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2-Minute Neuroscience: Myelin

Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics

in 2 minutes or less. In this installment I will discuss myelin.

Myelin is an insulating layer that surrounds the axons of neurons. Composed primarily of

lipids, myelin helps to prevent action potentials, which are the electrical signals that travel

along axons, from decaying due to electrical current leaking out through the axonal membrane.

Myelinated axons thus conduct action potentials more quickly and efficiently than unmyelinated

axons, and because of this many neurons in the nervous systems are myelinated.

Myelin does not cover an axon fully. Instead there are intermittent gaps in the myelin

where the axon is exposed to extracellular space. These gaps are called nodes of ranvier

and the sections of myelin that are adjacent to the nodes of ranvier are called internodes.

The nodes of ranvier are rich in sodium channels, which open in response to an action potential

traveling down an axon, allowing positive sodium ions to rush in. This influx of sodium

ions rejuvenates the action potential and helps to prevent it from dying out as it proceeds

along the axon. Because the nodes of Ranvier are not myelinated, however, the action potential

slows down at each node and speeds up as it travels along the myelinated internode. This

gives the appearance that an action potential is jumping from node to node, which is known

as saltatory conduction.

Myelin is formed by glial cells, but the particular type of glial cell responsible for myelinating

axons is different in the peripheral and central nervous systems. In the peripheral nervous

system, glial cells called schwann cells form myelin. Each schwann cell wraps around one

segment of an axon many times to form one internode. In the central nervous system,

oligodendrocytes form myelin. One oligodendrocyte can produce dozens of internodes on multiple

axons.

Because myelin is white, myelinated axons appear white and make up what is known as

the white matter of the brain.

We can see the importance of myelin to healthy neural functioning when we look at diseases

where myelination is disrupted. In multiple sclerosis, for example, deficiencies in myelin

disrupt neuronal communication, resulting in a variety of physical and psychiatric symptoms.