Unexpected Synergy in Bilateral Symmetry

Working on one side of the body affects the other side, even if it is idle. How can that be?

Our left and right halves of the body can work alone or in tandem. For instance, you can do bicep curls with one arm, or with both arms. But what if working on one side of the body makes the other side healthier?

Arm Yourself for Action

Exercising one arm has twice the benefits (Medical Xpress). Australian scientists found a surprising “cross-transfer effect” with patients who were unable to exercise one arm.

New research from Edith Cowan University (ECU) has revealed that training one arm can improve strength and decrease muscle loss in the other arm—without even moving it.

The team, part of an international study, performed controlled experiments on 30 participants who had one arm immobilized for eight hours a day for four weeks. Some of the participants performed exercises on the free arm. Those who performed eccentric exercises (those focused on when the contracting muscle is lengthening) showed the most benefit to the inactive arm. Professor Ken Nosaka said that the findings “challenge conventional rehabilitation methods” and may help those who have temporarily lost the use of one of their arms or legs. It’s a new way of thinking about exercise:

Professor Nosaka said the group who used a heavy dumbbell to perform only eccentric exercise on their active arm showed an increase in strength and a decrease in muscle atrophy, or wastage, in their immobilized arm….

“This group also had just two percent muscle wastage in their immobilized arm, compared with those who did no exercise who had a 28 percent loss of muscle. This means that for those people who do no exercise, they have to regain all that muscle and strength again.”

Like all vertebrates, humans have bilaterial symmetry.

How can an immobilized arm get stronger? The arms are united by a single body and brain. Something in the brain must be capable of transferring the benefit of the active arm into the inactive arm. This may be related to the phenomenon of “ghost limbs” when a person loses an arm or leg yet still has sensations of the limb being there. Is the brain trying to activate repair mechanisms in a phantom limb that should be there?

The Eyes Have It

Gene Therapy in One Eye Improves Vision in Both Eyes (The Scientist). Here’s another case where two sides of the body share a benefit given to one. Ophthalmologists in Denmark, hoping to treat a disease called Leber hereditary optic neuropathy (LHON), gave one eye a repaired mitochondrial gene by injection with a viral carrier. They were surprised that the untreated eye responded in tandem.

Each of 37 patients received the therapeutic virus via a single injection into the fluid within one eye six to 12 months after the onset of vision loss. They also got a sham treatment in the other eye: a surgeon pressed the eye with a blunt cannula to simulate an injection.

“We thought that, if there was going to be an effect, it would be isolated to that eye and then the other one would be the perfect internal control,” Yu-Wai-Man tells The Scientist. “But as it turns out, that wasn’t the case.”

With a slight delay in the sham-treated eye, both eyes started to improve. By 96 weeks after treatment, 29 of the patients had gained visual acuity in both eyes and reported increases in their quality of life.

It has been known that loss of vision in one eye sometimes results in blindness in the other eye. What was surprising was to find a “bilateral effect” on the second eye when the first eye was treated.

“Clearly, further investigations are needed to understand the underlying mechanisms of how the interocular diffusion of viral DNA vector occurs and whether there are other mechanisms by which the optic nerves directly communicate,” Bin Li, an ophthalmologist at Tongji Hospital in China who was not involved in the study, writes in an email to The Scientist. Li explains that his group has also reported that material injected in one eye can reach the other optic nerve.

The disease is caused by a point mutation in a gene within the mitochondrial genome. The scientists wanted to use the opposite eye as a control. To their surprise, the optic nerve of the second eye was healing along with the optic nerve of the treated eye. A noted science writer responded, ““The question then really becomes . . . why did you get that result?”

This would suggest foresight to keep both arms working in tandem to some degree; otherwise, the inactive arm could atrophy beyond repair. And for both eyes to respond to a genetic therapy is optimum for experiencing the complexities of the visual world. People can and do get by with one arm or eye, but to have both respond to exercise or therapy had to be a built-in response to a complex world.