The role of vagal afferent information and hypercapnia in control of the breathing pattern in chelonia.

The normal breathing pattern of the turtle, Chrysemys picta (Schneider), consists of periods of continuous breathing interspersed with periods of breath holding. During each ventilatory period respiratory frequency and tidal volume are controlled independently. There is a large variability in inspiratory and expiratory gas-flow rates yet tidal volumes are maintained within narrow limits by adjustments of the lengths of the active inspiratory and expiratory intervals. Lung volume information carried within the vagus nerve is responsible for the careful regulation of tidal volume as well as for modulation of the air flow rates and lowering of the threshold of the mechanism initiating expiration following breath holding. Increases in pulmonary minute ventilation during hypercapnia are caused by increases in respiratory frequency due solely to a shortening of the periods of breath holding. There is some increase in tidal volume but the breath length remains constant and thus the frequency of breathing within each ventilatory period also remains constant. After vagotomy, changes in minute ventilation due to hypercapnia stem primarily from changes in tidal volume while changes in respiratory frequency are greatly reduced.