Inhibition of Cardiac Accelerator Impulses by the Carotid Sinus. ∗

Abstract

It has been shown by Hering, 1 Heymans, 2 and others that stimulation of the afferent endings in the carotid sinus by an increase in the endosinusal pressure causes a slowing of the heart. Some have held that this is entirely due to a reflex stimulation of vagal fibers while others have presented evidence to show that there is a reflex inhibition of the cardiac accelerators as well. The second explanation involving a reciprocal mechanism has generally appeared to be the more probable. In the present investigation we have studied certain aspects of this reflex by observing the sympathetic impulses to the heart while varying the pressure in the carotid sinus.

The experiments were performed on cats under nembutal anesthesia. The carotid sinus on each side was isolated and perfused by way of the common carotid and external carotid arteries. Inasmuch as the nerve supply remained intact an increase of pressure within the sinus caused an increase in the number of afferent impulses going to the medullary centers. 3 The effect of these impulses on the cardiac accelerator discharge was determined by recording with a vacuum tube amplifier and oscillograph the sympathetic impulses in one of the small nerves running from the stellate ganglion to the heart.

The general nature of the cardiac sympathetic activity has been described 4 as a persistent tonic discharge of impulses grouped into large waves. We now find that as the pressure within the sinus is raised this discharge decreases until at about 125 to 150 mm. Hg. there is a complete inhibition of the sympathetic impulses.

If the pressure within the sinus be maintained at such a level the period of complete inhibition lasts for some seconds, after which there is an escape with a return of the sympathetic discharge. The duration of this period of inhibition is a function of the endosinusal pressure.