We tend to think of the body as roughly symmetrical: two lungs, two kidneys, two eyes, mirrored down the middle. A new study out of the University of Bristol suggests that when it comes to the nerve clusters controlling your heart, exercise doesn’t play by that rule at all.

Published in Autonomic Neuroscience: Basic and Clinical, the study looked at the stellate ganglia — paired bundles of sympathetic nerve cells sitting in the lower neck and upper chest. Think of them as the heart’s dimmer switch: they’re part of the “fight or flight” circuitry, sending the signals that speed the heart up under stress or physical exertion.

Ten weeks on a treadmill, then a very close look

Researchers put Wistar rats through 10 weeks of moderate treadmill training, then used 3D imaging and stereological analysis, a technique for precisely counting and measuring structures in tissue, to examine the stellate ganglia on both sides of the body, comparing trained animals to untrained controls.

What they found didn’t match the tidy, symmetrical picture you’d expect. The right stellate ganglion in trained rats gained roughly four times more neurons than the left, a jump not seen in the untrained group at all. But it was a strange trade-off: those new right-side neurons were smaller. Meanwhile, the left side told the opposite story, with far fewer new neurons but existing ones growing substantially larger, by about 1.8-fold. Overall, the volume of both ganglia actually shrank after training.

Why left and right would respond differently

This isn’t just a curiosity of rat anatomy. The left and right stellate ganglia don’t do identical jobs, cardiac innervation itself has a known left-right asymmetry, with the two sides influencing heart rhythm and contraction slightly differently. What this study adds is evidence that the nervous system’s physical structure adapts to training along those same asymmetric lines, rather than scaling up uniformly the way a muscle might.

Lead author Augusto Coppi, a senior lecturer in veterinary anatomy at Bristol, described the ganglia as remodeling “in a side-specific way” during regular moderate exercise, a previously hidden left-right pattern in what he called the body’s autopilot system for the heart.

Why this might matter for heart treatment

Stellate ganglia aren’t just an academic interest, cardiologists already target them clinically. Nerve blocks or surgical denervation of these ganglia are sometimes used to calm dangerous arrhythmias, hard-to-control angina, and Takotsubo cardiomyopathy (the aptly nicknamed “broken-heart syndrome,” a temporary heart condition often triggered by intense emotional or physical stress).

If the human nervous system shows the same lateral asymmetry — and responds to exercise the same way — it could help explain why some of these nerve-targeted treatments work better applied to one side than the other, and eventually help doctors choose which side to target more precisely.

The caveats

This is rat data, and the researchers are upfront that it’s early-stage. The next steps are to figure out how these structural changes actually affect heart function during exercise and rest, and whether the same left-right pattern shows up in larger animals, and eventually, humans, using non-invasive measurements.

Still, it’s a good reminder that “exercise is good for the heart” is a much richer story at the tissue level than the phrase lets on. The heart’s dimmer switch isn’t one dial — it’s two, and training seems to turn them in different directions.

Reference: Ladd, F.V.L., Barbosa, A.A., Cavalcanti, R.A.O., Melo, M.P., Loesch, A., Coppi, A.A. “Asymmetric neuroplasticity in stellate ganglia: Unveiling side-specific adaptations to aerobic exercise.” Autonomic Neuroscience: Basic and Clinical, 2025. DOI: 10.1016/j.autneu.2025.103338
https://www.autonomicneuroscience.com/article/S1566-0702(25)00100-6/abstract