What is Myelin?

SOURCE: msrc.co.uk

In order to understand Multiple Sclerosis and the role Myelin has to play in this disease , some basic facts about the nervous system in the body need to be addressed.

The body's nervous system is made up of two main components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Together, these two systems interact to carry and receive signals that are responsible for nearly every function within the body, including involuntary functions, those a person does not have to consciously think about, such as the beating of the heart or breathing, and voluntary functions such as walking or using your fingers on a computer keyboard to read and scroll up and down this page, which to some extent are consciously thought about .

The Central Nervous System (CNS) is made up of the brain and the spinal cord, and contains billions of specialized cells known as Neurons. Neurons have specific projections called Dendrites and axons that contribute to their unique function of transmitting signals throughout the body. Dendrites carry electrical signals to the Neuron, while axons carry them away from the Neuron.

The Peripheral Nervous System (PNS) consists of the rest of the Neurons in the body outside of the Central Nervous System (CNS) . These include the Sensory Neurons, which detect any sensory stimuli and alert the CNS of their presence, and Motor Neurons, which connect the CNS to the muscles and carry out instructions from the CNS for movement.

Myelin, is sometimes referred to as "white matter" because of its white, fatty appearance, it protects and insulates the axons. Myelin consists of a protective sheath of many different molecules that include both lipids (fatty molecules) and proteins. The protective sheath acts in a very similar way to that of the protective insulation that surrounds an electric wire; that is, it is necessary for the rapid transmission of electrical signals between Neurons. It does this by containing the electrical molecules within the Axon so that they are all properly transmitted to the next Neuron.

With the protective Myelin coat, Neurons can transmit signals at speeds up to 60 meters per second.

When the coat is damaged, as with Multiple Sclerosis, the maximum speed can decrease by ten-fold or more, since some of the signal is lost during transmission. This decrease in speed of signal transmission leads to significant disruption in the proper functioning of the nervous system.

Picture below from www.NLM.NIH.gov/

More about Myelin from Wikipedia.org

Myelin is an electrically-insulating dielectric phospholipid layer that surrounds only the axons of many neurons. It is an outgrowth glial cells: Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes supply it to those of the central nervous system. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates.^[1] Myelin was discovered in 1878 by Louis-Antoine Ranvier.

Composition of myelin

Myelin made by different cell types varies in chemical composition and configuration, but performs the same insulating function. Myelinated axons are white in appearance, hence the "white matter" of the brain.

Myelin is composed of about 80% lipid fat and about 20% protein. Some of the proteins that make up myelin are Myelin basic protein (MBP), Myelin oligodendrocyte glycoprotein (MOG), and Proteolipid protein (PLP). Myelin is made up primarily of a glycolipid called galactocerebroside. The intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.

Function of myelin layer

The main consequence of a myelin layer (or sheath) is an increase in the speed at which impulses propagate along the myelinated fiber. Along unmyelinated fibers, impulses move continuously as waves, but, in myelinated fibers, they hop or "propagate by saltation." Myelin increases resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50.^{[citation needed]} Myelination also helps prevent the electrical current from leaving the axon. When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.

Demyelination and Dysmyelination

Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including multiple sclerosis, acute disseminated encephalomyelitis, transverse myelitis, Alexander's disease, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré Syndrome and central pontine myelinosis. Sufferers of pernicious anaemia can also suffer nerve damage if the condition is not diagnosed quickly. Sub-acute combined degeneration of the cord secondary to pernicious anaemia can lead to anything from 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.

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 (TNF)^[2] or interferon.

Heavy metal poisoning may also lead to demyelination. Even very small amounts of mercury have been shown to be particularly destructive to nerve sheaths.^[3]

Research to repair damaged myelin sheaths is ongoing. Techniques include surgically implanting oligodendrocyte precursor cells in the central nervous system and inducing myelin repair with certain antibodies. While there have been some encouraging results in mice (via stem cell implant), it is still unknown whether this technique can be effective in humans.^[4]

Dysmyelination on the other hand is different from the lesions producing process of active demyelination and is characterized by defective structure and function of myelin sheaths. Such defective sheaths often arise from genetic mutations affecting the biosynthesis and formation of myelin. Examples of human diseases where dysmyelination has been implicated include leukodystrophies (Pelizaeus-Merzbacher disease, Canavan disease, Phenylketonuria) and schizophrenia

Symptoms of Demyelination

Demyelination destruction or loss of the myelin sheath typically results in diverse symptoms. The symptoms are determined by the functions normally contributed by the affected neurons.

Damage to the myelin sheath disrupts signals between the brain and other parts of the body producing a range of symptoms. Symptoms are often heterogeneous — dependent on pathophysiology of demyelination — differing from patient to patient, and have different presentations upon clinical observation and in laboratory studies.

See also

References