The nature of the atrial receptors responsible for a reflex increase in activity in efferent cardiac sympathetic nerves.

In dogs anaesthetized with chloralose, distension of small balloons in the right upper and middle pulmonary vein-atrial junctions, to stimulate left atrial receptors, caused an increase in heart rate and an increase in activity in efferent sympathetic nerve fibers in cardiac branches of the right stellate ganglion. Cooling of the cervical vagi in steps reduced the magnitude of the responses in these sympathetic nerve fibres. In four dogs, the response in six preparations of sympathetic nerves was slightly reduced with the vagi at 18 degrees C and markedly reduced or abolished at 12 degrees C. In these nerves there was no significant response to distension of the balloons when the cervical vagi were cooled to 9 degrees C. The effect of cooling the vagi was the same as the previously shown effect of cooling on the increase in activity in myelinated afferent vagal fibres and the increase in heart rate during stimulation of atrial receptors. It is concluded that the increase in activity in efferent sympathetic cardiac nerve fibres during distension of small balloons in the pulmonary vein-atrial junctions involves receptors discharging into myelinated vagal nerve fibres; receptors which discharge into non-myelinated vagal nerves of afferent sympathetic nerve fibres are not involved in this response. Thus, the efferent sympathetic nerve fibres studied are likely to represent the efferent pathway of the response of an increase in heart rate to distension of the small balloons.