Why myelin sheath have gaps

Myelin was discovered in the midth century when scientists were observing neurons through a microscope, and they noticed a glistening white substance surrounding the axons. At the time, it was believed that the myelin was at the core of the axon, however, it was later found to be a substance which wraps around the axons of neurons. This insulation provides protection to these axons in the same way that electrical wires have insulation.

Myelin sheath is a low electrical condenser and has high electrical resistance which means it can act as an insulator without disrupting the electrical signals traveling down the axon. Since myelin sheath provides insulation to axons, this allows these axons to conduct electrical signals at a higher speed than if they were not insulated by myelin. Thus, the more thoroughly myelinated an axon is, the higher the speed of electrical transmission.

Similarly, myelin sheath around an axon is able to prevent electrical impulses from traveling through the sheath and out of the axon. It prevents the movement of ions into or out of the neuron, also known as depolarization. This means the current of action potential will only flow down the axon.

The more action potential, the more neurons will be able to communicate to each other, transfer electrical and chemical messages, and keep the brain healthy and functioning properly. Whilst the myelin sheath wraps around the axons, there are some small, uncovered gaps between the myelin sheath, which are called the nodes of Ranvier. These are specialised molecular structures created by the myelin sheath which contains clusters of voltage-sensitive sodium and potassium ion channels.

This type of conduction is important for electrical impulses to be formed quickly and means that less energy is required for the conduction of electrical signals. This is because less energy is needed in myelinated axons to conduct impulses. Myelination is the formation of a myelin sheath, therefore axons which are covered by this insulating sleeve of protection are said to be myelinated axons.

If an axon is not surrounded by myelin sheath, it is said to be unmyelinated. The more myelinated axons someone has, the quicker their responses to stimuli will be, due to myelin sheaths increasing the conduction of nerve impulses. Consequently, unmyelinated axons will mean that an individual will not have quicker responses. Myelin sheath is produced by different types of glia cells. Glia cells are located in the CNS and PNS, that work to maintain homeostasis, and provide support and protection for neurons.

The two types of glia cells that produce myelin are Schwann cells and oligodendrocytes. Schwann cells are located within the peripheral nervous system PNS whereas oligodendrocytes are located within the central nervous system CNS.

Schwann cells originate from the neural crest, which is a group of embryonic cells. As such, Schwann cells will first start to myelinate axons during foetal development.

Schwann cells are surrounded by sheets of tissue known as basal lamina. The outside of the basal lamina is covered in a layer of connective tissues known as the endoneurium. The endoneurium contains blood vessels, macrophages, and fibroblasts. Finally, the inner surface area of the lamina layer faces the plasma membrane of the Schwann cells. For the myelin sheath to be created by Schwann cells in the PNS, the plasma membrane of these cells needs to wrap itself around the axons of the neuron concentrically, spiralling to add membrane layers.

This plasma membrane contains high levels of fat which is essential for the construction of myelin sheath. Sometimes, as many as revolutions of Schwann cell spirals around the axons of the neurons. Within the CNS, oligodendrocytes are the glia cells which also create myelin sheath. Oligodendrocytes are star-shaped cells which have about 15 arms coming out of their cell body, meaning it is able to myelinate multiple axons at one time.

In a similar fashion to Schwann cells, oligodendrocytes spiral around the axons of neurons to form a myelin sheath. One can visualize this structure arising from Figure if the glial cell process were pulled straight up and the myelin layers separated at the intermediate period line.

A diagram similar to Figure but showing one Schwann cell and its myelin sheath unrolled from a peripheral axon. The sheet of PNS myelin is, like CNS myelin, surrounded by a tube of cytoplasm and has additional tubes of cytoplasm, which make up the more Schmidt-Lantermann clefts are structures where the cytoplasmic surfaces of the myelin sheath have not compacted to form the major dense line and, instead, contain Schwann or glial cell cytoplasm Fig.

These regions are common in peripheral myelinated axons but rare in the CNS. These inclusions of cytoplasm are present in each layer of myelin. The clefts can be visualized in the unrolled myelin sheet as tubes of cytoplasm similar to the tubes making up the lateral loops but in the middle regions of the sheet, rather than at the edges Fig. In the PNS , myelination 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.

These cells line up along the axons with intervals between them; the intervals become the nodes of Ranvier. Myelin formation in the peripheral nervous system. A: The Schwann cell has surrounded the axon, but the external surfaces of the plasma membrane have not yet fused in the mesaxon. B: The mesaxon has fused into a five-layered structure and spiraled once more Before myelination, the axon lies in an invagination of the Schwann cell Fig.

