Changes in antidromic latencies of medullary respiratory neurons in hypercapnia and hypoxia.

We evaluated mechanisms underlying changes in discharge frequencies of medullary respiratory neurons. This evaluation was made by determining variations in antidromic latencies; these variations reflect changes in membrane potentials. In decerebrate, vagotomized, paralyzed, and ventilated cats, activities of the phrenic nerve and single respiratory neurons were monitored in hyperoxic normocapnia, hyperoxic hypercapnia, and/or normocapnic hypoxia. Axonal projections were defined as bulbospinal or laryngeal by antidromic activation. At normocapnic hyperoxia, antidromic latencies fell to minima during periods of spontaneous neuronal activity, with maxima occurring between neuronal bursts. In hypercapnia or hypoxia, these minima were not altered, whereas maximum latencies typically rose for neurons whose discharge frequencies increased. However, the increased frequencies most strongly correlated with increases in the difference between maximum and minimum latencies. No such correlation was evident for neurons whose discharge frequencies declined. We conclude that the overall change of membrane potential primarily defines neuronal discharge frequencies. Changes in membrane potentials induced by peripheral and central chemoreceptor afferents and by direct actions of hypercapnia and hypoxia are discussed.