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First discovered by the French Histologist Louis-
Myelination is the term used to describe the coating of axons with myelin to form the myelin sheath.
The process begins at the cranial nerves during the fifth month of pregnancy and continues throughout a person’s life [1]
Myelin -
The tertiary structure of myelin is very complex and has many loops which all promote the strength of the myelin sheath.
Dr. Jean Martin Charcot -
Special types of neuronal cells called glial cells are responsible for the formation of myelin.
This electrically-
Recurring inflammatory actions eventually causes scarring.
Symptoms resulting from multiple scarring vary considerably from person to person.
.
- Oligodendrocytes Cells -
are involved in the formation of myelin in the central nervous system (CNS brain and spinal regions) - Schwann Cells -
are involved in the formation of myelin in the peripheral nervous system (PNS the body)
Along the myelinated nerve fibres gaps in the sheath (Nodes of Ranvier) occur at
evenly-
The impulse moves from node to node in a wave like movement.
Lesions - also known as plaques, are patches of inflammation in the central nervous
system (CNS) in which the axons of neurons - (nerve cells) - have been partly stripped
of their myelin. Lesions tend to be randomly distributed in the CNS white matter.
The neurons of the white matter are responsible for sending communication impulses within the CNS and from the CNS to the rest of the body.
Demyelinated axons do not function efficiently and it is these lesions that give rise to the symptoms in various diseases.
As the disease progresses, the axons themselves can become scarred.
In relapsing-
Special maintenance cells called glial cells are responsible for the repair of the damaged nerves.
One type of glial cell in the Central Nervous System (CNS), called an oligodendrocyte, lays down new myelin and another type, called an astrocyte lays down scar tissue.
At the cellular level, what happens at the site of a lesion is very complex and varied.
Immune system component cells -
A complex mix of signalling molecules known as cytokines and chemokines mediates the destruction.
As well as damaged or destroyed myelin the Oligodendrocytes often die.
Nervous system components
- The Neuron (nerve cell) -
A neuron has a nucleus, cell body, dendrites and axon. Illustration - Dendrites extend a short distance from the cell body -
they bring information into the cell body - they have no myelin sheath. - Axons vary in length. They carry impulses away from the cell body.
- Normally each neuron has one axon, however, some specialised cells have more than one axon.
- Individual axons are microscopic in diameter (typically about 1μm across), but may be up to multiple feet long.
- The longest axons in the human body -
are those of the sciatic nerve - which run from the base of the spine to the big toe of each foot. These single- cell fibres of the sciatic nerve may extend a metre or more. - Astrocytes -
are large, star- shaped, maintenance cells within the central nervous system. They provide nerve cells with nutrients and structural support and are also involved in laying down the scar tissue typical of multiple sclerosis lesions. Glial scar formation is induced following damage to the nervous system. In the central nervous system, this glial scar formation significantly inhibits nerve regeneration, which leads to a loss of function. Several families of molecules are released that promote and drive glial scar formation. Transforming growth factors B- 1 and - 2, interleukins, and cytokines all play a role in the initiation of scar formation. The inhibition of nerve regeneration is a result of the accumulation of reactive astrocytes at the site of injury and the up regulation of molecules that are inhibitory to neurite extension outgrowth. - Leukocytes -
(White Blood Cells) - are cells of the immune system defending the body against both infectious disease and foreign materials. - Macrophages (Greek: "big eaters", from makros "large" + phagein "eat") They are cells
within the tissues that originate from specific white blood cells called monocytes.
Their function is to engulf and then digest cellular debris and pathogens either
as stationary or mobile cells, and to stimulate lymphocytes and other immune cells
to respond to the pathogen. Human macrophages are about 21 microns in diameter. Their
role is to phagocytose -
engulf and then digest - cellular debris and pathogens either as stationary or mobile cells, and to stimulate lymphocytes and other immune cells to respond to the pathogen. - Mast cell -
(or mastocyte), is a resident cell of several types of tissues and contains many granules rich in histamine and heparin. They play an important protective role as well as being intimately involved in wound healing and defence against pathogens. - T helper cells -
are a sub- group of lymphocytes (a type of white blood cell or leukocyte) that plays an important role in establishing and maximizing the capabilities of the immune system. It is this diversity in function and their role in influencing other cells that gives T helper cells their name.
How the nervous system works
Neurons (nerve cells) interconnect with each other and with body functions (muscles, skin, etc).
Connections between the nerve cells and elsewhere are made by axons, along which
messages (electrical - chemical impulses) from the nerve cell flow.
The Nervous System - is in two parts:
- The Central Nervous System (CNS) is composed of the brain and spinal cord, encapsulated by bone (skull) and vertebrae (spine). Tissue and fluid act as insulation for the brain and spinal cord.
- The Peripheral Nervous System (PNS) connects the rest of the body with the brain and spinal cord.
Neurons vary in their work
- They create awareness -
sensory neurons (afferent neurons) these relay messages from the skin, eyes, tongue, nose, ears into the central nervous system. - They create movement -
motor neurons (efferent neurons) relay impulses out from the central nervous system to the muscles and glands while Interneurons relay impulses between sensory and motor neurons.
Although the nervous system is extremely complex it has only three functions:
- Sensory Input (sensory neurons) -
provides information to the central nervous system and assists organs to adjust to the surrounding environment, both inside and outside the body. - Integration -
the central nervous system has to make sense of the input it is receiving. Interneurons integrate input from sensory nerves into the central nervous system. - Motor Output (motor neurons) -
respond to information received by the sensory neurons. The nervous system stimulates muscles and glands.
Myelination is an important process because it helps to increase the speed at which action potentials propagate (move with a pulsing wave like action) along axons efficiently [2] This importance is demonstrated by the various diseases that can occur from demyelination; which include:
- Adrenoleukodystrophy -
ALD (also known as Addison- Schilder Disease or Siemerling- Creutzfeldt Disease) is a rare, inherited disorder that leads to progressive brain damage, failure of the adrenal glands and eventually death. ALD is one disease in a group of inherited disorders called leukodystrophies. - Multiple Sclerosis -
An unpredictable disease of the central nervous system, multiple sclerosis symptoms can range from relatively benign to somewhat disabling, as communication between the brain and other parts of the body is disrupted. It is the nerve- insulating myelin that comes under assault. Multiple sclerosis - MS is a chronic disease of the central nervous system - CNS. It tends to be characterised by inflammation and degradation of myelin within the CNS. - Transverse Myelitis -
a neurological disorder caused by inflammation across both sides of one level, or segment, of the spinal cord. Myelitis refers to inflammation of the spinal cord and transverse describes the position of the inflammation, that is, across the width of the spinal cord. - Inflammation can damage or destroy myelin and may / will cause scars (sclerosis) of the axons leading to the disruption of the impulses from the nerve cell / cells and the various body components they control. [3]
Phospholipids are a class of lipids and are a major component of all cell membranes
-
- Barkovich -
2000 - Lehninger -
1968 - Loring -
2007 - Michelle, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna
Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood
Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-
13- 981176- 1. OCLC 32308337