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





How does ours nervous system operate therefore quickly and efficiently? The prize lies in a membranous structure dubbed myelin.

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

All information both to and from the body should be coordinated and transmitted simultaneously and an extremely quickly. The mind itself requires very fast speed to operate at even at the simplest level. How do the organic tissues of our body support such quick coordination of the brain, limbs, and also sensory input? They carry out so with nervous system tissue that imitates electrical wiring.

The nervous system is consisted of of two primary cell types: neurons and also glial cells. These cells connect with each various other to perform necessary tasks in the nervous system. The glial cells support neurons structurally and also maintain their irreversible neuronal integrity, and neurons regulate glial cell behavior. In this assistance of neurons, glial cell have end up being highly specialized. Glial cells, which deserve to be divided into several types, have various vital functions, such as providing structural support, expansion support, and insulation around the axon.

Why need to glial cells assistance neurons? Neurons are devoted cells the receive and send signals to other cells v fragile and also thin to move extensions referred to as axons. This axons prolong over distances long and also short come reach their target, eventually connecting neurons with various other nerve tissue, muscle tissue, or sensory guts (Figure 1A). For example, some motor neurons in the spinal cord have axons that exceed 1 m in length, connecting the spine to the lower limb muscles. These axons transmit signal to the target muscle in the form of electrical impulses called action potentials. However, the axons alone room not enough to create rapid conduction of the electric present necessary because that this signal to be sent. Glial cells space the an essential element for sustaining the message neurons send and receive almost everywhere the body. Lot like the insulation about the wires in electrical systems, glial cells type a membraneous sheath neighboring axons referred to as myelin, thereby insulating the axon. This myelination, together it is called, can greatly increase the rate of signals transmitted between neurons (known as action potentials). Indeed, the advancement of myelin allowed vertebrates to attain efficient nervous systems despite their big body size.

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

Depending on the location, different glial cell species make myelin in a various manner. Schwann cells make myelin in the peripheral nervous device (PNS: nerves) and also oligodendrocytes in the central nervous device (CNS: mind and spinal cord). In the PNS, one Schwann cell develops a solitary myelin sheath (Figure 1A). By contrast, in the CNS, the oligodendrocyte sends cell processes to myelinate multiple segment on countless axons (Figure 2). Although over there are numerous molecular or morphological differences in between nerve fibers in the PNS and CNS, the basic myelin sheath arrangement and also the electrophysiological attributes are basically the same.

Are every axons covered with myelin? No; they have the right to be either myelinated or unmyelinated. Myelinated axons are ensheathed along their entire length. The axon caliber (diameter) in mammalian PNS ranges from 0.1 μm to 20 μm, v unmyelinated axons being much less than 2 μm and also myelinated axons being more than 1–2 μm in diameter. In the CNS, nearly all axons with 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 D). Amazingly, a big myelinated axon may have actually up to 250 to 300 transforms of myelin wrapping roughly it. The ratio in between axon diameter and also that of the complete nerve fiber (axon and myelin) is 0.6–0.7, a ratio that is well preserved regardless the the axon caliber. The length of the myelin sheath along the axon is around 1 mm in the PNS. Between two adjacent myelin segments, over there are about 1-μm-long gaps called nodes the Ranvier (Figure 1A and also E). In ~ the nodes, the axon is exposed to the extracellular space.

How is the spiral pack of the myelin sheath around axons developed precisely and also appropriately? One mechanism has been determined in PNS myelination. In the PNS, neuregulin 1 kind III protein is express on the axon surface and interacts v glial ErbB receptors, and it has a pivotal duty for Schwann cabinet differentiation and also myelination (for review, view Nave & Salzer 2006). Unmyelinated autonomic neurons express low levels of neuregulin 1 kind III on the axon surface, whereas heavily myelinated axons express high levels.

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

One amazing question is: walk neuregulin-ErbB signaling regulate CNS myelination together well? Although numerous reports show that oligodendrocytes respond come neuregulin 1 in vitro, analyses that a series of conditional null mutant animals lacking neuregulin 1 confirmed normal myelination (Brinkmann et al. 2008). The is tho unclear just how myelination is regulated in the CNS.

How does myelin enhance the speed of activity potential propagation? the insulates the axon and assembles devoted molecular framework at the nodes of Ranvier. In unmyelinated axons, the action potential travels consistently along the axons. Because that example, in unmyelinated C fibers that conduct ache or temperature (0.4–1.2 μm in diameter), conduction velocity along the axon is 0.5–2.0 m/s (as rapid as friend walk or jog).

In contrast, among the myelinated nerve fibers, axons are mainly covered through myelin sheaths, and also transmembrane currents have the right to only occur at the nodes the Ranvier wherein the axonal membrane is exposed. Myelin is wealthy in lipids (approximately 80%) and can because of this act together an insulator (meaning high transverse resistance and also a low electric capacitance) along the internodal segments. For example, conduction velocity in the many thoroughly myelinated axons (12–20 μm in diameter) is 70–120 m/s (race auto speed), back other determinants such together axon caliber can influence this velocity.

