The mental benefits of exercise keep showing up, but more elusive has been an understanding of just how physical activity manages to work on the brain.

It's well known that exercise can produce a dramatic improvement in mental health and functioning. Studies have shown that exercise can boost the academic skills of young adults as well as the cognitive capacity of seniors.

High exposure to physical activity was sufficient to drive irisin, and its brain-protective effects, from the blood into the brain.

But no one knew exactly how physical activity contributed to learning enhancement. Some scientists hypothesized that exercise could stimulate learning by increasing blood flow to the brain. Others believed it was due to an alteration in a specific brain signal.

And now, a team of Harvard Medical School researchers may have just found the signal that scientists were searching for.

A new molecule, irisin, produced during exercise, is highly abundant in the brain after bouts of physical activity. Furthermore, it can exert a neuroprotective effect on brain cells.

Irisin is the product of a much larger protein called FNDC5 which is secreted by muscle cells during periods of intense physical activity. Previous studies have demonstrated that irisin induces some metabolic benefits of exercise, such as increases in energy expenditure and conversion of white fat into its healthier counterpart, brown fat.

Exercise begins a chain of events that leads to the brain. When researchers artificially induced levels of irisin in laboratory mice they found that it activated certain genes that have a critical role in learning and memory. Once of these was the gene that encodes the protein BDNF — brain-derived neurotrophic factor — which has previously been thought to regulate some of the effects of exercise on the brain.

BDNF has been shown to increase in the brain after certain exercises, yoga is one example, and may protect the brain against mental disorders such as depression and schizophrenia.

When the scientists genetically manipulated a different set of mice to develop low levels of irisin, the mice also exhibited low levels of BDNF and displayed noticeable deficits in learning and cognition.

Irisin is secreted by muscle cells, but the molecule crosses into the brain only after it accumulates in fairly high levels in the blood, the researchers found. In the study, mice were given a regimen of 30 days on a training wheel — equivalent to a high amount of exercise. This high exposure to physical activity was sufficient to drive irisin, and its brain-protective effects, from the blood into the brain.

Once irisin accumulated in the brain, it was enough to elevate levels of BDNF in the hippocampus, an area of the brain critical for learning and memory.

In terms of helping human memory, the next step, according to the researchers, is to deliver a stable form of the irisin protein into the brains of animals. If irisin can stimulate the pathways that protect against brain degeneration without intense physical activity, it's possible that even less fit older adults may gain a hedge against cognitive decline?

The study is published online in the Cell Metabolism.