WASHINGTON, May 17 (Xinhua) -- A study published on Thursday in the journal Cell revealed brain relies on an exquisite balance between the activity of two populations of neurons to carry out any sequential action like playing the piano or dancing the samba.
Scientists from Harvard Medical School attributed how brain achieves the remarkable feat to spiny projection neurons arranged into the direct and the indirect pathways in the striatum, a spiral-shaped region buried in the forebrain and a coordinating center for motor and action planning.
The findings could help researchers better understand conditions that dramatically impact movement, such as Parkinson's disease and Huntington's disease, and eventually develop new ways to treat them.
"We believe our observations set the stage for both unraveling how movement gets translated into desired action, and propel us forward in our ability to understand and, eventually, treat devastating neurodegenerative disorders where this process goes awry," said the study's senior author Sandeep Robert Datta, associate professor of neurobiology at Harvard Medical School.
Previously, some evidence suggested that the direct pathway selected and initiated the expression of actions, while the indirect pathway inhibited unwanted actions.
Other studies, however, have found that both pathways are often activated at the same time.
"That didn't make sense based on what we've long thought each pathway did," said the study's lead author Jeffrey Markowitz, a postdoctoral fellow in the university's department of Neurobiology.
The research team used a technology called motion sequencing to film animals' three-dimensional movements and used machine learning to precisely parse the movements into basic patterns lasting only a few hundred milliseconds apiece. The researchers dubbed those ultra-fast movements "syllables."
They found that the balance of activity between the two pathways differed. For some syllables, the direct pathway dominated and for others, the indirect pathway did.
Even for highly similar syllables, such as two different types of "scrunching," or curling up in a ball, the two pathways could be distinguished. Each syllable yielded a particular balance between the two pathways.
The relationship between neural activity and syllables was so pronounced that the researchers could successfully identify specific syllables expressed based on the pathway activity alone.
In another set of experiments, the scientists induced lesions in the striatums of a handful of mice.
After a week of recovery, they placed the mice into an arena-like space that had the scent of a fox wafting through one side.
With an inborn instinct to avoid predation, mice with an intact striatum immediately raced to the other side of the arena.
But mice with lesions in their striatums were also able to display all the separate syllables seen in normal mice, such as sniffing, running, rearing and turning, but their brains somehow failed to sequence these movements correctly, rendering the animals incapable of reaching the arena's opposite side.
"This underscores the importance of order in piecing movements together toward a desired outcome," Datta said. "Even if you're able to move your body correctly, if you can't put actions in the correct order, it's hard to do even the most basic of things."
If replicated in further studies, the findings could help inform new treatments for Parkinson's disease and Huntington's disease, conditions in which even basic movements become extremely difficult as these diseases progress, the researchers said.
Currently, Parkinson's disease is treated by giving patients a form of the neurotransmitter dopamine, which stimulates both the direct and indirect pathways. However, the efficacy of the treatment wanes over time. Still, there is still no effective treatment for Huntington's disease.
