In newborns of many species, breathing patterns are characterized by a very irregular rhythm interrupted by high-frequency respiratory periods (Mortola, 1984) and by the development of spontaneous apnoeas. Inspiratory activities of the phrenic nerve and diaphragm in newborns and particularly in preterm babies (Duron, Khater-Boidin & Wallois, 1991) consist of bursts of action potentials of short duration (50–60 ms) and low frequency (4–6 Hz). Moreover, in kittens, during eupnoea there exists relatively weak neuronal inspiratory activity (Bystrzycka, Nail & Purves, 1975; Marlot & Duron, 1976; Goldberg & Milic-Emili, 1977). In the newborn kitten, the inspiratory time is of short duration and the average duration of phrenic motor unit discharge does not exceed 500 ms (Duron & Marlot, 1979). The inspiratory time progressively lengthens during postnatal development, at the same time as the discharge pattern of phrenic motor units changes.

At birth, with regard to early inspiratory motor units (Hilaire, Monteau & Dussardier, 1972), we observed a very rapid increase in discharge frequency, which reached values of around 60 Hz, very clearly higher than those found in the adult animal. The end of the discharge is sudden, suggesting the intervention of powerful inhibitory mechanisms (Duron & Marlot, 1979). Moreover, in various experimental procedures (anaesthetized or decerebrate preparations), bilateral vagotomy, which in adults reinforces central inspiratory activity, induces prolongation and reinforcement of expiration in newborn animals (Marlot & Duron, 1979a). As shown in Fig. 32.1, bilateral vagotomy not only increases the expiratory time but also provokes the appearance of electrical activity in the expiratory muscles.