Describe the structure and function of the myelin sheath?

The myelin sheath is a fatty substance which surrounds and insulates neuron axons (i.e. it does not allow potassium or sodium ions to pass through it), which allows fast propagation of action potentials along neurones in the nervous system. Action potentials instead 'jump' along the axon, by a process known as saltatory conduction.  This makes passing information in both the central and peripheral nervous system faster and more efficient.