The heart does not operate entirely on its own. Two small clusters of nerves, positioned on either side of the lower neck, regulate the signals that make it beat faster or slower.
Exercise was long thought to influence both clusters in identical ways. New evidence, however, indicates that this is not the case.
After weeks of moderate training, the left and right sides displayed strikingly different structural adaptations. The contrast was so substantial that one side ultimately had four times the number of neurons as the other.
The heart’s autopilot
The body maintains the heartbeat without conscious control. The stellate ganglia are a pair of small nerve clusters located in the lower neck and upper chest.
They help regulate this automatic rhythm, accelerating the heart during stress, physical exertion or sudden alarm.
Researchers at the University of Bristol, led by Dr A. Augusto Coppi, put rats through ten weeks of moderate aerobic treadmill exercise.
Using 3D imaging, they then compared the trained animals with rats that had not exercised.
Before this research, no study had published a detailed 3D examination of the ganglia following training.
The findings challenge the longstanding belief that exercise causes the nervous system to adapt equally on both sides of the body.
A lopsided response
Among the trained rats, the right stellate ganglion contained approximately four times as many neurons as the left.
No comparable difference was seen in untrained animals. The imbalance emerged only after running, meaning it resulted from exercise rather than an existing baseline characteristic.
Neuroplasticity describes the way nerve tissue reorganises itself. Although it has been extensively investigated in the brain, it has received little attention in the circuit that controls the heart.
Previous studies had already established that aerobic exercise reduces resting heart rate and makes beat-to-beat variation more stable.
One study of older trained athletes, for instance, identified substantially greater heart rate variability than among untrained people of the same kind.
Yet none of this earlier work suggested that the two sides of the body might develop differently. The Bristol findings call that view into question.
Cells grew, cells shrank
At cell level, training produced changes that went in opposing directions on the two sides.
Neurons in the left ganglion became larger, reaching almost double their earlier volume. Neurons on the right instead became smaller.
Training also reduced the overall volume of both clusters, with the decline more pronounced on the right.
The right-hand cluster contained a denser concentration of slightly smaller neurons within less space. On the left, neurons were larger but less numerous.
Exercise alone causing structural changes in opposite directions was contrary to conventional expectations.
Until this research, nobody had reported training driving the two sides of a cardiac nerve cluster into mirror-image adaptations.
A clinical pattern
This left-right difference is more than an interesting observation. Clinicians already target the stellate ganglia for certain severe cardiac conditions.
Nerve blocks and surgical treatments reduce activity in these clusters to help control persistent arrhythmias and severe chest pain.
These approaches are also sometimes used for broken heart syndrome, a temporary type of heart failure brought on by intense emotional or physical shock.
The heart’s dimmer switch
A review of stellate ganglion blocks in people with drug-resistant arrhythmias found that the treatment reduced rhythm disturbances for one to three days.
Historically, most reported cases have focused on the left side, based on assumptions that it has the dominant role in controlling 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,” said Coppi.
In time, this pattern could enable clinicians to make more precise decisions about which side to target with a procedure.
Implications for humans
The study involved rats, and Coppi has been clear about its limits. It remains unknown whether the same uneven remodelling occurs in larger animals or people.
Future research is intended to connect these structural adaptations with functional effects.
The key issue is whether the increased neuron count on the right and expanded cells on the left produce measurable differences in cardiac behaviour at rest and during exertion.
The researchers will also investigate the pattern in larger animals and humans, using non-invasive methods that monitor the heart’s nerve signals without an operation.
Should the asymmetry also be found in human tissue, its significance could extend beyond exercise physiology.
A rewired baseline
For many years, researchers assumed that exercise altered the heart’s nerve wiring in the same way on both sides.
These new results suggest the reverse. Aerobic training seems to drive the two clusters in different directions: the right gains more neurons, while the left develops larger ones.
The discovery may matter to clinicians treating disorders associated with excessive nerve signalling.
The success of nerve interventions already varies according to the side they target. This research suggests a biological explanation for that difference.
A paper examining the role of nerve surges in broken heart syndrome illustrates the consequences when these signals overwhelm an otherwise healthy heart. The Bristol study identifies something the field has not previously observed.
Meaning in movement
Exercise seems to asymmetrically remodel cardiac control nerves, producing more neurons on the right and larger neurons on the left.
This may offer clinicians another factor to consider when tailoring care for heart-rhythm disorders and stress-related heart conditions.
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