On the relation between expiratory duration and subsequent inspiratory duration.

To characterize respiratory interphase relationships in dogs, inspiratory duration (TI) or expiratory duration (TE) was systematically altered by electrical activation of vagal afferents, and the effect on subsequent TE or TI values was measured from the phrenic discharge. A linear TI-subsequent TE relationship was found. Following a vagally mediated prolongation of TE, 1) TI was prolonged, and the TE-subsequent TI relationship was curvilinear, 2) the threshold for inspiratory termination by phasic vagal inputs was increased, 3) the amplitude of the time course of the phrenic discharge was reduced so that the peak discharge reached the same level at inspiratory termination independent of TI, 4) the effect on TI prolongation persisted for several breaths whereas the effect was minimal on subsequent TE values, and 5) for tonic inputs the direct shortening of TI was nearly offset by the indirect lengthening of TI. These studies suggest the existence of slow central mechanisms that provide adaptation to elevated levels of vagal input in the control of TI. These mechanisms may also be responsible for the interphase timing relationships.