What is the function of the myelin sheath?

Neurones can either be myelinated - they are sorrounded by a myelin sheath, or unmyelinated - they are not surrounded by a myelin sheath.
The myelin sheath is made up of Schwann cells and there are gaps in the sheath called the Nodes of Ranvier.
The main functions of the myelin sheath are:
1) It acts as an electrical insulator for the neurone - it prevents electrical impulses travelling through the sheath.
2) The sheath prevents the movement of ions into or out of the neurone/ it prevents depolarisation.
3) It speeds up conduction/ transmission of an electrical impulse in the neurone - impulses cannot travel through the sheath (the sheath acts as an electrical insulator), instead, impulses 'jump' from a gap in the myelin sheath to another gap (it jumps from one Node of Ranvier to another Node). This is a process called Saltatory Conduction.