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You are watching: Which part of a neuron is often encased by a fatty myelin sheath

Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and also Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.


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Correspondence come Pierre Morell, Neuroscience Center, college of phibìc Carolina in ~ Chapel Hill, Chapel Hill, phibìc Carolina 27599-7250.

The myelin sheath is a greatly extended and modified plasma membrane wrapped about the nerve axon in a spiral fashion <1>. The myelin membranes originate from and also are a part of the Schwann cell in the peripheral nervous device (PNS) and the oligodendroglial cell in the central nervous mechanism (CNS) (see Chap. 1). Each myelin-generating cell furnishes myelin for just one segment of any given axon. The periodic interruptions where short portions that the axon room left uncovered by myelin room the nodes that Ranvier, and they are crucial to the functioning of myelin.


Myelin facilitates conduction

Myelin is an electrical insulator; however, its duty of facilitating conduction in axons has actually no precise analogy in electrical circuitry. In unmyelinated fibers, impulse conduction is propagated by regional circuits the ion existing that circulation into the active an ar of the axonal membrane, with the axon and out through surrounding sections that the membrane (Fig. 4-1). These neighborhood circuits depolarize the adjacent piece the membrane in a continuous, sequential fashion. In myelinated axons, the excitable axonal membrane is exposed come the extracellular room only in ~ the nodes the Ranvier; this is the place of sodium channels <2>. As soon as the membrane at the node is excited, the local circuit created cannot flow through the high-resistance sheath and, therefore, flows the end through and depolarizes the membrane in ~ the following node, which might be 1 mm or farther far (Fig. 4-1). The low capacitance of the sheath method that small energy is compelled to depolarize the staying membrane in between the nodes, which outcomes in local circuit dispersing at an boosted speed. Energetic excitation that the axonal membrane jumps from node come node; this kind of advertise propagation is called saltatory conduction (Latin saltare, “to jump”). Such activity of the tide of depolarization is much much more rapid 보다 in unmyelinated fibers. Furthermore, because only the nodes the Ranvier space excited during conduction in myelinated fibers, Na+ flux into the nerve is much much less than in unmyelinated fibers, whereby the entire membrane is involved. An example of the advantage of myelination is obtained by compare of two various nerve fibers, both of which conduct at 25 m/sec in ~ 20°C. The 500-mm diameter unmyelinated large axon that the squid needs 5,000 time as much energy and also occupies around 1,500 times as much space as the 12-mm diameter myelinated nerve in the frog.


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Figure 4-1

Impulse conduction in unmyelinated (top) and also myelinated (bottom) fibers. Arrows present the circulation of action currents in neighborhood circuits into the active an ar of the membrane. In unmyelinated fibers, the circuits circulation through the nearby piece the membrane, (more...)


Conduction velocity in myelinated fibers is proportional to the diameter, while in unmyelinated yarn it is proportional come the square root of the diameter. Thus, differences in power and an are requirements in between the two species of fiber space exaggerated at greater conduction velocities. If nerves were not myelinated and also equivalent conduction velocities were maintained, the person spinal cord would must be as big as a good-sized tree trunk. Myelin, then, facilitates conduction when conserving an are and energy <3>.


Myelin has actually a characteristics ultrastructure

Myelin, and many the its morphological features, such together nodes the Ranvier and also Schmidt-Lantermann clefts, have the right to be seen conveniently with light microscopy (Fig. 4-2). More insight originates from biophysical research studies of frameworks with parallel axons: sciatic nerve as representative the the PNS and optic nerve or tract as representative the the CNS. Myelin, as soon as examined by polarized light, exhibits both a lipid-dependent and a protein-dependent birefringence. Low-angle X-ray diffraction studies of myelin carry out electron-density plots of the repeating unit that present three peaks which correspond to protein plus lipid polar groups and also two troughs which exchange mail to lipid hydrocarbon chains. The repeat distance varies somewhat depending upon the species and whether the sample is from the CNS or the PNS. Thus, the results from these two approaches are consistent with a protein—lipid—protein—lipid—protein structure, in i beg your pardon the lipid part is a bimolecular leaflet and nearby protein great are different in part way. Data because that mammalian optic nerve show a repeat distance of 80 Å (Fig. 4-3). This spacing deserve to accommodate one bimolecular class of lipid (about 50 Å) and also two protein class (about 15 Å each). The key repeating unit of 2 such unify unit membrane is twice this figure, or 160 Å <5>. Although that is useful to think that myelin in terms of alternate protein and lipid layers, this ide has been modified somewhat to be compatible through the “fluid mosaic” design of membrane structure, which consists of intrinsic transmembrane proteins and extrinsic proteins.


