The internodes are the myelin segments and the gaps between are referred to as<unk> s. The size and the spacing of the internodes vary with the fiber diameter in a curvilinear relationship that is optimized for maximal conduction velocity. The size of the<unk> s 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<unk>  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<unk>  and is constricted in its diameter. The decreased axon size reflects a higher packing density of neurofilaments in this region, which are less heavily phosphorylated and are transported more slowly. Vesicles and other organelles are also increased at the<unk> s, which suggest that there is a bottleneck of axonal transport in both directions as well as local axonal-glial signaling. When a longitudinal section is made through a myelinating Schwann cell at the<unk> , three distinctive segments are represented: the stereotypic internode, the paranodal region, and the<unk>  itself. In the internodal region, the Schwann cell has an outer collar of cytoplasm, a compact myelin sheath, and inner collar of cytoplasm, and the axolemma. At the paranodal regions, the paranodal cytoplasm loops contact thickenings of the axolemma to form septate –like junctions. In the<unk>  alone, the axolemma is contacted by several Schwann microvilli and contains a dense cytoskeletal undercoating. Differences in the central and peripheral nervous systems Although freeze fracture studies have revealed that the nodal axolemma in both the CNS and PNS is enriched in intra-membranous particles (IMPs) compared to the internode, there are some structural differences reflecting their cellular constituents. In the PNS, specialized microvilli project from the outer collar of Schwann cells and come very close to nodal axolemma of large fibers. The projections of the Schwann cells are perpendicular to the<unk>  and are radiating from the central axons. However, in the CNS, one or more of the astrocytic processes come in close vicinity of the<unk> s. Researchers declare that these processes stem from multi-functional astrocytes, as opposed to from a population of astrocytes dedicated to contacting the<unk> . On the other hand, in the PNS, the basal lamina that surrounds the Schwann cells is continuous across the<unk> . Composition The<unk>  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<unk>  in the central and peripheral nervous systems mostly consist of αNaV1.6 and β1 subunits. 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<unk> s in the CNS and interaction with this molecule enhances the surface expression of Na+ channels. Ankyrin has been found to be bounded to βIV spectrin, a spectrin isoform enriched at<unk>  and axon initial segments. The PNS<unk> s are surrounded by Schwann cell microvilli, which contain ERMs and EBP50 that may provide a connection to actin microfilaments. Several extracellular matrix proteins are enriched at<unk> , including tenascin-R, Bral-1, and proteoglycan NG2, as well as phosphacan and versican V2. At CNS<unk> s, the axonal proteins also include contactin; however, Schwann cell microvilli are replaced by astrocyte perinodal extensions. Molecular organization The molecular organization of the<unk> s corresponds to their specialized function in impulse propagation. The level of sodium channels in the<unk>  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<unk> . 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<unk> s. This highly asymmetric distribution of voltage-gated sodium and potassium channels is in striking contrast to their diffuse distribution in unmyelinated fibers. The filamentous network subjacent to the nodal membrane contains cytoskeletal proteins called spectrin and ankyrin. The high density of ankyrin at the<unk> s may be functionally significant because several of the proteins that are populated at the<unk> s share the ability to bind to ankyrin with extremely high affinity. All of these proteins, including ankyrin, are enriched in the initial segment of axons which suggests a functional relationship. Now the relationship of these molecular components to the clustering of sodium channels at the<unk> s is still not known. Although some cell-adhesion molecules have been reported to be present at the<unk> s inconsistently; however, a variety of other molecules are known to be highly populated at the glial membranes of the paranodal regions where they contribute to its organization and structural integrity.
Node of Ranvier
Sarcoplasmic reticulum
Projection fiber
External limiting membrane
Neuroeffector junction
Internal limiting membrane
Neuromere
Basal dendrite