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1. In anaesthetized dogs, the activity from left atrial receptors with vagal afferent fibres has been examined during distension of small balloons in the pulmonary veins or the application of pulsatile pressures to an isolated perfused pouch of the left atrium.

Extracellular recordings were made from 164 neurones in the nucleus tractus solitarius and dorsal motor vagal nucleus of the chloralose‐anaesthetized cat. 139 neurones were excited synaptically and 25 non‐synaptically by electrical stimulation of cardiac and pulmonary vagal branches. Synaptically excited neurones fall into two populations, one activated solely by myelinated afferent fibres and a second activated solely by non‐myelinated afferent fibres. 94 neurones were synaptically excited by afferent fibres in a single vagal branch while 45 were excited by stimulation of two or three branches. Neurones responding to volleys in myelinated afferent fibres were located in both medial and lateral regions of the nucleus tractus solitarius whilst those excited by non‐myelinated afferent fibres were restricted to the medial region. Consistent differences in the locations of neurones excited by stimulation of either cardiac or pulmonary or by single or several branches could not be distinguished.

1. Activation of left atrial receptors by distension of balloons at the pulmonary vein—atrial junctions caused an increase in the cardiac sympathetic nerve activity, a decrease in the renal sympathetic nerve activity, and no change in the lumbar and splenic sympathetic nerve activity. 2. The cardiac and renal nerve responses to balloon distension were abolished by vagal blockade in the neck. 3. The results provide additional evidence that the reflex increase in the heart rate following stimulation of the left atrial receptors is mediated by selective activation of cardiac sympathetic nerve fibres. 4. The inhibitory effect of atrial receptor activation on the renal sympathetic nerve activity suggests that a neural factor, possibly involving renal haemodynamics, may be a component of the diuretic response to the activation of the left atrial receptors. 5. The results provide further evidence for the selective nature of the pattern of reflex responses to left atrial receptor activation and show for the first time a directionally fractionated response from sympathetic efferent neurones.

1. A rapid increase in pressure in a vascularly isolated perfused carotid sinus has been shown to inhibit a reflex response in efferent sympathetic nerves of the dog evoked by electrical stimulation of the radial nerve.