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In this cross-section of a human head, the green fibers represent myelin-sheathed axons traveling from the cortex through the brain and down the spine. (Credit: Image courtesy of UCLA)

Had Griffey been 40, it could be argued, he might not have made the throw in time. That’s because in middle age, we begin to lose myelin — the fatty sheath of “insulation” that coats our nerve axons and allows for fast signaling bursts in our brains.

Reporting in the online version of the journal Neurobiology of Aging, Dr. George Bartzokis, professor of psychiatry at the UCLA Semel Institute for Neuroscience and Human Behavior at UCLA, and his colleagues compared how quickly a group of males ranging in age from 23 to 80 could perform a motor task and then correlated their performances to their brains’ myelin integrity. The researchers found a striking correlation between the speed of the task and the integrity of myelination over the range of ages. Put another way, after middle age, we start to lose the battle to repair the myelin in our brain, and our motor and cognitive functions begin a long, slow downhill slide.

The myelination of brain circuits follows an inverted U-shaped trajectory, peaking in middle age. Bartzokis and others have long argued that brain aging may be primarily related to the process of myelin breakdown.

“Studies have shown us that as we age, myelin breakdown and repair is continually occurring over the brain’s entire ‘neural network,'” said Bartzokis, who is also a member of UCLA’s Ahmanson–Lovelace Brain Mapping Center and the UCLA Laboratory of Neuro Imaging. “But in older age, we begin losing the repair battle. That means the average performance of the networks gradually declines with age at an accelerating rate.”

The researchers proposed that cognitive, sensory and motor processing speeds are all highly related to this decline. To test their hypothesis, they used one of the simplest and best understood tests of central nervous system processing speed: how fast an individual can tap their index finger.

It’s well known that the speed of a movement increases with the frequency of neuronal action potential (AP) bursts in the brain. AP is an electrical discharge that travels over the axons connecting nerves, whether it’s Ken Griffey Jr.’s brain ordering his arm to throw or the brain telling a finger to tap. Fast movements require high-frequency AP bursts that depend on excellent myelin integrity over the entire axon network involved in controlling that movement.

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