Equipping People To Make Sense Of What They Are Told
Myelin was first discovered by the French Histologist Louis-Antoine Ranvier in 1878.
This event was the foundation of understanding how the process of myelination occurs
and its importance in the nervous system.
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 - is composed of about 80% fat and 20% protein. These proteins include myelin
basic protein, proteolipid protein and myelin oligodendrocyte glycoprotein.
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 -1825 to 1893, was the first person to scientifically explain
and document this disease.
Special types of neuronal cells called glial cells are responsible for the formation
of myelin.
This electrically-insulating phospholipid covering surrounding the axons of neurons
within the central nervous system (CNS) expose the axon to its surrounding environment
when demyelination occurs causing the wave of impulse signals from the neuron moving
along the axon to become weakened or interrupted as a direct result of demyelination.
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)
Node of Ranvier
Along the myelinated nerve fibres gaps in the sheath (Nodes of Ranvier) occur at
evenly-spaced intervals, enabling an especially rapid mode of electrical impulse
propagation called saltation. Only at these nodes can ion exchange take place, and
as a result myelinated nerve axons conduct their impulses much more rapidly than
an unmyelinated nerve axon.
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-remitting MS, there may be significant recovery as the inflammation
dies down. In progressive forms of MS, recovery can be less significant.
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 - leukocytes especially Helper T-cells, Macrophages
and possibly mast cells appear to be involved.
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.
Oligodendrocyte cells are responsible for the formation of myelin sheaths surrounding
axons. They are not confined to the white matter and within grey matter they are
often closely associated with nerve cells. They have relatively small amounts of
cytoplasm around the nucleus but have several processes which wrap themselves around
axons to form an insulating sheath composed of a variable number of layers of cell
membrane, through which very little current leeks. Nodes of Ranvier are composed
of gaps between myelin sheets formed by different oligodendrocytes and at these points
almost all of the sodium channels responsible for nerve conduction exist.
A single oligodendrocyte can myelinate several axons (usually 10 - 25). The formation
of the myelin sheath is the result of a process from an oligodendrocyte spiralling
around an axon so that the cytoplasm is extruded until the opposite membranes meet,
thus forming a multi-layered lipoprotein coat with a node of Ranvier at each end.
An internode is the section between two nodes of Ranvier.
The thickness of the myelin sheath is related to the diameter of the axon, as is
the internodal length.
Axons impel electrical / chemical signals away from the cell body in a wave like
motion. The impulse travels along an axon by jumping from one node of Ranvier to
the next, a process known as saltatory conduction. The presence of a myelin sheath,
therefore both speeds up the conduction process and reduces the amount of energy
used. The faster conducting axons have the thickest myelin sheath and the longest
internodes.
The speed that a nerve impulse travels along an axon is also proportional to the
internodal length and since this is proportional to the diameter of an axon, the
larger the diameter of the latter the faster the speed of conduction.
Loss of myelin, known as demyelination, results in disturbance of the ability to
transmit a nerve impulse through the demyelinated segment and this has serious consequences
for function.
Because a single oligodendrocyte cell supports many axons as well as producing myelin
for the segments of axons they support the loss of a single oligodendrocyte cell
can result in loss of myelin from many axon segments.
Some remyelination can occur but this is limited in amount. Remyelination can be
very efficient but varies with its cause and with age. It appears to be less efficient
when the white matter in the same place is repeatedly damaged and the longer the
process of demyelination lasts.
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
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.
Phospholipids are a class of lipids and are a major component of all cell membranes
- Lipids are broadly defined as any fat-soluble naturally-occurring molecule, such
as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins (such as vitamins
A, D, E and K).
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
