Researchers compared how quickly a group of males aged 23 up to 80 could perform a motor task. They found that the speed of the task and the integrity of myelination were correlated over the range of ages.
The myelination of brain circuits follows an inverted U-shaped trajectory, peaking in middle age. "Studies have shown that as we age, myelin breakdown and repair are continually occurring over the brain's entire 'neural network,'" said lead researcher George Bartzokis. "In older age, we begin losing the repair battle. That means the average performance of the networks gradually declines."
The researchers proposed that cognitive, sensory and motor processing speeds are all related to this decline. The hypothesis was checked with a test: how fast an individual can tap the index finger.
It is 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. Fast movements require high-frequency AP bursts that depend on excellent myelin integrity.
In the study, each of the 72 participants had a magnetic resonance imaging (MRI) scan that measured the myelin integrity in the vulnerable wiring of their brain's frontal lobes. The maximum finger-tapping speed (number of taps within ten seconds) was measured just before the MRI measure was obtained.
This showed: finger-tapping speed and myelin integrity measurements were correlated and "had lifespan trajectories that were virtually indistinguishable," said Bartzokis. Both peaked at 39 years of age and declined with an accelerating trajectory thereafter.
"The nearly identical trajectory across the lifespan for both measures of myelin integrity and fine motor speed supports the notion that myelin health underlies maximum AP burst frequency," Bartzokis said. The research suggests that the myelin breakdown process should also reduce all other brain functions for which performance speed is dependent on higher AP frequencies, such as memory; and that myelin breakdown is a biological process of aging underlying the erosion of physical skills and cognitive decline, including the onset of age-driven disorders as Alzheimer's disease.
MEDICA.de; Source: University of California, Los Angeles (UCLA)