Activities of the phrenic and internal intercostal nerves show that the central nervous rhythm of respiration consists of 3 phases: inspiratory, postinspiratory and expiratory. The discharge patterns of medullary respiratory neurones of the anaesthetized, paralysed cat can be correlated with these phases of the central respiratory cycle, and the postsynaptic activity of individual cells can be analysed to obtain information about the populations of neurones converging upon them. Inferences are drawn about respiratory neurone connectivity and a theory is developed that the respiratory network primarily employs inspiratory-related neurones and that medullary expiratory neurones are less important for the rhythmogenesis of respiration. It is suggested that the inspiratory network consists of a ramp generating excitatory loop network of interneurones whose discharge is brought to an end ('off-switched') by inhibitory late-inspiratory interneurones. The discharge pattern of the latter type of neurone is explained by inhibition arriving from early-inspiratory interneurones. Subsequent to 'off-switching' the ramp generator is assumed to be immediately gated by a very powerful postinspiratory inhibition whereas expiratory activity seems to be disfacilitated at this time. This is the period when 'passive' (stage 1) expiration occurs. Following this interposed postinspiratory phase 'active' (stage 2) expiration may begin, depending on the amount of excitatory inflow to the inspiratory ramp generator. When expiratory neurones are activated the inspiratory system is again synaptically inhibited and the frequency of ventilation is markedly slowed.

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