Nodes of Ranvier

Structure

Myelinated axons are covered by myelin sheath, a modified plasma membrane enveloping the axon. In the peripheral nervous system (PNS) this plasma membrane originates from the Schwann cells, while in the central nervous system (CNS) from the oligodendroglial cells. The main difference between these two types of cells lies in the fact that each oligodendroglial cell can insulate multiple axons or segments of axons while each Schwann cell forms only a single myelin sheath. Despite their molecular and structural differences, the two types of myelin sheath present similar morphological features and functions.

Part of these morphological similarities are four distinct repetitive regions between successive parts of the myelin sheath:

• the node of Ranvier,
• the paranode,
• the juxtaparanode and
• the internode.

Sometimes, the paranode and the juxtaparanode are described as part of the node of Ranvier.

The nodes may lack myelin sheath, but the axon is still covered by either the microvilli of the Schwann cells in the PNS or by processes of the astrocytes in the CNS. In this 1μm long area, the number of voltage-gated sodium ion channels is increased compared to adjacent regions along the axon. Furthermore, the node contains a variety of molecules like ankyrins, cell adhesion molecules and gangliosides in their axonal membrane. Ankyrins serve the anchoring of membrane molecules to the axonal cytoskeleton. Cell adhesion molecules permit the adhesion of the glial to the axon. Finally, gangliosides are lactosylsphingolipids that contain one or more sialic acids and provide cell surface recognition sites for various molecules for processes like cell growth.

The paranode is directly adjacent to the node on each side and plays a crucial role in the compartmentation within the axon. More specifically, in this area, the myelin sheath forms a septate-like junction with the axonal membrane, a structure that limits the movement of ion channels in the membrane and that of the molecules inside the axon.
As for the juxtaparanode and the internode, the first is characterized by an increased number of voltage-gated potassium ion channels and the latter is fully covered in myelin which serves the rapid transmission of the action potential.

The morphology of neurons is important for the nervous system. Learn more about the anatomy of the nervous system with our beginner-friendly quizzes and labeled digrams.

Function

The role of myelin in both the CNS and the PNS is to insulate the nerve fiber and, as a result, to accelerate the conduction of the action potential. In this process, nodes of Ranvier are important for the amplification of the action potential. More specifically node’s voltage-gated sodium ion channels open when an action potential reaches the area. The massive influx of Na⁺ inside the axoplasm leads to the depolarization of the axolemma. This new depolarization is then transmitted to the next node in order to be reamplified. In between the nodes, the myelin insulates the axoplasm from the extracellular fluid and few channels are present. As a result, the depolarization of the axolemma is conducted passively without the necessity of constantly regenerating the action potential, as observed in unmyelinated fibers. Meanwhile, the voltage-gated potassium ion channels of the juxtaparanode open and the outflow of K⁺ initiates the process of repolarization. The regression of the action potential to the previous node is prevented thanks to the refractory period of the latter, as all the voltage-gated sodium channels are already opened.

This type of conduction from one node to the adjacent is called saltatory conduction due to this “jumping like” transmission of the impulse. This phenomenon permits speeds of over 100m/sec in comparison to 0,5-2m/sec in the successive conduction of the unmyelinated fiber. For an unmyelinated nerve fiber to reach a speed comparable to the myelinated it needs to have a diameter of approximately 1mm, which is impossible in the dense environment of the nervous system. At the same time, the regeneration of the active potential only in the nodes of Ranvier provides a more energy efficient way of transmission in comparison to the unmyelinated fibers because less Na+ is required to be pumped out.