Physical and Cognitive Declines Increase with Age
Due to Slow Decay of Nerve Insulation
Myelin breakdown is a process of aging underlying
the erosion of physical skills and cognitive ability
Oct. 19, 2008 - During this year's baseball
playoffs, Chicago White Sox outfielder Ken Griffey Jr., 38, threw a
picture-perfect strike from center field to home plate to stop an
opposing player from scoring. The White Sox ultimately won the game by a
single run and clinched the division title. Had Griffey been even 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.
The myelin insulating layer, made up of protein and
fatty substances, forms around nerves, including those in the brain and
spinal cord. The purpose of the myelin sheath is to allow rapid and
efficient transmission of impulses along the nerve cells. If the myelin
is damaged, the impulses are disrupted. This can cause diseases like
multiple sclerosis. (MedlinePlus)
New research has 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.
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 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
AhmansonLovelace Brain Mapping Center and the UCLA Laboratory of Neuro
Imaging.
In older age, we begin losing the battle
"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.
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 (the number of taps over a period of 10 seconds)
was measured just before the MRI measure was obtained.
The results supported what the researcher had
suspected, that finger-tapping speed and myelin integrity measurements
were correlated and "had lifespan trajectories that were virtually
indistinguishable," according to Bartzokis. And yes, they both peaked at
39 years of age and declined with an accelerating trajectory thereafter.
Bartzokis said these observations are consistent
with the hypothesis that "maximum motor speeds depend upon high
frequency AP bursts that, in turn, depend on the myelin integrity of the
neural networks involved in the task."
"Beginning in middle age," he said, "the process of
age-related myelin breakdown slowly erodes myelin's ability to support
the very highest frequency AP bursts. That may well be why, besides achy
joints and arthritis, even the fittest athletes retire and all older
people move slower than they did when they were younger."
Results are Striking
"The results are pretty striking," Bartzokis said.
"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."
Significantly, 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,
including memory; it also supports the suggestion that myelin breakdown
is a biological process of aging underlying the erosion of physical
skills and cognitive decline, including the onset of such age-driven
disorders as Alzheimer's disease.
There is, however, some good news, according to
Bartzokis.
"Since in healthy individuals brain myelin
breakdown begins to occur in middle age, there is a decades-long period
during which therapeutic interventions could alter the course of brain
aging and possibly delay age-driven degenerative brain disorders such as
Alzheimer's," he said.
"Non-invasive, serial evaluations of myelin
integrity could be used to monitor the effects of new and current
treatments that may slow the process of myelin breakdown as early as
midlife."
Editors Notes:
Other authors of the study included Po H. Lu,
Kathleen Tingus, Mario F. Mendez, Aurore Richard, Douglas G. Peters,
Bolanle Oluwadara, Katherine A. Barrall, J. Paul Finn, Pablo Villablanca,
Paul M. Thompson, and Jim Mintz. The authors report no conflict of
interest.
The study was supported by the National Institutes
of Health, the RCS Alzheimer's Foundation, Sidell-Kagan Foundation; and
the U.S. Department of Veterans Affairs.
The Semel Institute for Neuroscience and Human
Behavior at UCLA is an interdisciplinary research and education
institute devoted to the understanding of complex human behavior,
including the genetic, biological, behavioral and sociocultural
underpinnings of normal behavior, and the causes and consequences of
neuropsychiatric disorders. In addition to conducting fundamental
research, institute faculty members seek to develop effective treatments
for neurological and psychiatric disorders, improve access to mental
health services, and shape national health policy regarding
neuropsychiatric disorders.
Keep up with the latest news for senior citizens, baby
boomers
Oct. 19, 2008 - During this year's baseball
playoffs, Chicago White Sox outfielder Ken Griffey Jr., 38, threw a
picture-perfect strike from center field to home plate to stop an
opposing player from scoring. The White Sox ultimately won the game by a
single run and clinched the division title. Had Griffey been even 40, it
could be argued, he might not have made the throw in time.