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q1
Answer:
1=dendrites
2=Node of ranvier
3=synapse
q2=Nodes of Ranvier are absolutely essential for rapid, faithful, and efficient AP conduction in the nervous system. Although these structures have traditionally been viewed simply as passive contributors to AP propagation, recent work has begun to show that nodes play an active role in regulating neuronal excitability. That nodal properties are plastic remains an exciting and provocative idea. To uncover potential mechanisms regulating nodal plasticity will require a concerted effort to understand the mechanisms of node of Ranvier assembly during development, disassembly during disease or injury, and reassembly during repair. Finally, the discovery that node-associated proteins play prominent roles in cognitive disorders and other diseases or injuries emphasizes the importance of these excitable domains in nervous system function, and the need to understand their roles in regulating the structure and function of myelinated axons.
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Glial Cell Development
Laura Fontenas, Sarah Kucenas, in Reference Module in Life Sciences, 2017
SCs and Establishment of the Nodes of Ranvier
Nodes of Ranvier are at the core of saltatory conduction along myelinated axons (Fig. 1(d)). They contain all of the molecular machinery responsible for the propagation of action potentials along myelinated nerves (Black et al., 1990). If myelin is necessary for the fast propagation of action potentials by insulating the axons, the nodes of Ranvier are all the more important for this purpose as they regenerate the action potentials along their entire course by allowing current to enter the axolemma through voltage-gated sodium channels (Black et al., 2002). Although the components of the nodes are well established, the molecular mechanisms that give the nodes their functional organization in vivo have remained poorly understood until recently. Investigations in zebrafish have defined a number of proteins involved in node of Ranvier formation and function. A study showed the requirement of the N-ethylmaleimide sensitive factor (NSF), a key protein involved in membrane fusion, for node of Ranvier organization along myelinated axons (Pogoda et al., 2006; Woods et al., 2006). This study, involving a neuronal factor, highlights the crucial dialog between neurons and their associated glia, which are required for the proper organization of myelinated axons and is consistent with a previous study conducted in rat (Vabnick et al., 1996). This report also demonstrated that neuronal activity and PNS myelination is different from the CNS, as elimination of action potentials or synaptic release does not affect myelin gene expression nor node of Ranvier organization (Woods et al., 2006).
It has always been an intriguing question whether myelin is responsible for creation of nodes or, if on the contrary, nodes are responsible for the shape of the myelin internodes (Fig. 1(d)). A report shed light on this mystery by demonstrating that in numerous mutants lacking SCs (e.g., erbb2, erbb3, sox10/cls), a significant number of aberrant clusters of sodium channels were distributed all along peripheral axons, suggesting that SCs control sodium channel clustering (Voas et al., 2009). In gpr126 mutants, where SCs are arrested at the promyelinating stage, a significant decrease in the number of clusters could be seen in comparison to embryos lacking SCs, but clustering still existed. Taken together, these data show how important the interaction between neurons and glia are for axonal organization.
Many studies have contributed to enrich our understanding of peripheral myelination and have revealed key genes and proteins involved in SC migration, differentiation, and in the initiation of myelination. Additionally, different behaviors of SCs have been observed depending on the nerve they are associated with. These differences now raise a new question. Do SCs that associate with the sensory and motor nerves really have the same identity? Future studies will be needed to dissect whether these two subpopulations are molecularly and functionally distinct.
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Answer:
Names in figure: 1. Dendrites
2.Node of ranvire
3.Axon terminal