Astrocytes shown to regulate signal speeds of neurons.
It is known that the brain is composed of neurons, which have tail-like extensions known as axons. Axons are wrapped in layers of myelin, which serve as insulation to increase the speed of signals relayed by neurons. Gaps between segments of myelin are called nodes of Ranvier, and the number and width of these gaps can also regulate transmission speed. However, it is still unclear just how myelin helps synchronize signals that come from different areas of the brain. Now, a study from researchers at the NIH shows that astrocytes alter the transmission speed of neurons by changing the thickness of myelin and the width of gaps of nodes of Ranvier. The team state that researchers used to think that myelin could not be thinned except when destroyed in demyelinating diseases, such as multiple sclerosis, however, their study suggests that 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 show that 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, has been considered static, and it would need to be dynamic to achieve this. The current study shows shows that dynamic mature myelin thickness and length can be reversibly regulated by astrocytes at the node of Ranvier.
The current study focuses on perinodal astrocytes, which frequently touch nodes of Ranvier throughout the brain. Results in mice and rats show that these astrocytes regulate adhesion molecules which connect myelin to axons. Data findings show that when these molecules are cut by the enzyme thrombin, myelin detaches from the axon, layer by layer.
Results show that when the ability of perinodal astrocytes to regulate thrombin was blocked, thinner myelin sheaths and wider nodes of Ranvier were observed; in turn, these changes reduced the signal speeds of neurons by approximately 15%, which was enough to impair reflexes of mice in a vision-based test.
The team surmise their findings suggest that astrocytes, by regulating signal speeds via the myelin sheath, play an important role in how the brain processes information, and they propose that blocking thrombin may help stabilize myelin. For the future, the researchers state that thrombin inhibitors are already approved by the FDA for other uses, and they are now testing their idea in a mouse model of multiple sclerosis.