Just like the electrical wires in the national grid, the electrical connections between brain cells, as shown in this picture, have to be well insulated. If this insulation is lost, neurons lose their ability to communicate efficiently. This is what happens in several neurological diseases including multiple sclerosis (MS).
This colorful picture shows the wiring in a developing brain. Axons (red) are the cables that neurons use to transmit their information, often over relatively long distances and taking highly circuitous routes. The other colors represent different areas of the brain.