Node of Ranvier

The nodes of Ranvier are the gaps formed between myelin sheath cells along the axons. The sodium channels are densely localized at the nodes of Ranvier. 

The transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

Node of Ranvier The small gap between adjacent segments (internodes) of the myelin sheath. 

The axon emerges from a neuron as a slender thread and frequently does not branch until it nears its target. In contrast to the dendrite and the soma, the axon is myelinated frequently, thus increasing its efficiency as a conducting unit. Myelin, a spirally wrapped membrane (see Ch. 4), is laid down in segments, or internodes, by oligodendrocytes in the CNS and by Schwann cells in the PNS. The naked regions of axon between adjacent myelin internodes are known as nodes of Ranvier (see below). 

Myelination in the PNS is preceded by invasion of the nerve bundle by Schwann cells, rapid multiplication of these cells and segregation of the individual axons by Schwann cell processes. Smaller axons (<1 pm), which will remain unmyelinated, are segregated several may be surrounded by one Schwann cell, each within its own pocket, similarly to the single axon shown in Figure 4-10A. Large axons (>1 pm) destined for myelination are enclosed singly, one cell per axon per internode. These cells line up along the axons with intervals between them the intervals become the nodes of Ranvier. 

The action potential is propagated by local spread of depolarization 99 Membranes at nodes of Ranvier have high concentrations of Na+ channels 99... 

Throughout neurons and glia, enriched in growth cones and in membrane cytoskeleton Co-distributed with most MFs Enriched in membrane cytoskeleton Membrane cytoskeleton, distinct forms in axon, dendrite and nodes of Ranvier Growing neurites... 

A third myelin inhibitory protein, OMgp, is a GPI-linked protein expressed by oligodendrocytes [18], OMgp is a relatively minor component of myelin, believed to be localized to the paranodal loops, next to the node of Ranvier. OMgp contains a leucine-rich repeat (LRR) domain and a C-terminal domain with serine/threonine repeats. Like MAG, OMgp is also found in the PNS. Like Nogo, OMgp is also expressed in adult neurons. 

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