Myelin
Myelin is a lipid-rich substance that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses are passed along the axon. The myelinated axon can be likened to an electrical wire with insulating material around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single long section and, in general, each axon comprises multiple long myelinated sections separated from each other by short gaps called nodes of Ranvier.
Myelin is formed in the central nervous system by glial cells called oligodendrocytes and in the peripheral nervous system by glial cells called Schwann cells. In the CNS, axons carry electrical signals from one nerve cell body to another. In the PNS, axons carry signals to muscles and glands or from sensory organs such as the skin. Each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon. Myelin reduces the capacitance of the axonal membrane. On a molecular level, in the internodes it increases the distance between extracellular and intracellular ions, reducing the accumulation of charges.
The discontinuous structure of the myelin sheath results in saltatory conduction, whereby the action potential "jumps" from one node of Ranvier, over a long myelinated stretch of the axon called the internode, before "recharging" at the next node of Ranvier, and so on, until it reaches the axon terminal. Nodes of Ranvier are the short unmyelinated regions of the axon between adjacent long myelinated internodes. Once it reaches the axon terminal, this electrical signal provokes the release of a chemical message or neurotransmitter that binds to receptors on the adjacent post-synaptic cell at specialised regions called synapses.
This "insulating" role for myelin is essential for normal motor function, sensory function and cognition, as demonstrated by the consequences of disorders that affect it, such as the genetically determined leukodystrophies; the acquired inflammatory demyelinating disorder, multiple sclerosis; and the inflammatory demyelinating peripheral neuropathies. Due to its high prevalence, multiple sclerosis, which specifically affects the central nervous system, is the best known disorder of myelin.
Development
The process of generating myelin is called myelination or myelinogenesis. In the CNS, cells called oligodendrocyte precursor cells differentiate into mature oligodendrocytes, which form myelin. In humans, myelination begins early in the 3rd trimester, although only little myelin is present in either the CNS or the PNS at the time of birth. During infancy, myelination progresses rapidly, with increasing numbers of axons acquiring myelin sheaths. This corresponds with the development of cognitive and motor skills, including language comprehension, speech acquisition, crawling and walking. Myelination continues through adolescence and early adulthood and although largely complete at this time, myelin sheaths can be added in grey matter regions such as the cerebral cortex, throughout life.Species distribution
Myelin is considered a defining characteristic of the jawed vertebrates, though axons are ensheathed by a type of cell, called glial cells, in invertebrates. These glial wraps are quite different from vertebrate compact myelin, formed, as indicated above, by concentric wrapping of the myelinating cell process multiple times around the axon. Myelin was first described in 1854 by Rudolf Virchow, although it was over a century later, following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent.In vertebrates, not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated. Instead, they are ensheathed by non-myelinating Schwann cells known as Remak SCs and arranged in Remak bundles. In the CNS, non-myelinated axons intermingle with myelinated ones and are entwined, at least partially, by the processes of another type of glial cell called the astrocyte.
Composition
CNS myelin differs slightly in composition and configuration from PNS myelin, but both perform the same "insulating" function. Being rich in lipid, myelin appears white, hence the name given to the "white matter" of the CNS. Both CNS white matter tracts and PNS nerves each comprise thousands to millions of axons, largely aligned in parallel. Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which also contain other cell types including astrocytes and microglia in the CNS and macrophages in the PNS.In terms of total mass, myelin comprises approximately 40% water; the dry mass comprises between 60% and 75% lipid and between 15% and 25% protein. Protein content includes myelin basic protein, which is abundant in the CNS where it plays a critical, non-redundant role in formation of compact myelin; myelin oligodendrocyte glycoprotein, which is specific to the CNS; and proteolipid protein, which is the most abundant protein in CNS myelin, but only a minor component of PNS myelin. In the PNS, myelin protein zero has a similar role to that of PLP in the CNS in that it is involved in holding together the multiple concentric layers of glial cell membrane that constitute the myelin sheath. The primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin strengthen the myelin sheath. Cholesterol is an essential lipid component of myelin, without which myelin fails to form.
