Non-technical summary
The nerve cells or neurons running from your spine to your little toe may be over a metre long. That’s an extraordinary length for a cell that’s only around 50 microns (50 thousandths of a millimetre) in diameter. Electrical activity in very small neurons can be transmitted by simple diffusion of charged particles (ions). This will clearly not work over any great distance – the signal will peter out long before it reaches the other side. One way of dealing with this problem is by making neurons bigger, which permits more ions to flow through the axon (the section of the neuron that transmits activity). The squid nervous system for example, contains a giant axon around 1mm in diameter. That may not sound particularly giant, but it’s 20,000 times the size of a typical motor neuron. Squid giant axons are thus useful for research – work on them that elucidated many fundamental electrical properties of neurons won two Cambridge researchers, Alan Hodgkin and Andrew Huxley, a Nobel Prize in 1963.
If you want to create a more complex nervous system, you need a very high density of neurons. A one millimetre diameter is no good. Evolution has thus furnished nerve cells with an exquisite living insulator – other cells wrap around them hundreds of times forming a structure called the myelin sheath. By insulating the neurons, fast and reliable transmission of electrical activity is possible over distance, while keeping neurons a manageable size. This is important not only in motor neurons but throughout the nervous system. Disruption of the myelin sheath is a hallmark of a number of neurological diseases, including Multiple Sclerosis.
The inside of a neuron, although specialised to conduct electrical signals, does not transmit them as well as say, copper wire. The signal degrades and must be periodically regenerated. Regular gaps in the myelin sheath called ‘nodes of Ranvier, are the nerve’s booster stations. Exquisitely specific molecular and cellular mechanisms are involved in the formation, function and maintenance of these nodes, billions of which are present in your nervous system and indispensable to its function: your every thought, sensation and movement.
Introduction
The evolutionary growth of the nervous system has required a high concentration of axons conducting rapidly over distance1. Size constraints preclude increasing axon-diameter to increase speed of conduction. The myelin sheath increases transmembrane resistance and reduces capacitance to increase conduction velocity2. This sheath has regular gaps called nodes of Ranvier. This paper examines the function of the nodes of Ranvier in facilitating rapid, efficient, reliable and unidirectional propagation of action potentials (APs) and relates this function to the nodes’ cellular and molecular structure.
Structure and function of myelinated axons
Myelin is composed of stacked lipid membrane layers containing myelin proteins. Details of the structure and development of the myelin are beyond the scope of this paper, several reviews are available1, 3, 4. It is produced by glial cells which enwrap axons, forming an electrical insulator. Myelin-ensheathed areas (internodes) are regularly interspaced with gaps in myelin – the nodes of Ranvier, around 1μm in length. These are the only areas in which the axon is in contact with the extracellular fluid. The internode distance bears a stereotyped, approximately 100-fold ratio to axon diameter5. Other axonal subdomains – the paranode and juxtaparanode - connect nodes and internodes. These areas play important roles in mediating axoglial contact, limiting electrical activity and restoring resting potentials but their function is largely beyond the scope of this paper.
In the central nervous system (CNS) the myelinating glia are oligodendrocytes, each of which may myelinate several internodes in several axons. In the peripheral nervous system (PNS), a Schwann cell myelinates a single internode of one axon. Nodal areas are contacted by Schwann cell microvilli in the PNS and by perinodal astrocytes in the CNS4.
A critical feature of myelinated axons is the restriction of sodium channels, and hence sodium fluxes to nodal areas, with almost no transmembrane ion flux at the well-insulated internodes. An AP at one node can thus rapidly, passively spread down the internode without diffusional loss of depolarization. On reaching the next node, the depolarization triggers a new AP6. As APs appear to jump from node to node, this process is called “saltatory conduction”. This system is several thousand-fold more energy efficient than increasing axon diameter6. It is achieved by high density clustering of specific channels (see below) and permits very reliable propagation, with a safety factor (the ratio between current generated and a stimulating current) of 5-87. Unidirectional propagation is ensured by the characteristics of voltage-gated sodium channels (Navs), whose inactivation lasts longer than internodal conduction.
Ion channels
The node of Ranvier may have evolved from the axon initial segment (AIS)8 and shares several characteristics with it: high-density clustering of Navs and similar cytosketal scaffolding, cellular adhesion molecules (CAMs) and extra-cellular matrix (ECM) proteins9. Specific ion channels are expressed in the AIS and the node. The CNS Nav is a heterotrimer composed of one α-subunit and two β-subunits10. Several forms of the pore-forming α-subunit exist; expression in the various axonal subdomains is subject to complex spatio-temporal regulation. Retinal ganglion cells have myelinated and unmyelinated regions and are thus useful for studying Nav subtype expression. During development of these cells, Nav1.2 is initially expressed in nonmyelinated regions and immature nodes; as myelination progresses the node expresses Nav1.611. It has been suggested that Nav subtype switching modifies nodal conduction characteristics to suit the high-frequency firing of myelinated axons5. Circumstantial evidence comes from studies of the axon initial segment (AIS). Here, Nav subtype expression is very tightly spatially regulated12 and profoundly influences electrical properties including AP threshold13. β-subunits of Nav have important roles in the node, including modulating sodium current, mediating channel delivery to the membrane and functioning as cell adhesion molecules (CAMs)10. In addition to the Nav, two voltage gated potassium channels are also clustered at nodes – Kv7.2 and Kv7.3, expressed by theKCNQ2/3 genes respectively14. Kv7.2/3 heteromers produce the M-current, a hyperpolarizing current that can be inhibited by cholinergic or muscarinic agonists and therefore may contribute to circuit plasticity. Kv7.2 homotetramers produce the slow nodal potassium current, a slow activating, non-inactivating current localised to the node which dampens excitability15. This may serve to attenuate aberrant neuronal activity thus increasing the reliability of nodal function; mutations in these channels and their localizing proteins are associated with excessive motor activity and seizures in mice and humans15, 16.
