Ranvierin

Ranvierin, or nodes of Ranvier, are gaps in the myelin sheath along vertebrate myelinated axons. They occur at regular intervals between internodal segments produced by oligodendrocytes in the central nervous system or by Schwann cells in the peripheral nervous system. Each node provides a site where the axonal membrane is exposed to the extracellular space, allowing rapid electrical changes to occur.

The node is typically about 1 micrometer long and contains a high density of voltage-gated sodium channels, along with other components of the axonal membrane. This arrangement supports saltatory conduction, in which action potentials are regenerated at successive nodes, enabling much faster nerve impulse transmission than would be possible along unmyelinated fibers. The region flanking the node, called the paranode, contains axo-glial junctions that help stabilize the node and regulate channel clustering. The juxtaparanodal region beyond the paranode houses potassium channels that contribute to repolarization and membrane potential stabilization.

Formation and maintenance of nodes involve coordinated interactions between the axon and glial cells. Specific axonal proteins and glial adhesion molecules organize sodium channel clustering at the node and establish the distinct nodal, paranodal, and juxtaparanodal domains. Disruption of these structures can impair conduction and is implicated in demyelinating conditions.

Historically, the nodes are named after Louis-Antoine Ranvier, who described them in the late 19th century. The study of ranvierin continues to illuminate fundamental aspects of nervous system function and disease.