Exploring Degeneracy of Brainstem Networks that Generate the Respiratory Rhythm

Purpose There is ongoing debate as to what constitutes the necessary and sufficient anatomical substrates and cellular mechanisms for respiratory rhythm and pattern generation. Much of the literature is supportive of the pre‐Bötzinger complex (pre‐BötC) as the kernel of respiratory rhythm generation, but there is additional research suggesting the presence of other putatively rhythmogenic kernels within the respiratory network. The modular organization of the respiratory network, lends itself to the concept that anatomically and functionally discrete modules of the network may be capable of producing similar function (Degeneracy). In the present study we investigate the rhythmogenic capacity of a prioiri transected in‐situ arterially perfused brainstem preparations. Methods A priori transections were made at the stage of surgery, prior to reperfusion at four different levels: level 1: 1.5 mm caudal to obex, level 2: at obex, level 3: rostral to pre‐BötC, 1.5 mm rostral to obex, level 4: rostral to parafacial respiratory group (pFRG), 3mm rostral to obex. Respiratory motor activity was recorded from phrenic, vagus and abdominal nerves. Results Preparations transected at level 1 exhibited transient burst‐like activity upon initial reperfusion but did not produce any stable rhythmic respiratory bursting following re‐oxygenation of the tissue. Level 2 preparations exhibited short transient periods of sporadic rhythmic activity following re‐oxygenation. Level 3 preparations displayed stable rhythmic bursting but respiratory bursts showed low amplitudes. Preparations transected at level 4 showed robust rhythmic bursting of normal amplitude but displayed prolonged burst durations compared to preparation with a fully intact ponto‐medullary brainstem (n=6). In addition, n=3 of these preparations transected at level 4, showed a dual rhythm, with 2 separate burst identities (low and normal amplitude bursts). The high amplitude bursts were preceded by low amplitude pre‐BötC bursts with a latency of 0.64 ± 0.067 s. The latter suggesting that the low amplitude pre‐BötC rhythm triggers the final robust motor output seen at level 4. Across all transection levels 1–4, all respiratory activity recorded was synchronised across all motor outputs. Thus all preparations were lacking a normal motor sequence of inspiration, postinspiration and expiration. Conclusions No stable rhythmic respiratory activity was observed following re‐oxygenation of brainstem tissue transected at or caudal to the obex. Therefore we discount the hypothesis of a degenerate rhythmogenic circuit in the caudal medulla. We conclude that the pre‐BötC is necessary and sufficient to produce an intrinsic respiratory rhythm. The synchrony of respiratory motor activity in a priori transected perfused brainstem preparations suggest that sequential patterning of respiratory motor activity may depend on more rostral pontine brainstem areas. Support or Funding Information Authors work is supported by a University of Melbourne PhD international research scholarship