WASHINGTON (AP) — Think achy joints are the main reason we slow down as we get older? Blame the brain, too: The part of the brain in charge of motion may start a gradual downhill slide starting as early as age 40.
How fast you can throw a ball or run or swerve a steering wheel depends on how speedily brain cells fire off commands to muscles. Fast firing depends on good insulation for your brain's wiring.
Now new research suggests that in middle age, even healthy people begin to lose some of that insulation in a motor-control part of the brain — at the same rate that their speed subtly slows.
And while that may sound depressing, keep reading. The research points to yet another reason to stay physically and mentally active: An exercised brain may spot fraying insulation quicker and signal for repair cells to get to work.
To Dr. George Bartzokis, a neurologist at UCLA who led the work, the brain is like the Internet. Speedy movement depends on bandwidth, which in the brain is myelin, a special sheet of fat that coats nerve fibers.
Healthy myelin — good thick insulation wound tightly around those nerve fibers — allows prompt conduction of the electrical signals the brain uses to send commands. Higher-frequency electrical discharges, known as "actional potentials," speed movement — any movement, from a basketball rebound to a finger tap.
But while myelin builds up during adolescence, when does production slow enough that we fall behind in the race to repair fraying, older insulation?
Enter the new research. First, Bartzokis recruited 72 healthy men, ages 23 to 80, to perform a simple test: How fast they tapped an index finger. Anyone can do this; it doesn't depend on strength or fitness.
Researchers counted how many taps the men made in 10 seconds, recording the two fastest of 10 attempts. Then, brain scans checked for myelin in need of repair in the region that orders a finger to tap.
Strikingly, tapping speed and myelin health both peaked at age 39. Then both gradually declined with increasing age, the researchers reported last month in the journal Neurobiology of Aging.
That doesn't mean the rest of the brain is equally affected. Bartzokis has some evidence that myelin starts to fray a decade or so later in brain regions responsible for cognitive functions — higher-level thinking — than in motor-control areas.
Bartzokis’ ultimate goal is to fight Alzheimer's disease. The connection: Building memories requires high-frequency electrical bursts, too, and Bartzokis' earlier research suggests an Alzheimer's-linked gene may thwart myelin repair.
But the new research has broader implications because it sheds light on normal aging, says Dr. Zoe Arvanitakis, a neurologist at Chicago's Rush University Medical Center.
"We knew at some age you peak and there's a sense it would disintegrate as you grow older. But we didn't have a sense of where that age would be," says Arvanitakis, who next wants to see if myelin and cognitive functions show a similar trajectory.
While much more research is needed, Bartzokis has some practical advice: keep active and treat high blood pressure, high cholesterol and diabetes – all these steps have already been deemed important for good brain health. But Keep in mind that physical and mental activity also may stimulate myelin repair, while unused neural pathways wouldn't send out a help signal for the system to repair itself.