Myelinated axons are a portion of a neuron, or nerve cell, that is encapsulated by a fatty layer called the myelin sheath. Much like the rubber coating on an electrical cord, the myelin sheath insulates and protects the axon of the nerve cell as well as conducts the electrical impulse along the nerve. Nerves are made up of a network of neurons, specialized cells that send electrical signals either from the central nervous system—the brain and spinal cord — to the muscles or from the body’s various tissues back to the central nervous system. Myelinated axons conduct these signals from nerve cell to nerve cell along this pathway, ensuring that the message quickly gets where it needs to go.
A single neuron is made up of a cell body known as the soma, which contains the nucleus of the nerve cell, as well as the axon, which is like a tail or cord stretching from the soma to the soma of the next neuron. At the end of the axon are small branches called axon terminals. These connect to similar branches protruding from the soma of the subsequent neuron known as dendrites. The axon is long and skinny and acts like a kind of cellular electrical cord that plugs into the dendrites of the next cell.
Like any electrical cord, the conducting material within must be enclosed by an outer layer. The rubber encapsulating an electrical cord serves to protect the wire as well as move the electrical current along so that it cannot transfer to other surfaces with which it comes into contact. Similarly, myelinated axons are those that are protected by a layer of myelin known as the myelin sheath, 40 percent of which is made from water, 42 to 51 percent from fats, and nine to 18 percent from proteins.
Beginning in the womb as the fetus is still growing, the formation of myelinated axons occurs all the way into adolescence. In peripheral neurons, nerve cells making up the nerves found between the spinal cord and the body’s tissues, myelin is produced by support cells along the axon known as Schwann cells. Brain and spinal cord neurons, on the other hand, produce myelin by way of oligodendrocytes. Oligodendrocytes throw out cellular projections known as processes that encircle the axon to form the myelin sheath.
Both of these cell types serve the same function: to create an insulating layer around the axon that enhances the conductive potential of the neuron. In other words, myelinated axons can deliver the nervous system impulse quickly from cell to cell because the myelin itself as well as the solution in which nerve cells are suspended are highly effective conducting materials. Additionally, by encasing the axon much like the wire enclosed in an electrical cord, the myelin sheath prevents the electrical signal from escaping and thereby transferring to other cells, ensuring that it stays its course on its way to or from the central nervous system.