The parafacial respiratory group and the control of active expiration

• Published in: Respiratory physiology & neurobiology Vol. 265; pp. 153 - 160
• Main Authors: Pisanski, Annette, Pagliardini, Silvia
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.07.2019
• Subjects: Abdominal Muscles - physiology; Active expiration; Animals; Brainstem development; Exhalation - physiology; Expiratory abdominal muscles; Fetal Development - physiology; Humans; Hypercapnia - physiopathology; Hypoxia - physiopathology; Parafacial respiratory group (pFRG); Respiratory Center - physiology; Respiratory Center - physiopathology; Sleep; Sleep, REM - physiology; Parafacial respiratory group (pFRG); Expiratory abdominal muscles; Sleep; Brainstem development; Active expiration
• ISSN: 1569-9048; 1878-1519; 1878-1519
• DOI: 10.1016/j.resp.2018.06.010

•The parafacial respiratory group (pFRG) has a key role in the generation of active expiration and recruitment of expiratory abdominal muscles.•Expiratory abdominal muscle activity is present in REM sleep and it is associated with reduced respiratory variability, increased tidal volume and minute ventilation in adulthood and through post-natal rat development.•The pFRG neurons are under GABAergic and Cholinergic control and are activated in conditions of high metabolic demand (hypercapnia and hypoxia).•Mounting experimental evidence suggests that pFRG and RTN are two physiologically distinct nuclei in the parafacial region. Breathing at rest is typically characterized by three phases: active inspiration, post-inspiration (or stage 1 expiration), and passive expiration (or stage 2 expiration). Breathing during periods of increased respiratory demand, on the other hand, engages active expiration through recruitment of abdominal muscles in order to increase ventilation. It is currently hypothesized that different phases of the respiratory rhythm are driven by three coupled oscillators: the preBötzinger Complex, driving inspiration, the parafacial respiratory group (pFRG), driving active expiration and the post-inspiratory Complex, driving post-inspiration. In this paper we review advances in the understanding of the pFRG and its role in the generation of active expiration across different developmental stages and vigilance states. Recent experiments suggest that the abdominal recruitment varies across development depending on the vigilance state, possibly following the maturation of the network responsible for the generation of active expiration and neuromodulatory systems that influence its activity. The activity of the pFRG is tonically inhibited by GABAergic inputs and strongly recruited by cholinergic systems. However, the sources of these modulatory inputs and the physiological conditions under which these mechanisms are used to recruit active expiration and increase ventilation need further investigation. Some evidence suggests that active expiration during hypercapnia is evoked through disinhibition, while during hypoxia it is elicited through activation of catecholaminergic C1 neurons. Finally, a discussion of experiments indicating that the pFRG is anatomically and functionally distinct from the adjacent and partially overlapping chemosensitive neurons of the retrotrapezoid nucleus is also presented.