Intermittent breathing in vertebrates

The respiratory control system of animals exhibiting intermittent breathing patterns allows significant fluctuations in all respiratory variables. The extent of these fluctuations can be quite large, particularly in those species with episodic breathing patterns. In these animals, blood gases and pH seem to be regulated within a homeostatic range. Pattern generation in these animals would seem, therefore, to be less dependent on a central rhythm generator and more closely dependent on inputs from peripheral receptor systems and higher brain stem centers. The evidence does suggest, however, that the medulla is the important region for central nervous system coordination of ventilation in these animals just as it is in rhythmic breathers. Peripheral input from mechanoreceptors associated with the lungs and chemoreceptors associated with the lungs and/or arterial blood appear important in regulating (a) the tidal volume of each breath, (b) the length of the ventilatory period, and (c) the length of the ventilatory pause. One of the more important conclusions that can be drawn from the data is that all three variables appear to be under separate control. The relative roles of the various receptor groups in the control of each respiratory variable remain unclear. Given the intermittent nature of the breathing, broad oscillations occur in the PO2, and to a lesser extent, PCO2 in lungs, blood, and presumably tissues, with accompanying pH fluctuations in body fluids. Although the functional significance of the two distinct ventilatory patterns seen in these animals remains unclear, the fact that the episodic breathing pattern can be converted to a single breath pattern experimentally, suggests that both patterns are manifestations of a common underlying control system. The data suggest that the episodic breathing pattern arises from a single breath pattern as a consequence of vagally modulated input from supramedullary centers in the brain. The similarities between the data collected from all vertebrates is striking. These similarities suggest that a common mechanism may underlie the various intermittent breathing patterns seen under physiologic conditions in all vertebrates. The conversion of continuous breathing to intermittent breathing in some fish in normoxic or hyperoxic water, as well as in squirrels entering hibernation, further suggests that both continuous and intermittent breathing are manifestations of a common control system. Although it is still too early to do more than speculate, the possibility exists that intermittent breathing may be the consequence of a reduced metabolic rate (the one factor ectotherms and hibernating and sleeping endotherms have in common) such that animals no longer need to breath continuously to meet metabolic demands.(ABSTRACT TRUNCATED AT 400 WORDS)