Chemoreceptors and their reflexes with special reference to the fetus and newborn.

Such evidence as there is indicates that hypoxia excites peripheral chemoreceptors in the adult by causing a fall in the rate of oxidative phosphorylation and that this process is potentiated by a rise in PCO2 which most probably acts by altering intracellular pH. What is unclear is whether this occurs in Type I cells or in sensory fibres and whether either should be regarded as the primary receptor. There is also uncertainty about the role of noradrenaline and dopamine, which are stored and presumably secreted in the chemoreceptor complex: are they, respectively, excitatory and inhibitory transmitters or do they modulate an underlying chemoreceptive process? In the fetus, the carotid body chemoreceptors are largely insensitive despite an adequate chemical stimulus; they are activated at or shortly after birth by an unknown process and thereafter, in the newborn period, they appear to be involved in sensing chemical changes in arterial plasma and assisting in respiratory stability but there is some evidence that they only become fully effective some days or even weeks after birth. There is now compelling evidence that important if not unique central chemoreceptors, sensitive to changes in pH of brain extracellular fluid are located superficially in the ventro-lateral part of the medulla in the adult but in the fetus it appears that these receptors or their afferent discharge are in some way inhibited such that respiratory activity when it occurs is driven largely by supra-pontine influences, the so-called 'automatic' component of respiratory control being largely if not entirely suppressed. At birth, this component is clearly activated since breathing is continuous and sensitive to chemical stimulation. It is probable that in a proportion of newborn, activation of this component is either imperfect or delayed which would lead to respiratory instability or even failure and this would be more likely to occur in quiet sleep than in rapid-eye-movement sleep or with the newborn awake. A more complete understanding of the changes in sensitivity of the central and peripheral chemoreceptors and their reflexes at birth is required before the reasons for inadequate respiratory control in the newborn period can be appreciated.