The nodes of Ranvier are regular breaks in the myelin sheath that surrounds the axonal extension of nerve cells (neurons).

Nodes of Ranvier links:
Structure and Function of Nerve Cells
Intercellular Interactions of Axons and Myelinating Glia at the Nodes of Ranvier
Neuron-Glia Interactions at the Node of Ranvier
Electrophysiology
NCMIR: Journal Abstracts

MS Glossary
All About Multiple Sclerosis