Central and direct vagal dependent control of expiratory duration in anaesthetized rabbits.

In anaesthetized rabbits, total or partial (only inflation reflex nearly abolished) DC current vagal block was performed during inspiration (ITB and IPB), or expiration (ETB and EPB), or throughout the breathing cycle (CTB and CPB). During CTB inspiratory (Ti) and expiratory duration (Te) increased as after vagotomy. With ITB Ti equally Tivag; Te increased, but remained shorter than Tevag. During ETB, Ti was unchanged, Te increased, but remained shorter than Tevag. The sum of deltaTe during ITB and ETB equalled deltaTevag. During CPB and IPB, Ti and Te behaved as during ITB. With EPB, Ti was unchanged and Te shortened. Preferential stimulation of large myelinated fibers in the central vagal stumps during expiration lengthened Te. Inspiratory stimulation shortened both Ti and Te, restored breath timing of ETB, but not that of pre-vagotomy control. Hence, Te of eupneic breaths should depend on a central mechanism relating Te to preceding Ti and on expiratory vagal discharge, having both a small lengthening (from stretch receptors) and a variable shortening effect (from irritant receptors). Both central and peripheral mechanisms are affected by CO2 breathing.