The Vagus Nerve and Exercise: A Paradigm Shift
New research challenges the long-held belief that the vagus nerve shuts down during exercise, revealing its crucial role in maintaining heart health. Discover how this "rest-and-digest" nerve actually amps up during physical activity, supporting your cardiovascular system.
DR T S DIDWAL MD
11/29/20235 min read
Role of the autonomic nervous system (ANS) in regulating the cardiovascular response during exercise
When exercise begins, the central nervous system (CNS) generates a cardiorespiratory pattern (central command) that aligns with the somatomotor signal. Central command then triggers a reduction in parasympathetic activity to the heart, an elevation in ventilation rate, and likely contributes to the recalibration of the arterial baroreflex to higher pressures. While the specific central neural connections initiating changes in parasympathetic and sympathetic outflows during exercise are not fully understood, both central command and arterial baroreflexes play crucial roles in the sympathoexcitatory response at the onset of exercise. In various animal models and humans, there is a redistribution of cardiac output to the active muscles during exercise. This involves an increase in sympathetic outflow to visceral organs, leading to elevated vascular resistance in these regions. Conversely, resistance to blood flow in the exercising muscles decreases. Initially, the heightened resistance in non-exercising regions is a result of the upward adjustment of the arterial baroreflex. As exercise persists, an additional rise in vascular resistance may occur due to the activation of chemosensitive muscle afferents.
New Research
A new study published in Circulation Research challenges the traditional assumption that cardiac vagal activity decreases during exercise and proposes that it actually increases, playing a crucial role in maintaining cardiac function. The researchers conducted chronic direct recordings of cardiac vagal nerve activity, cardiac output, coronary artery blood flow, and heart rate in conscious adult sheep during treadmill exercise. They confirmed cardiac innervation of the left cardiac vagal branch and used pharmacological blockers to investigate the role of acetylcholine and vasoactive intestinal peptide (VIP) in cardiac function during exercise.
Increase in Cardiac Vagal Nerve Activity During Exercise: Contrary to conventional belief, the study found that directly recorded cardiac vagal nerve activity increased during exercise. This challenges the idea of complete vagal withdrawal during physical activity.
Crucial Role of Cardiac Vagus in Maintaining Heart Function: Denervation of the left cardiac vagal branch resulted in a significant reduction in coronary artery blood flow, cardiac output, and heart rate during exercise. This highlights the essential role of cardiac vagal activity in sustaining cardiac function during physical exertion.
Importance of Vasoactive Intestinal Peptide (VIP): The study demonstrated that the dynamic modulation of coronary artery blood flow during exercise is mediated by VIP, a neuropeptide, rather than acetylcholine. Inhibition of VIP during exercise had similar effects on coronary artery blood flow as vagal denervation.
Confirmation of Cardiac Vagal Innervation: Neural innervation from the cardiac vagal branch was observed at major cardiac ganglionic plexi and within the fat pads associated with the coronary arteries. This direct confirmation reinforces the role of the cardiac vagus in modulating heart function.
Implications for Understanding Exercise Physiology: The findings challenge the simplistic view of sympathetic and parasympathetic nervous systems acting in a binary opposition during exercise. Understanding the dynamic contribution of the parasympathetic (vagal) system, particularly through VIP mediation, provides new insights into the complex regulation of cardiovascular responses during physical activity.
The Revelation of CVNA Increase
The cornerstone of this study lies in the revelation that cardiac vagal nerve activity (CVNA) experiences a surge during exercise. Contrary to historical beliefs that an increased heart rate necessitates a withdrawal of vagal tone, the study asserts that the correlation is more complex than previously thought. Most measures of vagal tone have, until now, been indirect, making this direct recording during exercise a pioneering feat.
CVNA's Impact on Coronary Artery Blood Flow
Moving beyond heart rate regulation, the research uncovers that CVNA plays a crucial role in modulating coronary artery blood flow during exercise. While past studies have hinted at the importance of cardiac vagal activity during physical exertion, this research provides the first direct evidence of CVNA during exercise in conscious animals.
Vagal Cotransmitter VIP: Orchestrator of Increased Blood Flow
A significant player in this orchestration is the vagal cotransmitter VIP. Mediating the surge in coronary artery blood flow observed during exercise, VIP emerges as a key regulator. This finding not only adds a new dimension to our understanding of vagal control during exercise but also highlights the intricate web of signalling pathways involved.
Unraveling the Synergy: Vagal and Sympathetic Nerve Activity During Exercise
In a paradigm-shifting hypothesis, the study suggests that, during exercise, both vagal and sympathetic nerve activity witness an increase. This simultaneous coactivation is posited to have a synergistic effect on maintaining optimum cardiac function. The unexpected vagally-mediated tachycardia observed in sheep with left cardiac vagal branch denervation challenges conventional assumptions, opening avenues for further investigation into the mechanisms at play.
Acetylcholine's Limited Role and VIP's Regulatory Power
Contrary to previous assumptions, the study finds that acetylcholine, often equated with vagal blockade, has no significant effect on cardiac function during exercise beyond heart rate control. On the other hand, VIP emerges as a potent regulator of coronary artery blood flow, showcasing its vital role in maintaining perfusion during physical activity.
Implications for Exercise Physiology: Redefining the Norms
This study not only challenges but also redefines the existing norms in exercise physiology. The elevation of cardiac vagal nerve activity during exercise, contrary to historical perspectives, showcases the dynamic interplay between the sympathetic and parasympathetic nervous systems in orchestrating a healthy, pulsating heart. Traditionally, the surge in heart function during exercise was attributed solely to sympathetic nervous system control, sidelining the parasympathetic counterpart. However, our large animal exercise model study establishes, for the first time, the critical role of the cardiac parasympathetic nervous system during exercise. While historical perspectives dictated a minimal role for the parasympathetic nervous system during exercise, this study aligns with recent data challenging this assumption. The complexities of studying vagal control in the heart may have contributed to varying conclusions, but our direct cardiac vagal branch nerve recordings offer clarity. The coronary circulation undergoes acute adaptations to ensure optimal perfusion during exercise. This study sheds light on the specific role of cardiac vagal nerve activity during this process. Understanding this modulation becomes paramount, given the links between reduced exercise capacity, diminished vagal tone, and conditions such as heart failure.
Key points
CVNA increases during exercise and is important for maintaining cardiac function.
VIP is the principal vagal neurotransmitter responsible for mediating coronary artery blood flow during exercise.
Selective denervation of the cardiac vagal branch attenuates heart function during exercise.
Pharmacological blockade of VIP during exercise reduces coronary artery blood flow to a similar level as cardiac vagal denervation.
Cardiac vagal nerve activity plays an important role in maintaining coronary artery blood flow during exercise.
Concluding Insights: A Gateway to Cardiovascular Health
As we unravel the intricate interplay between CVNA, VIP, and exercise, a clearer picture emerges. This study propels us toward a deeper understanding of how these pathways influence cardiac perfusion during exercise and, by extension, cardiovascular health. The implications are profound, providing a foundation for future research aimed at promoting and enhancing cardiovascular well-being.
References
Shanks J, Pachen M, Chang JWH, George B, Ramchandra R. Cardiac vagal nerve activity increases during exercise to enhance coronary blood flow. Circ. Res. 2023;133(7):559-571. doi: 10.1161/CIRCRESAHA.123.323017
Bishop, V. S. (2003). Exercise and the Autonomic Nervous System. Primer on the Autonomic Nervous System (Second Edition), 183-184. https://doi.org/10.1016/B978-012589762-4/50048-7
Image: Wikimedia Commons
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