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





How does ours nervous device operate therefore quickly and also efficiently? The prize lies in a membranous structure referred to as myelin.

You are watching: Gaps in the myelin sheath are called

All our activities — eating, walking, talk — are managed by our brains, the center of the worried system. The mind receives large amounts of details from external our body via our five senses (vision, sound, taste, touch, and also smell), integrates this information, and also orders our muscle to take action. How is all that completed so efficiently? The prize lies in a membrane structure dubbed myelin.

All information both to and from the body need to be coordinated and transmitted concurrently and an extremely quickly. The brain itself requires very fast speed to operate at also at the simplest level. Exactly how do the biological tissues of our body support such fast coordination the the brain, limbs, and sensory input? They carry out so through nervous device tissue the imitates electric wiring.

The nervous mechanism is comprised of two major cell types: neurons and also glial cells. These cells communicate with each other to perform vital tasks in the worried system. The glial cells assistance neurons structurally and also maintain their long-term neuronal integrity, and also neurons regulate glial cell behavior. In this support of neurons, glial cells have become highly specialized. Glial cells, which have the right to be separated into number of types, have various essential functions, together as giving structural support, growth support, and also insulation approximately the axon.

Why need to glial cells support neurons? Neurons are devoted cells the receive and also send signal to various other cells v fragile and thin cellular extensions referred to as axons. This axons expand over ranges long and short to reach your target, eventually connecting neurons with various other nerve tissue, muscle tissue, or sensory organs (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. This axons transmit signal to the target muscle in the form of electrical impulses called action potentials. However, the axons alone are not enough to create rapid conduction the the electric present necessary for this signal to it is in sent. Glial cells space the an essential element for supporting the message neurons send and receive everywhere the body. Lot like the insulation about the wires in electric systems, glial cells type a membraneous sheath bordering axons referred to as myelin, thereby insulating the axon. This myelination, together it is called, can greatly increase the speed of signals transmitted in between neurons (known as activity potentials). Indeed, the development of myelin enabled vertebrates to attain efficient nervous systems in spite of their huge body size.

What precisely is myelin? Myelin is a concentrically laminated membrane structure surrounding an axon approximately which lamellae (or cellular protrusions) repeat radially at a duration of about 12 nm (Waxman, Kocsis & Stys 1995; Sherman & Brophy 2005). The myelin lamella is created by fusion of the apposed inner leaflets of the plasma membrane in glial cells, v no intervening cytoplasm (Figure 1B).

Depending top top the location, different glial cell species make myelin in a different manner. Schwann cells do myelin in the peripheral nervous device (PNS: nerves) and oligodendrocytes in the main nervous device (CNS: brain and spinal cord). In the PNS, one Schwann cell develops a single myelin sheath (Figure 1A). By contrast, in the CNS, the oligodendrocyte sends cell processes to myelinate multiple segments on many axons (Figure 2). Although over there are number of molecular or morphological differences in between nerve fibers in the PNS and CNS, the straightforward myelin sheath arrangement and the electrophysiological qualities are essentially the same.

Are all axons extended with myelin? No; they have the right to be either myelinated or unmyelinated. Myelinated axons room ensheathed along their whole length. The axon caliber (diameter) in mammalian PNS ranges from 0.1 μm come 20 μm, with unmyelinated axons being much less than 2 μm and also myelinated axons being more than 1–2 μm in diameter. In the CNS, practically all axons through diameters better than 0.2 μm room myelinated. In cross section, the myelinated axon shows up as a practically circular profile surrounding by a spirally wound multilamellar sheath (Figure 1C and also D). Amazingly, a big myelinated axon may have up to 250 to 300 turns of myelin wrapping around it. The ratio between axon diameter and that of the full nerve fiber (axon and also myelin) is 0.6–0.7, a proportion that is well kept regardless the the axon caliber. The length of the myelin sheath follow me the axon is roughly 1 mm in the PNS. In between two nearby myelin segments, over there are approximately 1-μm-long gaps dubbed nodes the Ranvier (Figure 1A and E). At the nodes, the axon is exposed to the extracellular space.

How is the spiral pack of the myelin sheath roughly axons developed precisely and also appropriately? One mechanism has actually been established in PNS myelination. In the PNS, neuregulin 1 form III protein is express on the axon surface and also interacts with glial ErbB receptors, and also it has actually a pivotal function for Schwann cabinet differentiation and also myelination (for review, view Nave & Salzer 2006). Unmyelinated autonomic neurons express short levels that neuregulin 1 type III top top the axon surface, whereas greatly myelinated axons refer high levels.

Without neuregulin 1 type III, Schwann cell in culture derived from these mutant mouse cannot myelinate neurons in the spinal cord (dorsal root ganglion neurons). Intriguingly, in usually unmyelinated fibers, required expression of neuregulin 1 kind III in the postganglionic yarn of sorry neurons get an impressive in society can be forced to myelinate. Thus, the level the neuregulin 1 type III on the PNS axons is a an essential instructive signal for myelination. Furthermore, above the threshold, the myelin formation is associated with the amount of neuregulin 1 kind III presented by the axon to the Schwann cell. Lessened expression that neuregulin 1 kind III leader to a thinner than typical myelin sheath in the heterozygous mutant mice of this molecule. In contrast, transgenic mice that overexpress neuregulin 1 become hypermyelinated.

One amazing question is: does neuregulin-ErbB signaling regulate CNS myelination as well? Although numerous reports display that oligodendrocytes respond come neuregulin 1 in vitro, analyses of a collection of conditional null mutant animals lacking neuregulin 1 verified normal myelination (Brinkmann et al. 2008). That is still unclear how myelination is regulation in the CNS.

