What is the myelin sheath?

The myelin sheath is a layer of insulating, fatty material that encases some neurones. The sheath is composed of Schwann cells that have gaps called nodes of Ranvier between them. Potassium and sodium ions can only diffuse in and out of the neurone at these gaps. This allows saltatory or 'jumping' conduction where the local currents are elongated as the sodium ions have to diffuse further along the neurone before the action potential can occur. The action potential appears to 'jump' from one node of Ranvier to another.