Impulses in Cardiac Sympathetic Nerves. ∗

Abstract

A better understanding of the mechanism of the reflex regulation of heart rate and blood pressure is to be gained from a study of the afferent nerve impulses from the circulatory system and of the efferent impulses to the heart and blood vessels. Previous communications have described the nature of the nervous discharge from the arch of the aorta and from the carotid sinus, 1 , 2 and its relation to blood pressure. On the efferent side, sympathetic impulses concerned in maintaining the tone of the blood vessels have been recorded. 3 The present report is concerned with the activity of the sympathetic fibers to the heart.

One of the small nerve twigs running to the heart from the stellate or inferior cervical ganglion in a cat under urethane anesthesia was freed from the surrounding tissue and cut close to the heart, all other cardiac sympathetic fibers remaining intact. The nerve was then slung onto electrodes and the action potentials, after amplification, were recorded by means of an oscillograph. Figure 1A shows a typical discharge. It will be observed that the impulses tend to come in bursts but in general we have not been able to identify the frequency of these volleys with the heart rate or respiratory rhythm, although it has been shown 3 that in the case of sympathetic nerves carrying fibers to the blood vessels, there is usually a grouping that is synchronous with either the respiratory or cardiac rhythm. The records further show that under the conditions of these experiments there is normally a ”tonic” sympathetic discharge to the heart.

The relation of the discharge in the cardiac sympathetic fibers to the heart rate is shown by a comparison of Figures 1A and B. In A, the frequency of the heart beat was 132 per min.