The plasmalemma of the cell then surrounds the axon and joins to form a double-membrane structure that communicates with the cell surface. This structure, called the mesaxon, elongates around the axon in a spiral fashion Fig.

Thus, formation of myelin topologically resembles rolling up a sleeping bag; the mesaxon winds about the axon, and the cytoplasmic surfaces condense into a compact myelin sheath and form the major dense line. The two external surfaces form the myelin intraperiod line. In the CNS , the structures of myelin are formed by the oligodendroglial cell [ 7 ]. This has many similarities but also points of difference with respect to myelination in the PNS.

CNS nerve fibers are not separated by connective tissue, nor are they surrounded by cell cytoplasm, and specific glial nuclei are not obviously associated with particular myelinated fibers. CNS myelin is a spiral structure similar to PNS myelin; it has an inner mesaxon and an outer mesaxon that ends in a loop, or tongue, of glial cytoplasm Fig. Unlike the peripheral nerve, where the sheath is surrounded by Schwann cell cytoplasm, the cytoplasmic tongue in the CNS is restricted to a small portion of the sheath.

This glial tongue is continuous with the plasma membrane of the oligodendroglial cell through slender processes. One glial cell can myelinate 40 or more separate axons [ 8 ]. Myelin deposition in the PNS may result in a single axon having up to myelin layers; therefore, it is improbable that myelin is laid down by a simple rotation of the Schwann cell nucleus around the axon.

In the CNS , such a postulate is precluded by the fact that one glial cell can myelinate several axons. During myelination, there are increases in the length of the internode, the diameter of the axon and the number of myelin layers. Myelin, therefore, expands in all planes at once. Any mechanism to account for this growth must assume that the membrane system is able to expand and contract and that layers slip over each other. If CNS tissue is homogenized in media of low ionic strength, myelin peels off the axons and reforms in vesicles of the size range of nuclei and mitochondria.

Because of their high lipid content, these myelin vesicles have the lowest intrinsic density of any membrane fraction of the nervous system.

Procedures for isolation of myelin take advantage of both the large vesicle size and the low density [ 9 ]. In a widely used method, a homogenate of rodent nervous tissue, or dissected white matter in the case of larger animals, in isotonic sucrose 0. Mitochondria and synaptosomes sediment through the denser sucrose, and many of the smaller membrane fragments from other organelles remain in the 0. A crude myelin layer collects at the interface.

The major impurities, microsomes and axoplasm trapped in the vesicles during the homogenization procedure are released by subjecting the myelin to osmotic shock in distilled water.

The larger myelin particles can then be separated from the smaller, membranous material by low-speed centrifugation or by repeating the density gradient centrifugation on continuous or discontinuous gradients, usually of sucrose. Preparations of purified myelin can be subdivided further and arbitrarily into fractions of different densities by centrifugation on expanded continuous or discontinuous density gradients.

These fractions differ somewhat in composition. Demonstration of purity for a myelin preparation includes electron-microscopic appearance; however, the difficulty of identifying small membrane vesicles of microsomes in a field of myelin membranes and the well-known sampling problems inherent in electron microscopy make this characterization unreliable after a certain purity level has been reached. Markers characteristic of myelin include certain proteins, lipids and enzymes described in the following sections.

Although such assays are useful, like electron microscopy they are not sensitive to small amounts of impurities. Although all of these markers are low in purified myelin and set an outside limit for levels of contamination by other membranes, the actual contamination may be less than calculated by such methods since low levels of many different enzymes appear to be intrinsic to myelin. Myelin sheath: Myelin is a electrically insulating material that forms a layer, the myelin sheath, usually around only in the part of the cella called the axon it is Glenn Messina answered.

Aids in conduction: Myelin sheath is a substance covering the peripheral nerves acting almost like the plastic you see covering a basic electrical wire. It offers not onl See below: In peripheral nerves, damage to just the myelin coverings, recovery may take place over weeks, but anatomically, the nerve may not fully recover i Myelin can heal: Demyelinating diseases like multiple sclerosis create a situation where the myelin will not heal itself.

Traumatized nerves do heal but they only move View 1 more answer. Various mechanisms: The classic neurological diseases that involve myelin are Multiple Sclerosis, and Neuromyelitis Optica. Both Alzheimer's and Parkinson's involve nerv Philip Kern answered. Neuropathy: Hyperglycemia diabetes causes neuropathy. There are several forms of neuropathy, but most do not involve a problem with myelin.

Deterioration of m Mark Fisher answered. No: Surely you remember the old days when cars needed valve jobs. You couldn't fix valves by using a higher grade of gasoline. Same logic applies here. Natalie Sieb answered. It's one cause: Demyelenating disease is just one cause of nerve damage that can occur over the course of a lifetime.