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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.