At nodes, voltage-gated sodium channels are highly accumulated and space responsible for the generation of activity potentials. To induce and also maintain nodal sodium channel clusters, particular molecules are likewise enriched in ~ nodal axons, including cell adhesion molecules such together neurofascin 186 and also cytoskeletal and scaffolding protein such as bIV spectrin (Poliak & Peles 2003; Susuki & Rasband 2008). The myelin help assemble this nodal molecule organization. For example, during the breakthrough of PNS myelinated nerve fibers, a molecule dubbed gliomedin is secreted from myelinating Schwann cells then included into the extracellular matrix neighboring nodes, wherein it promotes assembly that nodal axonal molecules. Due to the existence of the insulating myelin sheath at internodes and voltage-gated sodium channels at nodes, the action potential in myelinated nerve yarn jumps indigenous one node come the next. This mode of travel by the action potential is called "saltatory conduction" and enables for rapid impulse propagation (Figure 1A).

The instance in the CNS is illustrated. Following demyelination, a demyelinated axon has actually two feasible fates. The normal an answer to demyelination, at the very least in most speculative models, is voluntarily remyelination including the generation of new oligodendrocytes. In part circumstances, remyelination fails, leaving the axons and even the entire neuron fragile to degeneration.
© 2008 Publishing team Franklink, R. J. M. & ffrench-Constant, C. Remyelination in the CNS: from biology to therapy. reviews Neuroscience 9, 839–855 (2008). All legal rights reserved.

What happens if myelin is damaged? The importance of myelin is underscored by the visibility of various diseases in i beg your pardon the primary problem is defective myelination. Demyelination is the condition in which preexisting myelin sheaths room damaged and subsequently lost, and also it is among the leading reasons of neurological condition (Figure 2). Primary demyelination have the right to be induced by numerous mechanisms, including inflammatory or metabolic causes. Myelin defects likewise occur by genetic abnormalities that affect glial cells. Nevertheless of its cause, myelin loss reasons remarkable nerve dysfunction since nerve conduction have the right to be slowed or blocked, bring about the damaged information networks between the brain and the human body or in ~ 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 connected with practical recovery (Franklin and also ffrench-Constant 2008). The myelin sheaths generated during remyelination are frequently thinner and shorter than those generated during developmental myelination. In part circumstances, however, remyelination fails, leaving axons and even the whole neuron vulnerable to degeneration. Thus, patients with demyelinating conditions suffer from assorted neurological symptoms.

The representative demyelinating disease, and also perhaps the most well known, is lot of sclerosis (MS). This autoimmune neurological disorder is caused by the dispersing of demyelinating CNS lesions in the entire mind and gradually (Siffrin et al. 2010). Patients with MS develop various symptoms, including visual loss, cognitive dysfunction, engine weakness, and pain. Around 80 percent the patients endure relapse and also remitting episodes of neurologic deficits in the early phase the the an illness (relapse-remitting MS). There are no clinical deteriorations in between two episodes. Approximately ten year after condition onset, around one-half of multiple sclerosis patients endure from progressive neurological degradation (secondary progressive MS). Around 10–15 percent that patients never ever experience relapsing-remitting episodes; your neurological status deteriorates continuously without any improvement (primary gradual MS). Importantly, the loss of axons and also their neurons is a significant factor identify long-term special needs in patients, back the primary reason of the disease is demyelination. Several immunodulative therapies space in usage to prevent brand-new attacks; however, there is no known cure because that MS.

Despite the significant outcome and also considerable result of demyelinating diseases on patients" lives and also society, tiny is known around the mechanism through which myelin is disrupted, just how axons degenerate ~ demyelination, or exactly how remyelination deserve to be facilitated. To establish brand-new treatments because that demyelinating diseases, a far better understanding that myelin biology and pathology is absolutely required.

How carry out we framework a research initiative to elucidate the mechanisms involved in developmental myelination and also demyelinating diseases? We must develop beneficial models to test drugs or to modify molecular expression in glial cells. One strong strategy is to usage a culture system. Coculture of dorsal source ganglion neurons and also Schwann cells deserve to promote efficient myelin development 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 watch the consequences in the culture dish.

Modeling demyelinating an illness in laboratory animals is commonly completed by treatment with toxins injurious come glial cells such as lysolecithin or cuprizone. Autoimmune conditions such together MS or autoimmune neuropathies have the right to be reproduced through sensitizing animals with myelin protein or lipids (Figure 3). Some mutant pets with defects in myelin proteins and lipids have been found or generated, providing useful disease models because that hereditary demyelinating disorders. Further research is required to understand myelin biology and pathology in detail and to establish new treatment techniques for demyelinating neurological disorders.

Myelin can significantly increase the rate of electric impulses in neurons because it insulates the axon and assembles voltage-gated salt channel clusters at discrete nodes follow me its length. Myelin damage reasons several neurological diseases, such as multiple sclerosis. Future studies for myelin biology and also pathology will carry out important hints for establishing new treatments for 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 come therapy. reviews Neuroscience 9, 839–855 (2008).

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

Poliak, S. & Peles, E. The neighborhood differentiation that myelinated axons at nodes of Ranvier. reviews Neuroscience 4, 968–980 (2003).

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

Siffrin, V. Et al. Many sclerosis — candidate mechanisms basic CNS atrophy. Patterns in Neurosciences 33, 202–210 (2010).

Susuki, K. & Rasband, M. N. Molecule mechanisms the node that Ranvier formation. Existing 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 university Press, 1995.