Exercise doesn't just strengthen your heart. It rewires it

Exercise does more than make the heart stronger. New research suggests it also physically reshapes the nerves that control the heart, revealing an unexpected difference between the left and right sides of the body.

Scientists at the University of Bristol (UK) discovered that moderate aerobic exercise changes the structure of nerve cells involved in regulating heart activity. Even more surprisingly, the changes were dramatically different on the two sides of the body. The findings, published in Autonomic Neuroscience, could eventually help doctors develop more precise treatments for irregular heartbeats, chest pain, angina, and stress-induced 'broken-heart' syndrome.

The discovery was made in rats, so further research is needed to determine whether the same changes occur in humans. Nevertheless, it offers a new perspective on how physical activity influences the nervous system that keeps the heart functioning.

The heart is regulated partly by the autonomic nervous system, a network of nerves that automatically manages essential functions such as heartbeat, breathing, and digestion. One branch of this system helps the heart beat faster when the body needs more oxygen, such as during exercise or stressful situations.

Small clusters of nerve cells called stellate ganglia play an important role in this process. Located on both sides of the lower neck and upper chest, these nerve hubs send signals that influence heart rate and other aspects of heart function.

Study lead author Dr. Augusto Coppi, Senior Lecturer in Veterinary Anatomy at the University of Bristol, said: "The discovery points to a previously hidden left-right pattern in the body's 'autopilot' system that helps run the heart.

"These nerve clusters act like the heart's dimmer switch and we've shown that regular, moderate exercise remodels that switch in a side-specific way. This could help explain why some treatments work better on one side than the other and, in future, help doctors target therapies more precisely and effectively."

To investigate how exercise affects these nerve clusters, the researchers examined rats that underwent 10 weeks of moderate aerobic training. Aerobic exercise is physical activity that increases heart rate and breathing, such as brisk walking, swimming, or cycling.

The study involved researchers from University College London (UCL) in the UK, the University of São Paulo (USP), and the Federal University of São Paulo (UNIFESP) in Brazil.

The team used advanced three-dimensional analysis techniques known as stereology to examine the nerve cells. This approach allows scientists to estimate the number, size, and distribution of cells within tissues.

The results revealed an unusual imbalance between the two sides of the body. In the trained rats, the cardiovascular nerve cluster on the right contained roughly four times as many neurons as the cluster on the left, a striking difference from the pattern observed in untrained rats.

The nerve cells also changed in size, but in opposite directions. Neurons on the left nearly doubled in size, while those on the right became slightly smaller.

These findings suggest that exercise does not affect all the nerves controlling the heart equally. Instead, it produces distinct structural changes on each side, potentially providing clues about how the nervous system adapts to regular physical activity.

Arrhythmias occur when the heart beats too quickly, too slowly, or irregularly. Angina is chest pain or discomfort typically caused by insufficient blood flow to the heart muscle. Another condition, commonly called 'broken-heart' syndrome, involves a sudden weakening of the heart muscle that is often triggered by intense emotional or physical stress.

In some patients, doctors treat these problems by reducing excessive nerve signals reaching the heart. Procedures such as stellate ganglion nerve blocks temporarily interrupt nerve activity, while denervation involves disrupting certain nerve pathways.

Dr. Coppi explained: "Irregular heart rhythms, known as arrhythmias, stress-induced 'broken-heart' syndrome, and certain types of chest pain are often treated by dialing down overactive stellate ganglia -- the paired small nerve hubs in the lower neck/upper chest area that send 'go faster' signals to the heart.

"By mapping how exercise changes these ganglia on each side, the study offers clues that could one day fine-tune procedures like nerve blocks or denervation to the side most likely to help. The findings are early-stage and in rats, so clinical studies would need to follow."

The research team plans to investigate whether the differences between the left and right nerve clusters influence how the heart behaves during rest and physical activity. They also intend to look for similar patterns in other animal models and eventually in humans.

If the same left-right differences are found in people, the discovery could help guide more personalized treatments for arrhythmias, difficult-to-treat angina, and stress-induced 'broken-heart' syndrome.

Dr. Coppi added: "Understanding these left-right differences could help us personalize treatments for heart rhythm disorders and angina. Our next step is to test how these structural changes map onto function and whether similar patterns appear in larger animals and humans."