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The Evolution of Myelin.

Primitive animals living in the dark recesses of the sea bed lead a quiet and relatively unchallenged life. When animals started to venture away from such protected areas in search of food, the need for flight from external danger became necessary. Evolutionary pressure resulted in the selection of features which favoured escape. This is where myelin came in. Myelin is an insulating material that is wrapped around the axons of nerve cells. Its job is to increase the speed of conduction of electrical signals along the nerve fibre to muscle targets. The result is faster reactions to danger and a better chance of survival. So what is myelin and how does it work?

Imagine you are an animal whose life is threatened by a predator. What you need for survival is a means of getting your nerves to stimulate your muscles really rapidly so that you can escape the danger as soon as possible. How can this be achieved?

Electrical signals move faster along a thicker nerve axon or fibre. So you may imagine that, as animals became bigger and more complex,



one answer to their need for quicker reactions to danger might be to increase the size of the axon diameter. This would allow faster transmission of signals through the nerves to the muscles and so a quicker escape reaction. However, increases in axon diameter require space. Obviously you cannot keep increasing the size of your nerves. They would bump into physical barriers such as the bones of the skull and spine and your overall body size. Nor can you become bigger and bigger to accommodate bigger nerves without sacrificing mobility. This is where myelin comes in. Thanks to myelin signals can be transduced faster through small axons and nerves and brains can remain compact.

Myelin is a complex mixture of proteins and fats produced by cells lying in close contact with the axon membrane called glial cells.

Each glial cell sends out a sheath-like process that wraps around an axon many times to produce a multilayered myelin sheath. (figure 3) This sheath is like insulating tape surrounding an electric wire.



Unlike the tape, however, the myelin sheath is discontinuous. Along the length of the axon there are gaps up to a millimetre apart where the axon is not covered by myelin. The gaps are called ‘Nodes of Ranvier’. This is where electrical signals originate. The signal jumps from one node to the next allowing it to move 100 times faster than in non-myelinated axons. So this was evolution’s clever answer to the need for rapid communication between brain and muscles resulting in a greater chance of survival for animals living in a dangerous world.

It is no surprise that errors in making myelin or wrapping it around the axon result in a number of neurological diseases. A well known example is multiple sclerosis (MS). Scientists are trying to understand the precise molecular interactions between the myelin producing glial cells and the axon. Hopefully, a proper understanding of these processes will lead to therapeutic answers to myelin-associated disorders.


Figure 3.
The process of myelination: a) A Schwann cell nestles itself around an axon. b) While continuously producing myelin, the Schwann cell winds a sheath-like protrusion around the axon yielding multiple layers. c) Finally, these layers are compacted into a tightly packed insulation coat. In the electron micrographic pictures, the myelin coat appears as a thick black line around the axon. A closer look shows the finer structure of the multilayered coat.