Studies concerning neural control of respiratory timing and tidal volume have focused predominantly on the control the inspiratory phase (von Euler, 1986). With application of lung inflation during the inspiratory phase, three distinct reflexes have been described: (1) inspiratory off-switch (Clark & von Euler, 1972), (2) reversible graded inhibition (Younes, Remmers & Baker, 1978) and (3) low-threshold facilitation (Bartoli et al., 1975; DiMarco et al., 1981). Each of these reflexes is mediated through vagal mechanisms presumably via pulmonary stretch receptors. After vagotomy, changes in lung volume result in only minor effects on phrenic nerve activity.

Previous studies related to the control of the amplitude of expiratory motor activity and expiratory time were based predominantly on the response to expiratory loads (Bishop, 1967; Bishop, Hirsch & Thursby, 1978). They demonstrated that increases in lung volume during the expiratory phase facilitate expiratory motor activity and prolong expiratory time (TE) (Bishop et al., 1978; Barrett et al., 1994). More recent studies, however, showed that expiratory muscle activities may be inhibited by pulsed lung inflation (Arita & Bishop, 1983; Bajic et al., 1992; Cohen, Feldman & Sommer, 1985; Younes, Vaillancourt & Milic-Emili, 1974), increases in lung volume (Chung et al., 1987; Fregosi, Bartlett & St John, 1990; Polacheck, Remmers & Younes, 1978) or by stimulation of vagal afferents (Haxhiu et al., 1988; Smith et al., 1990). On the basis of these new data, we further examined vagal influences on expiratory motor activity and timing.