SUMMARY1. 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.

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Afferent impulses in 61 fine myelinated or unmyelinated vagal fibers arising from endings in large blood vessels ("vascular endings") were recorded in anesthetized dogs and cats. The endings had a sparse, irregular spontaneous discharge, or they were quiescent. They were stimulated by capsaicin, phenyl diguanide, or veratridine injected into the bloodstream but not by sodium cyanide or hypoxia. Unlike arterial baroreceptors and atrial receptors, which are concentrated in localized areas, vascular endings were widely distributed in the thoracic aorta, the pulmonary artery, the brachiocephalic artery, the splenic artery, the atriovenous junctions, the atrial appendages, the inferior vena cava, and the hepatic vein. Conduction velocity in 28 fibers ranged from 0.8 to 11.0 m/sec (mean 3.1 m/sec), and in 19 of these fibers the velocity was less than 2.5 m/sec (i.e., the fibers were C-fibers). Vascular endings were stimulated by distending the vessels with balloons. Endings in the pulmonary artery and the aorta did not respond to pressures within the physiological range, but they were stimulated by abnormally high pressures (pulmonary artery 60-110 mm Hg, aorta 200-215 mm Hg). The physiological role of the vascular endings is unknown. Downloaded from 35. FIDONE, S.J., AND SATO, A.: Study of chemoreceptor and baroreceptor A-and C-fibers in the cat carotid nerve.

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