What is the role of a myelin sheath in neurones?

The role of the myelin sheath is to speed up the conduction of an action potential along neurones. It's made up of Schwann cells that wrap around neurones, forming an insulating layer with gaps inbetween called nodes. Once the action potential is initiated at the axon hillock, it passes down the neurone to reach the axon terminal and as it does so, the action potential appears to 'jump' from node to node. Action potentials are elicited at each node since there is no insulating myelin present. In non-myelinated neurones the speed of conduction is a lot slower as action potentials are elicited at each point until it reaches the axon terminal.