Myelin is a dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 of a neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

. It is essential for the proper functioning of the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. Myelin is an outgrowth of a type of glial cell
Glial cell
Glial cells, sometimes called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system...

. The production of the myelin sheath is called myelination. In humans, the production of myelin begins in the fourteenth week of fetal development, although little myelin exists in the brain at the time of birth.