Function
The main purpose of myelin is to increase the speed at which electrical impulses propagate along the myelinated fiber. In unmyelinated fibers, electrical impulses travel as continuous waves, but, in myelinated fibers, they "hop" or propagate by saltatory conduction. The latter is markedly faster than the former, at least for axons over a certain diameter. Myelin decreases capacitance and increases electrical resistance across the axonal membrane. It has been suggested that myelin permits larger body size by maintaining agile communication between distant body parts.Myelinated fibers lack voltage-gated sodium channels along the myelinated internodes, exposing them only at the nodes of Ranvier. Here, they are highly abundant and densely packed. Positively charged sodium ions can enter the axon through these voltage-gated channels, leading to depolarisation of the membrane potential at the node of Ranvier. The resting membrane potential is then rapidly restored due to positively charged potassium ions leaving the axon through potassium channels. The sodium ions inside the axon then diffuse rapidly through the axoplasm, to the adjacent myelinated internode and ultimately to the next node of Ranvier, triggering the opening of the voltage gated sodium channels and entry of sodium ions at this site. Although the sodium ions diffuse through the axoplasm rapidly, diffusion is decremental by nature, thus nodes of Ranvier have to be closely spaced, to secure action potential propagation. The action potential "recharges" at consecutive nodes of Ranvier as the axolemmal membrane potential depolarises to approximately +35 mV. Along the myelinated internode, energy-dependent sodium/potassium pumps pump the sodium ions back out of the axon and potassium ions back into the axon to restore the balance of ions between the intracellular and extracellular fluids.
Whilst the role of myelin as an "axonal insulator" is well-established, other functions of myelinating cells are less well known or only recently established. The myelinating cell "sculpts" the underlying axon by promoting the phosphorylation of neurofilaments, thus increasing the diameter or thickness of the axon at the internodal regions; helps cluster molecules on the axolemma at the node of Ranvier; and modulates the transport of cytoskeletal structures and organelles such as mitochondria, along the axon. In 2012, evidence came to light to support a role for the myelinating cell in "feeding" the axon. In other words, the myelinating cell seems to act as a local "fueling station" for the axon, which uses a great deal of energy to restore the normal balance of ions between it and its environment, following the generation of action potentials.
When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. However, the myelin layer does not ensure a perfect regeneration of the nerve fiber. Some regenerated nerve fibers do not find the correct muscle fibers, and some damaged motor neurons of the peripheral nervous system die without regrowth. Damage to the myelin sheath and nerve fiber is often associated with increased functional insufficiency.
Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.
Some studies have revealed that optic nerve fibers can be regenerated in postnatal rats. This regeneration depends upon two conditions: axonal die-back has to be prevented with appropriate neurotrophic factors, and neurite growth inhibitory components have to be inactivated. These studies may lead to further understanding of nerve fiber regeneration in the central nervous system.
Clinical significance
Demyelination
Demyelination is the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including multiple sclerosis, acute disseminated encephalomyelitis, neuromyelitis optica, transverse myelitis, chronic inflammatory demyelinating polyneuropathy, Guillain–Barré syndrome, central pontine myelinosis, inherited demyelinating diseases such as leukodystrophy, and Charcot–Marie–Tooth disease. Sufferers of pernicious anaemia can also suffer nerve damage if the condition is not diagnosed quickly. Subacute combined degeneration of spinal cord secondary to pernicious anaemia can lead to slight peripheral nerve damage to severe damage to the central nervous system, affecting speech, balance, and cognitive awareness. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. A more serious case of myelin deterioration is called Canavan disease.The immune system may play a role in demyelination associated with such diseases, including inflammation causing demyelination by overproduction of cytokines via upregulation of tumor necrosis factor or interferon.
Symptoms
results in diverse symptoms determined by the functions of the affected neurons. It disrupts signals between the brain and other parts of the body; symptoms differ from patient to patient, and have different presentations upon clinical observation and in laboratory studies.Typical symptoms include blurriness in the central visual field that affects only one eye, may be accompanied by pain upon eye movement, double vision, loss of vision/hearing, odd sensation in legs, arms, chest, or face, such as tingling or numbness, weakness of arms or legs, cognitive disruption, including speech impairment and memory loss, heat sensitivity, loss of dexterity, difficulty coordinating movement or balance disorder, difficulty controlling bowel movements or urination, fatigue, and tinnitus.