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Key points

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    Basic respiratory rhythm is generated and maintained by neurones located in the medulla oblongata.
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    This basic respiratory rhythm is changed by neurones in the region called the periaqueductal grey (PAG) located in the midbrain, in order to modulate breathing for behaviour and emotional expressivity.
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    How the PAG converts basic respiratory rhythm into behavioural breathing is not known.
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    In this study I investigated the influence of the PAG on two important populations of medullary neurones, the late-inspiratory (late-I) and post-inspiratory (post-I) neurones, which are thought to be involved in mediating rhythmic inspiration to expiration phase transition.
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    I show that the PAG modulates the activity of the medullary late-I and post-I neurones, and this modulation contributes to the conversion of basic breathing into behavioural breathing.

Abstract  Emotional reactions such as vocalization take place during expiration, and thus expression of emotional behaviour requires a switch from inspiration to expiration. I investigated how the midbrain periaqueductal grey (PAG), a known behavioural modulator of breathing, influences the inspiratory-to-expiratory phase transition. Contemporary models propose that late inspiratory (late-I) and post-inspiratory (post-I) neurones found in the medulla, which are active during the inspiratory-to-expiratory phase transition are involved in converting inspiration to expiration. I examined the effect of excitatory amino acid (d,l-homocysteic acid; DLH) stimulation of the PAG on the discharge function of late-I and post-I neurones. The data show a topographical organization of DLH-induced late-I and post-I neuronal modulation within the PAG. Dorsal PAG stimulation induced tachypnoea and caused excitation of both the late-I and post-I neurones. Lateral PAG induced inspiratory prolongation and caused an excitation of late-I neurones but inhibition of post-I neurones. Ventrolateral PAG induced expiratory prolongation and caused a persistent activation of post-I neurones. As well, PAG stimulation modulated both the late-I and post-I cells for least two–three breaths even prior to the change in respiratory motor pattern. This indicates that the PAG influences the late-I and post-I cells independent of pulmonary or other sensory afferent feedback. I conclude that the PAG modulates the activity of the medullary late-I and post-I neurones, and this modulation contributes to the conversion of eupnoea into a behavioural breathing pattern.