By:Keiichiro Susuki, M.D., Ph.D.(Dept. The Neuroscience, Baylor university of Medicine)© Education
Citation:Susuki,K.(2010)Myelin: A committed Membrane because that Cell Education3(9):59





How does ours nervous mechanism operate for this reason quickly and also efficiently? The price lies in a membranous structure dubbed myelin.

You are watching: Large-diameter nerve fibers conduct impulses much faster than small-diameter fibers.

All our activities — eating, walking, talk — are regulated by our brains, the center of the nervous system. The mind receives huge amounts of information from external our body via our five senses (vision, sound, taste, touch, and also smell), integrates this information, and orders our muscle to take it action. Exactly how is all that completed so efficiently? The answer lies in a membrane structure called myelin.

All information both to and from the body must be coordinated and also transmitted concurrently and really quickly. The mind itself requires incredibly fast speed to operate at even at the most basic level. Just how do the biological tissues of our body support such quick coordination the the brain, limbs, and sensory input? They execute so v nervous device tissue the imitates electric wiring.

The nervous system is made up of two main cell types: neurons and glial cells. These cells connect with each various other to perform important tasks in the nervous system. The glial cells assistance neurons structurally and also maintain their permanent neuronal integrity, and also neurons manage glial cell behavior. In this assistance of neurons, glial cells have come to be highly specialized. Glial cells, which can be separated into numerous types, have actually various crucial functions, together as giving structural support, expansion support, and insulation around the axon.

Why need to glial cells support neurons? Neurons are committed cells that receive and also send signal to other cells with fragile and thin to move extensions dubbed axons. This axons prolong over distances long and short to reach your target, eventually connecting neurons with other nerve tissue, muscle tissue, or sensory guts (Figure 1A). For example, part motor neurons in the spinal cord have actually axons the exceed 1 m in length, connecting the spine come the reduced limb muscles. These axons transmit signal to the target muscle in the type of electrical impulses called activity potentials. However, the axons alone room not enough to develop rapid conduction of the electric current necessary because that this signal to be sent. Glial cells room the crucial element for sustaining the message neurons send and receive almost everywhere the body. Lot like the insulation approximately the wires in electric systems, glial cells kind a membraneous sheath surrounding axons referred to as myelin, in order to insulating the axon. This myelination, as it is called, can considerably increase the rate of signal transmitted in between neurons (known as activity potentials). Indeed, the evolution of myelin allowed vertebrates to accomplish efficient concerned systems in spite of their big body size.

What specifically is myelin? Myelin is a concentrically laminated membrane structure neighboring an axon roughly which lamellae (or to move protrusions) repeat radially in ~ a duration of about 12 nm (Waxman, Kocsis & Stys 1995; Sherman & Brophy 2005). The myelin lamella is developed by combination of the apposed within leaflets that the plasma membrane in glial cells, v no intervening cytoplasm (Figure 1B).

Depending ~ above the location, various glial cell species make myelin in a various manner. Schwann cells do myelin in the peripheral nervous mechanism (PNS: nerves) and oligodendrocytes in the main nervous system (CNS: brain and spinal cord). In the PNS, one Schwann cell forms a single myelin sheath (Figure 1A). Through contrast, in the CNS, the oligodendrocyte sends out cell procedures to myelinate multiple segments on many axons (Figure 2). Although over there are number of molecular or morphological differences between nerve fibers in the PNS and also CNS, the straightforward myelin sheath arrangement and also the electrophysiological attributes are essentially the same.

Are every axons extended with myelin? No; they deserve to be one of two people myelinated or unmyelinated. Myelinated axons room ensheathed follow me their whole length. The axon caliber (diameter) in mammalian PNS arrays from 0.1 μm to 20 μm, v unmyelinated axons being much less than 2 μm and also myelinated axons being much more than 1–2 μm in diameter. In the CNS, almost all axons with diameters greater than 0.2 μm are myelinated. In overcome section, the myelinated axon shows up as a almost circular profile surrounding by a spirally wound multilamellar sheath (Figure 1C and D). Amazingly, a large myelinated axon may have actually up come 250 come 300 transforms of myelin wrapping around it. The ratio between axon diameter and also that of the full nerve fiber (axon and also myelin) is 0.6–0.7, a proportion that is well maintained regardless the the axon caliber. The size of the myelin sheath along the axon is around 1 mm in the PNS. Between two nearby myelin segments, over there are about 1-μm-long gaps referred to as nodes that Ranvier (Figure 1A and E). In ~ the nodes, the axon is exposed come the extracellular space.

