Assessment of Beat‐by‐Beat Control of Heart Rate by the Autonomic Nervous System: Molecular Biology Techniques Are Necessary, But Not Sufficient

Vagus nerve activity can change heart rate substantially within one cardiac cycle, and the chronotropic effects decay almost completely within one cardiac cycle after cessation of vagal activity. The ability of the vagus nerves to regulate heart rate beat by beat can be explained in large part by the speed at which the neural signal is transduced to a cardiac response and by the rapidity of the processes that restore the basal heart rate when vagal activity ceases. Currently, the question of whether the cardiac cells can transduce the sympathetic neural signals rapidly enough to implement beatwise regulation is controversial. Emphasis on the speed of the processes that initiate the responses may, however, be misplaced. Instead, the processes that terminate the responses to autonomic neural activity (especially those processes that remove the released neurotransmitters) are probably the main determinants of the ability of the vagal and sympathetic systems to affect beatwise control. The norepinephrine (NE) released from the sympathetic nerve endings is removed from the cardiac tissues much more slowly than is the acetylcholine that is released from the vagal terminals. As a consequence of the potential deleterious effects associated with the slow removal of NE, the cardiac neural control system has evolved such that the sympathetic nerves ordinarily release NE at a slow rate. Hence, changes in sympathetic neural activity can alter cardiac behavior only slightly from beat to beat. Hence, beatwise control of cardiac function would be negligible, regardless of how swiftly the sympathetic nerve impulse is transduced to a change in cardiac performance.