Astrocytes shown to regulate signal speeds of neurons.

The world’s greatest supercomputer, also known as the human brain is composed of neurons with tail-like extensions known as axons.  Axons are wrapped in layers of myelin, serving as insulation to increase the speed of signals relayed by neurons. Gaps between segments of myelin are called nodes of Ranvier, with the number and width of these gaps known to regulate transmission speed.  However, it is still unclear just how myelin helps synchronize signals originating in different areas of the brain.  Now, a study from researchers at the NIH shows astrocytes alter the transmission speed of neurons by changing the thickness of myelin and the width of gaps of nodes of Ranvier.  The team states researchers have in the past assumed myelin could not be thinned except when destroyed in demyelinating diseases, such as multiple sclerosis, however, their study suggests under normal conditions, the myelin sheath and structure of the nodes of Ranvier are dynamic, even in adults.  The opensource study is published in the journal PNAS.

Previous studies have indicated proper communication between brain regions, via impulses along white matter tracts, allows people to carry out complex cognitive and motor tasks. These impulses must arrive at relay points almost simultaneously for such communication to be effective. Myelin enables conduction of impulses and hence is a candidate for modulation of impulse conduction velocity, however, myelin structure, once formed is considered static, when it would need to be dynamic to achieve this.  The current study indicates dynamic mature myelin thickness and length can be reversibly regulated by astrocytes at the node of Ranvier.

The current study focuses on perinodal astrocytes shown to frequently touch nodes of Ranvier throughout the brain.  Results in mice and rats show these astrocytes regulate adhesion molecules connecting myelin to axons. Data findings show when these molecules are cut by the enzyme thrombin, myelin detaches from the axon, layer by layer.

Results show when the ability of perinodal astrocytes to regulate thrombin was blocked, thinner myelin sheaths and wider nodes of Ranvier were observed.  The lab states these changes then reduced the signal speeds of neurons by approximately 15%, a sufficient amount to impair reflexes of mice in a vision-based test.

The team surmises their findings suggest astrocytes play an important role in how the brain processes information by regulating signal speeds via the myelin sheath.  For the future, the researchers hypothesize blocking thrombin may help stabilize myelin in Multiple Sclerosis, and they are now testing their idea in a mouse model of multiple sclerosis.

Source: National Institutes of Health (NIH)

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