How is the spiral wrapping of the myelin sheath approximately axons created precisely and appropriately? One mechanism has been established in PNS myelination. In the PNS, neuregulin 1 kind III protein is express on the axon surface and interacts v glial ErbB receptors, and also it has actually a pivotal role for Schwann cell differentiation and myelination (for review, watch Nave & Salzer 2006). Unmyelinated autonomic neurons express short levels that neuregulin 1 form III ~ above the axon surface, whereas heavily myelinated axons to express high levels.

Without neuregulin 1 kind III, Schwann cell in culture derived from these mutant mouse cannot myelinate neurons in the spinal cord (dorsal root ganglion neurons). Intriguingly, in generally unmyelinated fibers, compelled expression of neuregulin 1 type III in the postganglionic fibers of forgiveness neurons get an impressive in culture can be required to myelinate. Thus, the level the neuregulin 1 kind III top top the PNS axons is a an essential instructive signal because that myelination. Furthermore, above the threshold, the myelin formation is correlated with the lot of neuregulin 1 kind III gift by the axon come the Schwann cell. Diminished expression of neuregulin 1 type III leader to a thinner than typical myelin sheath in the heterozygous mutant mouse of this molecule. In contrast, transgenic mice the overexpress neuregulin 1 end up being hypermyelinated.

One amazing question is: go neuregulin-ErbB signaling control CNS myelination together well? Although several reports show that oligodendrocytes respond to neuregulin 1 in vitro, analyses of a series of conditional null mutant animals lacking neuregulin 1 confirmed normal myelination (Brinkmann et al. 2008). It is tho unclear exactly how myelination is regulated in the CNS.

How go myelin boost the speed of activity potential propagation? it insulates the axon and also assembles specialized molecular framework at the nodes that Ranvier. In unmyelinated axons, the activity potential travels repetitively along the axons. For example, in unmyelinated C fibers the conduct pains or temperature (0.4–1.2 μm in diameter), conduction velocity along the axon is 0.5–2.0 m/s (as quick as friend walk or jog).

In contrast, among the myelinated nerve fibers, axons are largely covered through myelin sheaths, and also transmembrane currents can only take place at the nodes that Ranvier wherein the axonal membrane is exposed. Myelin is well-off in lipids (approximately 80%) and also can therefore act together an insulator (meaning high transverse resistance and also a low electric capacitance) follow me the internodal segments. Because that example, conduction velocity in the many thoroughly myelinated axons (12–20 μm in diameter) is 70–120 m/s (race vehicle speed), although other factors such as axon caliber can influence this velocity.

At nodes, voltage-gated sodium channels are highly built up and room responsible for the generation of activity potentials. Come induce and also maintain nodal salt channel clusters, specific molecules are likewise enriched in ~ nodal axons, consisting of cell adhesion molecules such as neurofascin 186 and cytoskeletal and scaffolding proteins such together bIV spectrin (Poliak & Peles 2003; Susuki & Rasband 2008). The myelin helps assemble this nodal molecule organization. Because that example, during the development of PNS myelinated nerve fibers, a molecule called gliomedin is secreted indigenous myelinating Schwann cell then integrated into the extracellular matrix neighboring nodes, whereby it disclosure assembly of nodal axonal molecules. Due to the existence of the insulating myelin sheath at internodes and voltage-gated sodium channels at nodes, the activity potential in myelinated nerve yarn jumps indigenous one node to the next. This mode of travel by the activity potential is referred to as "saltatory conduction" and enables for quick impulse propagation (Figure 1A).

The situation in the CNS is illustrated. Complying with demyelination, a demyelinated axon has actually two possible fates. The normal solution to demyelination, at least in most speculative models, is spontaneous remyelination entailing the generation of brand-new oligodendrocytes. In part circumstances, remyelination fails, leaving the axons and also even the whole neuron fragile to degeneration.
© 2008 Publishing team Franklink, R. J. M. & ffrench-Constant, C. Remyelination in the CNS: native biology to therapy. reviews Neuroscience 9, 839–855 (2008). All civil liberties reserved.

What happens if myelin is damaged? The prominence of myelin is underscored through the presence of various diseases in i beg your pardon the primary trouble is defective myelination. Demyelination is the condition in i m sorry preexisting myelin sheaths space damaged and subsequently lost, and it is among the leading causes of neurological condition (Figure 2). Major demyelination can be induced by numerous mechanisms, consisting of inflammatory or metabolic causes. Myelin defects additionally occur by hereditary abnormalities that affect glial cells. Regardless of that cause, myelin loss causes remarkable nerve dysfunction because nerve conduction deserve to be slowed or blocked, resulting in the damaged details networks in between the mind and the body or within the mind itself (Figure 3).

