When a nerve cell needs to send a signal to its specific target it sends an electrochemical impulse down its axon. For many nerve cells the signal can propagate much more quickly because the axon is covered by a special insulating material known as myelin.
The Structure of Myelin
Myelin is a highly ordered, fatty substance that consists of tightly wound, multiple wrappings of a specialized cell membrane around the axons of some nerves. In the brain and spinal cord (the central nervous system) the cell membrane that wraps the axons comes from cells known as oligodendrocytes. In nerves outside of the brain and spinal cord (the peripheral nervous system), the wrapping is derived from the cell membrane of Schwann cells.
As the membrane wraps around the nerve axons, it is wound very tightly, squeezing out most of the cytoplasmic contents that are normally present within the cell as the two opposite regions of cell membrane are forced together. Picture a balloon filled not with air but with a thick soup. Starting at the tip of the balloon, you hold the tip in place against a broomstick and start wrapping the balloon tightly around the broomstick. As it is wrapped, the soup is largely squeezed away from this area towards the other end of the balloon, as the two layers of balloon end up very close to each other.
What Does Myelin Do?
For the nerve cell axons that have myelin wrappings, the addition of this specialized “insulation” has a dramatic effect on the speed at which signals travel along the axon. Compared to a nerve axon without myelination (and there are many in the body), a myelinated axon can transmit a signal from one end of the axon to the other approximately 50 times faster than a comparable axon that is non-myelinated. This happens because of what is known as “saltatory conduction”. The wrappings do not completely cover the axon along its length but rather small gaps, otherwise known as the “Nodes of Ranvier”, are present. In essence the signal can jump from one node to the next along the axon because of specific concentrations of specialized proteins at the nodes, leading to more rapid signal conduction.
Myelin is an important contributor not only to the speed of neural communication but also to the overall health and function of the nervous system. In fact, the so-called “white matter” of the brain and spinal cord has its whitish appearance because of myelin. The importance of myelin is made apparent in a disease such as multiple sclerosis where the white matter is damaged, leading to numerous difficulties. Interestingly, changes in the white matter and myelination are also now suspected of being involved in the development of schizophrenia and other neuropsychiatric diseases, e.g. according to recent research from R.D. Fields. (full reference below).
Learn more about schizophrenia at the US National Institute of Mental Health (NIMH)
Fields RD. (2008) White matter in learning, cognition and psychiatric disorders. Trends Neurosci.; 31(7):361-370.
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