). Vertical dashed line represents start of XIIn burst. preBötC bursts can be divided into a pre-inspiratory (pre-I) phase and an inspiratory burst (I-burst) phase. Scale bar, 0.5 s. D, Cd²⁺ selectively eliminates preBötC and XII bursts, but burstlets persist (bottom). Scale bar, 10 s. Adapted from Kam et al. (2013a).