Following demyelination, the naked axon deserve to be re-covered by new myelin. This process is referred to as remyelination and is linked with sensible recovery (Franklin and ffrench-Constant 2008). The myelin sheaths generated throughout remyelination are commonly thinner and shorter than those generated during developmental myelination. In part circumstances, however, remyelination fails, leaving axons and also even the whole neuron breakable to degeneration. Thus, patients v demyelinating diseases suffer from miscellaneous neurological symptoms.

The representative demyelinating disease, and also perhaps the most well known, is multiple sclerosis (MS). This autoimmune neurological disorder is led to by the spreading of demyelinating CNS lesions in the entire brain and gradually (Siffrin et al. 2010). Patients v MS build various symptoms, including visual loss, cognitive dysfunction, motor weakness, and also pain. Around 80 percent of patients experience relapse and remitting illustration of neurologic deficits in the beforehand phase the the condition (relapse-remitting MS). There space no clinical deteriorations in between two episodes. Approximately ten years after an illness onset, around one-half of ms patients endure from steady neurological degradation (secondary steady MS). Around 10–15 percent of patients never ever experience relapsing-remitting episodes; their neurological condition deteriorates repetitively without any innovation (primary gradual MS). Importantly, the lose of axons and their neurons is a major factor determining long-term impairment in patients, return the primary reason of the condition is demyelination. Number of immunodulative therapies are in usage to prevent brand-new attacks; however, there is no recognized cure for MS.

Despite the severe outcome and considerable impact of demyelinating illness on patients" lives and also society, small is known about the mechanism whereby myelin is disrupted, exactly how axons degenerate after demyelination, or how remyelination deserve to be facilitated. To establish brand-new treatments for demyelinating diseases, a much better understanding the myelin biology and also pathology is absolutely required.

How execute we structure a research initiative to elucidate the mechanisms connected in developmental myelination and also demyelinating diseases? We have to develop beneficial models to check drugs or to change molecular expression in glial cells. One strong strategy is to usage a culture system. Coculture that dorsal source ganglion neurons and Schwann cells can promote reliable myelin formation in vitro (Figure 1E). Researchers can modify the molecule expression in Schwann cells, neurons, or both by miscellaneous methods, including drugs, enzymes, and introducing genes, and also can observe the after-effects in the culture dish.

Modeling demyelinating disease in laboratory animals is commonly completed by treatment through toxins injurious come glial cell such as lysolecithin or cuprizone. Autoimmune illness such as MS or autoimmune neuropathies deserve to be reproduced through sensitizing animals with myelin proteins or lipids (Figure 3). Some mutant animals with defects in myelin proteins and also lipids have actually been found or generated, offering useful condition models for hereditary demyelinating disorders. Additional research is compelled to recognize myelin biology and pathology in detail and to establish new treatment strategies for demyelinating neurological disorders.

Myelin can substantially increase the rate of electrical impulses in neurons since it insulates the axon and also assembles voltage-gated salt channel swarm at discrete nodes follow me its length. Myelin damage causes several neurological diseases, such as multiple sclerosis. Future studies for myelin biology and pathology will carry out important ideas for establishing new treatments for demyelinating diseases.

Brinkmann, B. G. Et al. Neuregulin-1/ErbB signaling serves unique functions in myelination the the peripheral and central nervous system. Neuron 59, 581–595 (2008).

Franklin, R. J. & ffrench-Constant, C. Remyelination in the CNS: indigenous biology come therapy. reviews Neuroscience 9, 839–855 (2008).

Nave, K. A. & Salzer, J. L. Axonal regulation the myelination through neuregulin 1. Present Opinion in Neurobiology 16, 492–500 (2006).

Poliak, S. & Peles, E. The local differentiation that myelinated axons in ~ nodes of Ranvier. evaluate Neuroscience 4, 968–980 (2003).

Sherman, D. L. & Brophy, P. J. Mechanisms of axon ensheathment and also myelin growth. reviews Neuroscience 6, 683–690 (2005).

Siffrin, V. Et al. Lot of sclerosis — candidate mechanisms underlying CNS atrophy. Fads in Neurosciences 33, 202–210 (2010).

Susuki, K. & Rasband, M. N. Molecular mechanisms that node of Ranvier formation. Present Opinion in cell Biology 20, 616–623 (2008).

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Waxman, S. G., Kocsis, J. D. & Stys, P. K., eds. The Axon: Structure, role and Pathophysiology. New York: Oxford college Press, 1995.