Nodes of Ranvier are uninsulated and highly enriched in ion channels, allowing them to participate in the exchange of ions required to regenerate the action potential. Nerve conduction in myelinated axons is referred to as saltatory conduction due to the manner in which the action potential seems to “jump” from one node to the next along the axon. This results in faster conduction of the action potential.
Many vertebrate axons are surrounded by a myelin sheath, allowing rapid and efficient saltatory (“jumping”) propagation of action potentials. The contacts between neurons and glial cells display a very high level of spatial and temporal organization in myelinated fibers. The myelinating glial cells - oligodendrocytes in the central nervous system (CNS), and Schwann cells in the peripheral nervous system (PNS) - are wrapped around the axon, leaving the axolemma relatively uncovered at the regularly spaced nodes of Ranvier.
Nodes Of Ranvier Structure
The internodes are the myelin segments and the gaps between are referred to as nodes. The size and the spacing of the internodes vary with the fiber diameter in a curvilinear relationship that is optimized for maximal conduction velocity1.
The size of the nodes span from 1–2 μm whereas the internodes can be up to (and occasionally even greater than)1.5 millimetres long, depending on the axon diameter and fiber type.
The structure of the node and the flanking paranodal regions are distinct from the internodes under the compact myelin sheath, but are very similar in CNS and PNS. The axon is exposed to the extra-cellular environment at the node and is constricted in its diameter.
The nodes of Ranvier Na+/Ca2+ exchangers and high density of voltage-gated Na+ channels that generate action potentials. A sodium channel consists of a pore-forming α subunit and two accessory β subunits, which anchor the channel to extra-cellular and intra-cellular components. The nodes of Ranvier in the central and peripheral nervous systems mostly consist of αNaV1.6 and β1 subunits.2
The extra-cellular region of β subunits can associate with itself and other proteins, such as tenascin R and the cell-adhesion molecules neurofascin and contactin. Contactin is also present at nodes in the CNS and interaction with this molecule enhances the surface expression of Na+ channels.
The molecular organization of the nodes corresponds to their specialized function in impulse propagation. The level of sodium channels in the node versus the internode suggests that the number IMPs corresponds to sodium channels.
Potassium channels are essentially absent in the nodal axolemma, whereas they are highly concentrated in the paranodal axolemma and Schwann cell membranes at the node.
The exact function of potassium channels have not quite been revealed, but it is known that they may contribute to the rapid repolarization of the action potentials or play a vital role in buffering the potassium ions at the nodes. This highly asymmetric distribution of voltage-gated sodium and potassium channels is in striking contrast to their diffuse distribution in unmyelinated fibers.
Salzer J. L. (1997). Clustering sodium channels at the node of Ranvier: close encounters of the axon-glia kind. Neuron. 18 (6): 843–846. doi:10.1016/S0896-6273(00)80323-2. ↩︎
Kaplan M.R.; Cho M.H.; Ullian E.M.; Isom L.L.; Levinson S.R.; Barres B.A. (2001). Differential control of clustering of the sodium channels Na(v)1.2 and Na(v)1.6 at developing CNS nodes of Ranvier. Neuron. 30 (1): 105–119. doi:10.1016/S0896-6273(01)00266-5 ↩︎