Brain slows at 40, starts body decline

WASHINGTON — Think achy joints are the main reason we slow down as we get older? Blame the brain, too: The part in charge of motion may start a gradual downhill slide at 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.

That helps explain why “it’s hard to be a world-class athlete after 40,” concludes Dr. George Bartzokis, a neurologist at the University of California at Los Angeles, who led the work.

To Bartzokis, 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.