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Figure 4-2

Light micrograph that a 1-μm Epon ar of rabbit peripheral nerve (anterior root) stained through toluidine blue. The myelin sheath appears as a thick black ring approximately the pale axon. ×600, prior to 30% reduction. (Courtesy of Dr. Cedric Raine.) (more...)


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Information worrying myelin structure is also easily accessible from electron-microscopic studies, i beg your pardon visualize myelin as a series of protein layers appearing as alternate dark and also less dark lines separated by lipid hydrocarbon chain which appear as unstained area (Figs. 4-4–4-7). There is asymmetry in the staining that the protein layers. The much less dark, or intraperiod, heat represents the very closely apposed external protein coats of the initial cell membrane; the membranes are not in reality fused because they deserve to be fixed as a dual line in ~ high resolution (Figs. 4-6 and 4-7). The dark, or significant period, heat is the fused, inside protein coat of the cell membrane. The repeat distances observed by electron microscopy are less than those calculated from the low-angle X-ray diffraction data, a an effect of the significant shrinkage that takes location after fixation and also dehydration. However, the distinction in periodicity between PNS and CNS myelin is maintained; peripheral myelin has an median repeat distance of 119 Å and main myelin, 107 Å.


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Figure 4-4

Electron micrograph of a single peripheral nerve fiber from rabbit. Keep in mind that the myelin sheath has actually a lamellated structure and also is surrounded by Schwann cabinet cytoplasm. The outer mesaxon (arrowhead) deserve to be watched at the reduced left. AX, axon. ×18,000. (more...)


Nodes the Ranvier. Two nearby segments the myelin on one axon are separated by a node the Ranvier. In this region, the axon is not spanned by myelin. In ~ the paranodal an ar and the Schmidt-Lantermann clefts, the cytoplasmic surfaces of myelin room not compacted and also Schwann or glial cabinet cytoplasm is had within the sheath. To visualize this structures, one may refer to numbers 4-8 and also 4-9, which display that if myelin were unrolled indigenous the axon, it would certainly be a flat, spade-shaped sheet surrounding by a pipe of cytoplasm. Thus, as displayed in electron micrographs of longitudinal part of axon paranodal regions, the major dense line created by apposition of the cytoplasmic deals with opens up in ~ the edges of the sheet, enclosing cytoplasm in ~ a loop (see Figs. 4-3 and 4-9). These loop-shaped terminations that the sheath in ~ the node are referred to as lateral loops. The loops form membrane complexes through the axolemma called transverse bands, vice versa, myelin in the internodal an ar is separated indigenous the axon by a void of periaxonal space. The transverse bands room helical structures that seal the myelin to the axolemma yet provide, through spaces between them, a tortuous route from the extracellular room to the periaxonal space.


Figure 4-8

A diagram reflecting the illustration of CNS myelin if it were unrolled indigenous the axon. One deserve to visualize this structure emerging from figure 4-3 if the glial cell procedure were pulled straight up and the myelin great separated at the intermediate duration line. (more...)


Schmidt-Lantermann clefts room structures whereby the cytoplasmic surface of the myelin sheath have actually not compacted to kind the significant dense heat and, instead, contain Schwann or glial cell cytoplasm (Fig. 4-9). These regions are usual in peripheral myelinated axons yet rare in the CNS. This inclusions that cytoplasm are present in each layer the myelin. The clefts can be visualized in the unrolled myelin sheet as tubes that cytoplasm similar to the tubes making up the lateral loops but in the middle regions the the sheet, quite than at the edge (Fig. 4-9).


Myelin is an expansion of a cabinet membrane

In the PNS, myelination is came before by intrusion of the nerve bundle by Schwann cells, fast multiplication of these cells and segregation the the separation, personal, instance axons by Schwann cell processes. Smaller axons (≤1 μm), i beg your pardon will stay unmyelinated, space segregated; several might be attached in one cell, each within its very own pocket, comparable to the structure displayed in number 4-10A. Large axons (≥1 μm) destined because that myelination room enclosed singly, one cell every axon per internode. This cells heat up along the axons through intervals between them; the intervals become the nodes the Ranvier.


Figure 4-10

Myelin formation in the peripheral nervous system. A: The Schwann cell has actually surrounded the axon, but the outside surfaces that the plasma membrane have not however fused in the mesaxon. B: The mesaxon has actually fused right into a five-layered structure and also spiraled once (more...)