Cytoskeletal architecture
The key structural characteristic of both the AIS and the nodes of Ranvier is the sodium channel density (1500/μm2) that allows rapid AP initiation and propagation7 From the early chordata on (i.e. cotemporal with the evolution of myelination), Nav and Kv7 channels have evolved specific domains for interaction with the cytoskeletal scaffolding/adaptor protein ankyrinG 15. AnkyrinG is concentrated at the AIS and nodes and is connected to the underlying cytoskeleton by βIV-spectrin17. This constitutes a spatially anchored adhesion complex for the appropriate ion channels
Cellular Adhesion Molecules and interactions
A key open question in the study of the node of Ranvier is how membrane proteins can be localized with such exquisite spatial specificity to particular axonal subdomains. Several distinct mechanisms involving glial and axonal structures may be at work, particularly given the structural differences betweens myelination in the CNS and PNS.
In the PNS, Schwann cell microvilli express a protein named gliomedin and the adhesion molecule NrCAM. The axon expresses the adhesion molecule NF-18614, 18. Gliomedin is cleaved from the Schwann cell surface, assembles into large multimers and is incorporated into the ECM3. Gliomedin binding is sufficient to concentrate axonal NF-186 at developing nodal areas; NF-186 subsequently binds AnkyrinG 19. As we have seen, AnkyrinG associates with βIV-spectrin and ion channels, resulting in ion channel clustering. As an additional mechanism, Schwann cells have been shown to actively force Navs away from internodes, ensuring that they are limited to the node and AIS20. This protects the speed and efficiency of saltatory conduction.
Study of nodal structure in the CNS is hampered by a paucity of suitable in vitro models, however the mechanisms are understood to be distinct from those in the PNS. The paranodal junction plays a significant role in the CNS as here, paranodal structures localize before nodal ones. Moreover, paranodal reconstitution in the absence of NF is able to induce channel clustering21. Paranodal activity is not the full story however – clustering occurs in the absence of a paranode. A longstanding finding - that non-contacting oligodendrocytes can trigger Nav clustering in cultured axons22 – raises the intriguing possibility that a glial-derived clustering factor exists that has not yet been identified3, 23.
Extracellular Matrix Molecules
The role of the ECM protein gliomedin in PNS ion-channel clustering has been discussed above. An extensive array of specialized ECMs is present at the CNS nodes of Ranvier. These include brevican, tenascin-R, V2-versican, oligodendrocyte-myelin glycoprotein (OMgp), Bral1 and phosphacan3, 14. Individual knockouts have demonstrated no single protein critical for node function or formation and they may act together redundantly. V2-Versican for example, is secreted by perinodal astrocytes and assembles tenascin-R and phosphacan18, 24. The resulting complex binds the CAMs contactin and NF-186 and thus may trigger channel clustering. ECM proteins may have additional roles including cluster stabilization, receptor modulation and ion buffering14. It has been proposed that Bral1 limits perinodal cation diffusion and pools extracellular sodium to facilitate saltatory conduction25, 26. Tenascin-R has been demonstrated to bind to the beta-subunits of Navs to potentiate sodium currents27.
Conclusions and future directions
Key open questions include : how ankyrinG restriction is achieved, how internode distance is fixed, factors preventing channel diffusion, the role of Nav subtype switching at the node and the precise role of the ECM proteins. The recent demonstration that functional Navs are only 3-times more concentrated at the AIS and node than at the soma (in contrast to previously believed 30-50 times)28 suggests other mechanisms, such as channel gating or subtype-ratio, may be crucial to nodal function29. Future research may reveal a much more plastic and dynamic node than previously considered. Remarkably, AIS position and length may be dynamically regulated by input30 and internodal distance may be dynamically regulated in the chick auditory system to achieve binaural coincidence detection31. The mechanisms by which this occurs are unknown but clearly require complex axoglial communication and may include regulation of nodal cytoarchitecture, channel localization, subtype specificity and perinodal ECM milieu. Clearly, the exquisite cellular and molecular structure of the node of Ranvier and its roles in promoting speed, reliability, efficiency and plasticity of action potential propagation are only beginning to be revealed.
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