The baroreceptor input to cardiac vagal motoneurones.

1. A study has been made of twenty-three cardiac vagal motoneurones (c.v.m.s) in the nucleus ambiguus of chloralose-anaesthetized cats.2. C.v.m.s were identified as described previously (McAllen & Spyer, 1978). They were either spontaneously active (five units) or induced to fire by the ionophoresis of DL-homocysteic acid (DLH). Evidence is presented that above threshold, the DLH dose is not critical for the ability to show a baroreceptor influence.3. The influence of the baroreceptors was determined by plotting pulse-triggered histograms of c.v.m. activity. If the aortic baroreceptors were denervated, the pulse synchronous activity disappeared reversibly on carotid occlusion (eight out of eight cases). In five out of fifteen cells studied, when aortic baroreceptor pathways were intact, a small degree of cardiac modulation survived carotid occlusion.4. Estimates of the central delay of the baroreceptor-vagal reflex were made by timing the c.v.m. response to both electrical stimulation of the sinus nerve and natural baroreceptor stimulation. When both methods were used on individual units, electrical stimulation invariably gave shorter values. Natural stimulation gave values that fell into two ranges (20-60 msec and 70-110 msec), suggesting the involvement of two pathways.5. Using higher currents, four c.v.m.s were induced to fire tonically during inspiration. Their activity in this phase showed a cardiac rhythm just as during expiration. Similarly, they responded to sinus nerve stimulation throughout the respiratory cycle.6. This result is discussed in relation to the observation of others that the baroreceptor-cardiac reflex is ineffective during inspiration. It is proposed that there is no need to invoke a ;gate' early in the pathway but that this phenomenon and the present observations are best explained by summation of influences at the c.v.m. cell membrane.