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

In contrast, amongst the myelinated nerve fibers, axons are mainly covered by myelin sheaths, and also transmembrane currents can only take place at the nodes the Ranvier wherein the axonal membrane is exposed. Myelin is affluent in lipids (approximately 80%) and can thus act together an insulator (meaning high transverse resistance and also a low electric capacitance) along the internodal segments. Because that example, conduction velocity in the most thoroughly myelinated axons (12–20 μm in diameter) is 70–120 m/s (race auto speed), return other factors such as axon caliber deserve to influence this velocity.

At nodes, voltage-gated sodium networks are highly accumulated and room responsible because that the generation of action potentials. To induce and maintain nodal sodium channel clusters, details molecules are also enriched at nodal axons, consisting of cell adhesion molecule such as neurofascin 186 and cytoskeletal and also scaffolding proteins such together bIV spectrin (Poliak & Peles 2003; Susuki & Rasband 2008). The myelin helps assemble this nodal molecule organization. For example, throughout the advancement of PNS myelinated nerve fibers, a molecule referred to as gliomedin is secreted indigenous myelinating Schwann cells then incorporated into the extracellular matrix neighboring nodes, whereby it promotes assembly that nodal axonal molecules. Because of the visibility of the insulating myelin sheath at internodes and also voltage-gated sodium channels at nodes, the action potential in myelinated nerve yarn jumps native one node to the next. This setting of take trip by the action potential is referred to as "saltatory conduction" and allows for fast impulse propagation (Figure 1A).

The case in the CNS is illustrated. Complying with demyelination, a demyelinated axon has actually two feasible fates. The normal an answer to demyelination, at least in most experimental models, is voluntary remyelination including the generation of new oligodendrocytes. In some 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: from biology come therapy. reviews Neuroscience 9, 839–855 (2008). All civil liberties reserved.

What wake up if myelin is damaged? The prestige of myelin is underscored through the visibility of various diseases in i beg your pardon the primary problem is defective myelination. Demyelination is the problem in i beg your pardon preexisting myelin sheaths are damaged and subsequently lost, and it is one of the leading reasons of neurological disease (Figure 2). Major demyelination have the right to be induced by several mechanisms, including inflammatory or metabolic causes. Myelin defects also occur by genetic abnormalities that affect glial cells. Regardless of that cause, myelin loss reasons remarkable nerve dysfunction since nerve conduction can be slowed or blocked, leading to the damaged information networks in between the brain and the body or in ~ the mind itself (Figure 3).

Following demyelination, the nude axon can be re-covered by brand-new myelin. This process is called remyelination and also is linked with useful recovery (Franklin and ffrench-Constant 2008). The myelin sheaths generated throughout remyelination are frequently thinner and much shorter than those generated during developmental myelination. In part circumstances, however, remyelination fails, leaving axons and even the entire neuron breakable to degeneration. Thus, patients v demyelinating conditions suffer from assorted neurological symptoms.

The representative demyelinating disease, and perhaps the most well known, is many sclerosis (MS). This autoimmune neurological disorder is led to by the dispersing of demyelinating CNS lesions in the entire brain and in time (Siffrin et al. 2010). Patients v MS construct various symptoms, including visual loss, cognitive dysfunction, engine weakness, and pain. About 80 percent of patients endure relapse and also remitting illustration of neurologic deficits in the at an early stage phase that the an illness (relapse-remitting MS). There room no clinical deteriorations in between two episodes. Roughly ten years after condition onset, about one-half of ms patients experience from gradual neurological degradation (secondary progressive MS). Around 10–15 percent the patients never experience relapsing-remitting episodes; your neurological status deteriorates repetitively without any innovation (primary gradual MS). Importantly, the ns of axons and also their neurons is a major factor identify long-term impairment in patients, although the primary cause of the disease is demyelination. Several immunodulative therapies room in usage to prevent new attacks; however, over there is no recognized cure because that MS.

Despite the major outcome and considerable impact of demyelinating illness on patients" lives and society, small is known about the mechanism by which myelin is disrupted, how axons degenerate after ~ demyelination, or how remyelination can be facilitated. Come establish brand-new treatments for demyelinating diseases, a better understanding the myelin biology and also pathology is absolutely required.

How do we structure a research initiative to elucidate the mechanisms associated in developmental myelination and demyelinating diseases? We should develop helpful models to check drugs or to modify molecular expression in glial cells. One solid strategy is to use a culture system. Coculture the dorsal source ganglion neurons and also Schwann cells have the right to promote reliable myelin formation in vitro (Figure 1E). Researchers have the right to modify the molecular expression in Schwann cells, neurons, or both by various methods, consisting of drugs, enzymes, and introducing genes, and also can observe the consequences in the society dish.

Modeling demyelinating disease in laboratory pets is commonly completed by treatment with toxins injurious to glial cell such as lysolecithin or cuprizone. Autoimmune diseases such together MS or autoimmune neuropathies can be reproduced by sensitizing animals with myelin proteins or lipids (Figure 3). Part mutant animals with defects in myelin proteins and lipids have actually been uncovered or generated, offering useful condition models for hereditary demyelinating disorders. Further research is forced to understand myelin biology and also pathology in detail and also to establish new treatment methods for demyelinating neurological disorders.

Myelin can considerably increase the rate of electrical impulses in neurons due to the fact that it insulates the axon and assembles voltage-gated salt channel swarm at discrete nodes along its length. Myelin damage causes several neurological diseases, such together multiple sclerosis. Future research studies for myelin biology and also pathology will carry out important clues for establishing brand-new treatments because that demyelinating diseases.

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

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

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

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

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

Siffrin, V. Et al. Many 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 cabinet Biology 20, 616–623 (2008).

See more: Spider-Man: Shattered Dimensions Suits, Alternate Costumes

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