The nodes of Ranvier allow an action potential to propagate quickly down an axon. The nodes of Ranvier are small gaps that are approximately 1 micrometer wide. These gaps form on axons between the myelin sheath.
Myelin sheaths are comprised mostly of fat cells. Fat is a good insulator and does not allow for quick propagation of electrical signals through action potentials. The nodes of Ranvier are free of myelin sheath cells. They allow action potentials to pick up speed so they can propagate down the axon and reach the target cell or tissue. The rapid conduction of action potentials from one node to another is known as saltatory conduction.