I want to emphasize that the vagus has effects upon both the sinus node and the A-V node. It slows sinus discharge, and it depresses, delays, or blocks atrioventricular transmission. It is also known that under certain conditions that discharge of vagal fibers to the heart may be phasic. From the physiological standpoint, it ought to be phasic, particularly at slow heart rates. The reason it should be phasic is that when the systolic pulse wave arrives at the baroreceptor regions of the aorta and of the carotid sinuses, it elicits a discharge of afferent fibers in those regions which goes to the central nervous system and leads to enhancement of vagal activity
