Abstract
Breathing is vital for survival but also interesting from the perspective of rhythm generation. This rhythmic behavior is generated within the brainstem and is thought to emerge through the interaction between independent oscillatory neuronal networks. In mammals, breathing is composed of three phases – inspiration, post-inspiration, and active expiration – and this article discusses the concept that each phase is generated by anatomically distinct rhythm-generating networks: the preBötzinger complex (preBötC), the post-inspiratory complex (PiCo), and the lateral parafacial nucleus (pF L), respectively. The preBötC was first discovered 25 years ago and was shown to be both necessary and sufficient for the generation of inspiration. More recently, networks have been described that are responsible for post-inspiration and active expiration. Here, we attempt to collate the current knowledge and hypotheses regarding how respiratory rhythms are generated, the role that inhibition plays, and the interactions between the medullary networks. Our considerations may have implications for rhythm generation in general.
Highlights
Rhythms and oscillations function at the core of many brain processes[1,2]
20% of preBötzinger complex (preBötC) neurons can be classified as pacemakers, as defined by their tendency to burst in the absence of synaptic input at a period and burst duration similar to the duty cycle of the in vitro respiratory rhythm[38,45,46,47]
Light stimulation of channelrhodopsin-expressing Dbx[1] neurons in the preBötC simultaneously evokes inspiratory population activity in the contralateral preBötC and hyperpolarizes a postinspiratory post-inspiratory complex (PiCo) neuron[15]. When this experiment is repeated in the absence of inhibition, light stimulation both activates an inspiratory population burst and depolarizes the PiCo neuron. These results suggest that, under baseline conditions, the preBötC imparts an inhibitory influence on PiCo
Summary
Rhythms and oscillations function at the core of many brain processes[1,2]. For example, rhythmic spinal circuits control locomotor gait[3,4], thalamic oscillations detect attentional state[5,6], cerebellar rhythms are important for motor coordination[7,8], and circadian rhythms entrain our biological clocks to a 24-hour cycle[9,10]. The “pacemaker hypothesis”, in its strictest interpretation, is the idea that excitatory pacemaker cells play an obligatory role in driving the inspiratory rhythm When this experiment is repeated in the absence of inhibition, light stimulation both activates an inspiratory population burst and depolarizes the PiCo neuron Taken together, these results suggest that, under baseline conditions, the preBötC imparts an inhibitory influence on PiCo. when inhibition is blocked, it unmasks a concurrent excitatory influence of preBötC onto PiCo. This work lays the foundation for beginning to understand the dynamic interplay between the three independent rhythm generators. Further studies are needed that probe the interactions between the pFL and PiCo
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