The effect on breathing of abruptly stopping carotid body discharge.

In bilaterally vagotomized, decerebrate or pentobarbitone anaesthetized cats, intense carotid body discharge (FET O2 ca. 0.075) was abruptly removed by injections centrally of 100% O2-equilibrated Ringer into both external carotid arteries. In an inspiration the injections usually shortened that inspiration, reduced its volume and prolonged the immediately following expiration. Early in expiration they prolonged that expiration, but later in expiration they shortened it. The inspiratory results can be reconciled with von Euler's model of the inspiratory off-switch if the off-switch acts early because a reduction in chemoreceptor input lowers its threshold more rapidly than it reduces the input to it. The threshold falls to half of its final value in about one second. The respiratory centres respond to decreases in carotid body activity nearly as quickly as to increases, and expiration can be altered independently of the preceding inspiration. We present a simple model of the control of expiratory duration.