Nodes of Ranvier

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 .

The Nodes of Ranvier are the gaps between the myelin insulation of Schwann cells which insulate the axon of neuron. 

The Node of Ranvier is the 1-2 micrometre gap between the glial cells of the myelin sheath. These glial cells are called Schwann cells and they help to electrically insulate the neuron. Node of Ranviers are only present when the axon of a neuron is myelinated. Myelination allows for an increase rate of action potential transmission due to action potentials "jumping" between Node of Ranviers, this is called saltatory conduction.

The movement of sodium ions to depolarise the membrane can only occur at the Node of Ranvier due to that fact that voltage-gated sodium ion channels are only able to open and determine the movement of sodium ions at the Node of Ranvier. The Schwann cells of the myelin sheath block the movement of sodium ions elsewhere along the axon.

However, in Multiple Sclerosis the myelin sheath is degraded which leads to demyelination. This allows for action potentials to move as current loops instead of by saltatory conduction, which slows the transmission of action potentials and therefore a decrease in reaction time.