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The Other Brain
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Lisa Johnson
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7/2/2012
November of 2010 was an exciting time, in fact, a revolutionary time in the study of the
function, or lack of as in the case of neurological illness, of the brain. For many years the
neurons were given credit for being the star of the show when it came to processing information
and storing memories. Neurons, which are impulse conducting cells, are made up of a cell body
with one or more dendrites and a single axon. A dendrite is a tree like extension of the cell that
pulls impulses from adjacent cells into the cell body. The axon can be found on the other end of
the cell. It is a long nerve fiber which pulls impulses away from the neuron’s cell body. These
flashy, electrical impulses coming in and out of the neurons kept the attention of neuroscientists,
while the “silent, dull majority” of cells, known as Glia, were overlooked.
Glia were first noticed back in 1856 by a German biologist named Rudolf Virchow. He
happened upon a “putty-like substance” full of nerve cells which he called neuroglia, which
meant “nerve glue” in Greek. Ten years later, after further observations of the neuroglia, a
German anatomist named Otto Karl Deiters noticed the nerve cells were tailless; meaning, they
were missing an axon and therefore could not be neurons. Over the next century, different
explanations of these odd looking cells were presented. A common thread between the
explanations was the conviction that Glia served as passive counterparts to the neurons’ active
central role in the brain. Because it was thought that Glia’s only purpose was to support the
neurons, its research was neglected for almost 100 years. This thought process, however, is
changing, and changing rapidly.
Neuroscientists are fascinated by this turning point in brain research. R. Douglas Fields,
Ph. D., Editor-in-Chief of the journal “Neuron Glia Biology”, is calling this new frontier “The
Other Brain.” The Glia are just beginning to be explored and already these findings of their
functions are leading to new treatments of diseases. Diseases ranging from Alzheimer’s,
Multiple Sclerosis, and chronic pain to spinal cord injuries and brain cancer are all benefitting
from these new findings.
Through experiments conducted during the 1980s and 1990s, scientists found that glial
cells are not passive. They found that glial cells not only respond to a neuron’s chemical signal,
but they actually send their own chemical signal to neurons, which in turn influence the neuron’s
activities. In short, Glia had been talking- to each other and to neurons- all along. The
difference is that Glia only communicate chemically, while neurons communicate both
chemically and electrically. They now realize that glial cells are involved in all aspects of
neuron function. They have found that glial cells are more intricate and involved in many more
processes than neurons. They help maintain the brain’s environment, respond to injuries,
regulate neurotransmitters and synapses (chemical substances that transmit nerve impulses and
the junction they pass to communicate with neurons, muscle cells, or gland cells) and they can
even become neurons.
By 2010, researchers had discovered four types of Glia and by 2011 they had discovered
an additional type, bringing it to five. Astrocyte, Microglia, Oligodendrocyte, Schwann and the
newest, NG2 are the five types of glial cells discovered so far. Astrocytes, the most common
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glial cell in the brain, are star shaped and fill the spaces between neurons. They are the energy
source, as well as the maintainer of their chemical environment. Microglia are little cells that are
in healing and defending the brain from disease. They are the private immune cells of the brain.
NG2 Cells can transform into Oligodendrocytes and Astrocytes, as well as neurons. This newest
cell discovery is leading to even more questions when it comes to distinguishing the difference
between the glial and neuron cells.
The last two glial cell types (Oligodendrocyte and Schwann) are both involved in
creating the myelin sheaths that protect axons. Myelin is a substance rich in lipids (80%) and
protein (20%) that forms layers around the nerve fibers and acts as insulation. A myelinated
axon’s electrical impulse transmission is 50 times faster than a bare axon. The speed of impulse
transmissions has a huge impact on information processing in the neural circuit. The octopus-like
Oligodendrocyte cell can create many myelin sheaths at once by wrapping the tips of its tentacles
around several axons. The Oligodendrocytes usually myelinate 10 to 15 axons simultaneously,
but can process up to 50 axons at once. Meanwhile, the Schwann cells are only able to myelinate
one axon at a time. The Oligodendrocytes myelinate the axons found in the Central Nervous
System (brain, spinal cord, and optic nerves), while the Schwann cells process in the Peripheral
Nervous System. Because Schwann cells are the only Glia outside the brain and spinal cord,
they have a range of roles in addition to creating myelin. Most notably, are the roles such as
chemical clean ups and nerve regeneration.
The importance of the myelin insulation is apparent in people who suffer from Multiple
Sclerosis (MS). MS is an autoimmune disorder that causes the immune system to produce
antibodies that attack the myelin sheaths covering the axons. Because these attacks leave holes
or lesions in the myelin and disrupt the electrical impulse transmissions, many MS patients are
left with serious impairments in the sensory and motor function. Scientists have discovered that
the body can heal some of these lesions naturally by stimulating Oligodendrocytes in the area or
by recruiting younger Oligodendrocytes from further away locations, to begin making new
myelin at the damaged site. Since 2001, a number of experimental studies have implanted
Schwann cells in an attempt to begin remyelination in MS patients. Studies have also shown
positive results for Schwann cell transplantation for spinal cord patients. The cells would help in
aiding regrowth and remyelination of damaged Central Nervous System (CNS) axons. It took
scientists years to understand why damaged nerves in the body’s Peripheral Nervous System
(PNS) can heal over time, while nerves in the CNS could not. The difference is the myelin
creating cells. Remember, the CNS has Oligodendrocyte cells that only create myelin while the
PNS has Schwann cells that can support the repair of damaged neurons, provide vital proteins
that protect nerve cells after injury, encourage new axons to grow and reconnect with proper
structures, and still create myelin. This is why scientists have filed an application to begin the
first phase of a new cell transplantation technique. They hope neurons will grow and repair
damaged connections by transplanting the Schwann cell, with all its additional healing
properties, to support and guide them.
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This seems to be a hopeful time for patients needing help with the remyelination process.
As this new dimension of brain science involving the Glia is unfolding, and as neuroscientists
begin to explore it, they are gaining an invaluable new understanding of the brain. This
understanding is quickly leading to new treatments for these types of diseases. R. Douglas Fields
summed it up nicely when he called this new frontier of neuroscience “The Other Brain.”
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Literature Cited
Ferris Jabr; Know Your Neurons: Meet the Glia
Scientific American; May 18, 2012 Brainwaves Blog Network
R. Douglas Fields; Glia; The New Frontier in Brain Science
Scientific American: November 4, 2010 Guest Blog
R. Douglas Fields; Preliminary Human Experiments to Test Safety of Nerve Cell
Transplants for Spinal Cord Paralysis
Scientific American: October 19, 2011 Mind News
Peter Stern; Glee for Glia
Science; November 5, 2010 ; Vol 330 no. 6005 pg. 773
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