myelin sheath

noun

: the insulating covering that surrounds an axon with multiple spiral layers of myelin, that is discontinuous at the nodes of Ranvier, and that increases the speed at which a nerve impulse can travel along an axon

called also medullary sheath

Examples of myelin sheath in a Sentence

Recent Examples on the Web
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Some clinicians have also observed low vitamin E levels in patients, though more research is needed to determine how widespread this is. Fats Fats are essential for nervous system repair and help maintain the myelin sheath,the fatty layer that protects nerve cells, Spechler said. Mark Weinstein, MSNBC Newsweek, 27 Apr. 2025 Consistently training your brain will help boost your cognitive processes over time, because the myelin sheath—the layer of protein that coats your nerves—thickens. Ayana Underwood, SELF, 14 Feb. 2024 That's because, in the vast majority of spinal cord injuries, the cord is not completely severed — rather, the myelin sheath that protects the nerve cells is damaged or destroyed. Eliza Strickland, Discover Magazine, 12 Oct. 2010 Sensory receptors in the skin, which lack the speed-boosting insulating layer called a myelin sheath, are among the slowest, at 1 mph. Valerie Ross, Discover Magazine, 14 May 2011 In earlier work, Hauser and his colleagues found that the tissue damage in MS is primarily directed by B cells, which attack the myelin sheath protecting nerves. Lydia Denworth, Scientific American, 13 Jan. 2022 In this case, your immune system mistakenly attacks your myelin and damages the myelin sheath. Sara Gaynes Levy, SELF, 7 Mar. 2022 In the case of MS, the body attacks its myelin sheath, a protective covering for the nerves in the brain and spinal cord. Jocelyn Solis-Moreira, Health.com, 21 Jan. 2022 In the disease, inflammation damages the myelin sheath that insulates nerve cells, ultimately disrupting signals to and from the brain and causing a variety of symptoms, from numbness and pain to paralysis. Lydia Denworth, Scientific American, 13 Jan. 2022

Word History

First Known Use

1891, in the meaning defined above

Time Traveler
The first known use of myelin sheath was in 1891

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Cite this Entry

“Myelin sheath.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/myelin%20sheath. Accessed 2 May. 2025.

Kids Definition

myelin sheath

noun
: a layer of myelin surrounding the axons of some neurons

Medical Definition

myelin sheath

noun
: the insulating covering that surrounds an axon with multiple spiral layers of myelin, that is discontinuous at the nodes of Ranvier, and that increases the speed at which a nerve impulse can travel along an axon

called also medullary sheath

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