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| Myelin Speeds Up Thought |
Most neurons have a fatty outer layer called myelin, which insulates and protects the axons of neurons. In this way, myelin is like the plastic that surrounds electric wires. Myelin is actually made up of two special classes of glial cells, called the oligodendroglia and Schwann cells, which wrap themselves around the axon much like a jellyroll. Between these cells there are small gaps in the myelin sheath called the Nodes of Ranvier. Action potentials are able to jump from one node to the next one down the neuron incredibly rapidly. For this reason, impulses will travel down a myelinated neuron faster than they will across an unmyelinated neuron. In myelinated neurons, action potentials usually travel at over 100 meters per second, which is about half the speed of sound. In about one-hundredth of a second, an action potential can travel from the brain to the base of the spinal cord of an adult. Though seemingly instantaneous, this rate is still on the order of a million times slower than electricity.
Several degenerative diseases are due to the loss of myelin in certain neurons. The loss of muscle coordination that people with multiple sclerosis face is due to the degeneration of the myelin sheath in classes of neurons that are involved in the movement of muscles. The disease is suspected to be an autoimmune disorder - the immune system attacks the myelin sheaths. While MS is usually strikes first in early adulthood, many other diseases that are due to myelin degeneration occur in infancy or early childhood.