The nerves that are insulated by a sheath or myelin are called myelinated nerve fibres whereas the nerves which lack such sheath are called non-myelinated nerve fibres. Nodes of Ranvier are microscopic gaps found within myelinated axons. Their function is to speed up propagation of Action potentials along the axon via saltatory conduction and it is absent in non-myelinated axons. Collateral branches of the nerve fibre arise at the nodes of Ranvier which is absent in non-myelinated axons. In a myelinated nerve fibre, the action potential is conducted from one node to another whereas in an non-myelinated nerve fibre, the action potential is not conducted from node to node. It is carried along the whole length of the nerve fibre. If the axon is covered with myelin sheath, the nerve impulse is faster and in the unmyelinated nerve fibre impulse rate is slow.