Before myelination, the axon lies in an invagination of the Schwann cell (Fig. 4-10A). The plasmalemma the the cell climate surrounds the axon and joins to type a double-membrane structure that communicates through the cabinet surface. This structure, dubbed the mesaxon, elongates around the axon in a spiral fashion (Fig. 4-10). Thus, development of myelin topologically each other rolling up a resting bag; the mesaxon winds about the axon, and also the cytoplasmic surface condense into a compact myelin sheath and type the major dense line. The two exterior surfaces type the myelin intraperiod line.

In the CNS, the frameworks of myelin are created by the oligodendroglial cell <7>. This has many similarities but likewise points of distinction with respect to myelination in the PNS. CNS nerve fibers are not separated by connective tissue, nor space they surrounding by cabinet cytoplasm, and certain glial nuclei space not obviously linked with specific myelinated fibers. CNS myelin is a spiral structure similar to PNS myelin; it has an within mesaxon and also an external mesaxon the ends in a loop, or tongue, the glial cytoplasm (Fig. 4-3). Unequal the peripheral nerve, whereby the sheath is surrounded by Schwann cell cytoplasm, the cytoplasmic tongue in the CNS is limited to a small portion of the sheath. This glial tongue is consistent with the plasma membrane the the oligodendroglial cell with slender processes. One glial cell can myelinate 40 or more separate axons <8>.

Myelin deposition in the PNS may an outcome in a single axon having up come 100 myelin layers; therefore, that is improbable that myelin is laid under by a an easy rotation of the Schwann cell nucleus around the axon. In the CNS, together a postulate is creates by the reality that one glial cell deserve to myelinate number of axons. During myelination, over there are boosts in the length of the internode, the diameter of the axon and the variety of myelin layers. Myelin, therefore, expands in all planes in ~ once. Any type of mechanism come account for this development must assume that the membrane mechanism is able come expand and contract and that layers slip over each other.


Myelin can be diverted in high yield and purity through conventional techniques of subcellular fractionation

If CNS tissue is homogenized in media of low ionic strength, myelin peels off the axons and reforms in motor of the size variety of nuclei and mitochondria. Since of your high lipid content, these myelin vesicles have actually the lowest intrinsic density of any membrane portion of the nervous system. Actions for isolation that myelin take advantage of both the huge vesicle size and also the low density <9>.

In a widely supplied method, a homogenate the rodent worried tissue, or dissected white issue in the situation of bigger animals, in isotonic sucrose (0.3 M) is layered directly onto 0.85 M sucrose and also centrifuged in ~ high speed. Mitochondria and synaptosomes sediment with the denser sucrose, and also many the the smaller membrane pieces from various other organelles stay in the 0.3 M sucrose layer. A crude myelin layer collection at the interface. The major impurities, microsomes and also axoplasm trapped in the vesicles during the homogenization procedure space released through subjecting the myelin come osmotic shock in distilled water. The larger myelin particles have the right to then be separated from the smaller, membranous product by low-speed centrifugation or through repeating the thickness gradient centrifugation on continuous or discontinuous gradients, typically of sucrose. Prepare of purified myelin have the right to be subdivided further and also arbitrarily into fractions of various densities by centrifugation ~ above expanded constant or discontinuous thickness gradients. These fractions differ somewhat in composition.

Demonstration that purity because that a myelin preparation consists of electron-microscopic appearance; however, the an obstacle of identifying tiny membrane engine of microsomes in a ar of myelin membranes and the famous sampling troubles inherent in electron microscopy make this characterization unreliable ~ a specific purity level has actually been reached.

Markers characteristic of myelin include certain proteins, lipids and enzymes explained in the adhering to sections. Although such assays room useful, choose electron microscopy they room not perceptible to little amounts the impurities. If purity the a myelin ready is an issue, the is important to assay air pollution of myelin by various other subcellular fractions utilizing markers such together succinic dehydrogenase because that mitochondria; Na,K-ATPase and also 5′-nucleotidase for plasma membranes; NADH-cytochrome-C reductase for microsomes; DNA because that nuclei; RNA for nuclei, ribosomes and microsomes; lactate dehydrogenase because that cytosol; β-glucosidase for lysosomes; and also acetylcholinesterase for neuronal fragments. Although every one of these markers room low in purified myelin and set an outside limit for levels of pollution by various other membranes, the really contamination might be much less than calculation by such methods since low level of numerous different enzymes appear to it is in intrinsic come myelin.

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Peripheral nerve myelin deserve to be secluded by comparable techniques, but especially vigorous homogenization problems are required due to the fact that of the huge amounts the connective tissue and, sometimes, adipose tissue existing in the nerve. The slightly lesser density of PNS myelin needs some convey of gradient ingredient to prevent loss that myelin.