New evidence of myelin's essential role in learning and
retaining new practical skills, such as playing a musical
instrument, has been uncovered by UCL research.
Myelin is a fatty substance that insulates the brain's wiring
and is a major constituent of 'white matter'. It is produced by the
brain and spinal cord into early adulthood as it is needed for many
developmental processes, and although earlier studies of human
white matter hinted at its involvement in skill learning, this is
the first time it has been confirmed experimentally.
The study in mice shows that new myelin must be made each time a
skill is learned later in life and the structure of the brain's
white matter changes during new practical activities by increasing
the number of myelin-producing cells. Furthermore, the team say
once a new skill has been learnt, it is retained even after myelin
production stops. These discoveries could prove important in
finding ways to stimulate and improve learning, and in
understanding myelin's involvement in other brain processes, such
as in cognition.
For a child to learn to walk or an adult to master a new skill
such as juggling, new brain circuit activity is needed and new
connections are made across large distances and at high speeds
between different parts of the brain and spinal cord. For this,
electrical signals fire between neurons connected by 'axons' -
thread-like extensions of their outer surfaces which can be viewed
as the 'wire' in the electric circuit. When new signals fire
repeatedly along axons, the connections between the neurons
strengthen, making them easier to fire in the same pattern in
future. Neighbouring myelin-producing cells called oligodendrocytes
(OLs) recognise the repeating signal and wrap myelin around the
active circuit wiring. It is this activity-driven insulation that
the team identified as essential for learning.
The team demonstrated that young adult mice need to make myelin
to learn new motor skills but that new myelin does not need to be
produced to recall and perform a pre-learned skill. They tested the
ability of mice to learn to run on a complex wheel with irregularly
spaced rungs. The study looked at thirty-six normal mice and
thirty-two mice with a drug-controlled genetic switch to prevent
new OLs and myelin from being made. They found the mice that were
prevented from producing new myelin could not master the complex
wheel, whereas those that could produce myelin did learn, with
differences between the two groups' abilities seen after only two
hours of practice.
A second experiment looked at mice that were first allowed to
learn to run on the complex wheel before being treated with the
drug to prevent further myelin production. When the mice were later
re-introduced to the complex wheel, they were immediately able to
run at top speed without having to spend time re-learning. This
shows that the inability to make new myelin did not affect the
mouse's running ability and that new myelin is not required to
remember and perform a skill once learned; it is required only
during the initial learning phase.
Lead researcher, Professor Bill Richardson, Director of the UCL
Wolfson Institute for Biomedical Research, said: "From earlier
studies of human white matter using advanced MRI technology, we
thought OLs and myelin might be involved in some way in skill
learning, so we decided to attack this idea experimentally. We were
surprised how quickly we saw differences in the ability of mice
from each group to learn how to run on complex wheel, which shows
just how fast the brain can respond to wrap newly-activated
circuits in myelin and how this improves learning. This rapid
response suggests that a number of alternative axon pathways might
already exist in the brain that could be used to drive a particular
sequence of movements, but it quickly works out which of those
circuits is most efficient and both selects and protects its chosen
route with myelin.
"We think these findings are really exciting as they open up
opportunities to investigate the role of OLs and myelin in other
brain processes, such as cognitive activities (like navigating
through a maze), to see if the requirement for new myelin is
general or specific to motor activity. I'm keen to find out the
precise sequence of changes to OLs and myelin during learning and
whether these changes are needed more in some parts of the brain
than others, which might shed light on some of the mysteries still
surrounding how the brain adapts and learns throughout life."
The paper, Motor
skill learning requires active central myelination, is
published in Science.