Posts Tagged ‘Axon’

Our Neurons Make Up Only 15 Percent of Our Brain Cells

May 4, 2011

So what makes up the rest of our brain cells—glial cells. When I was a graduate student no one had a good idea what glial cells did. Glia comes from the Greek word for glue, so the best bet was the glial cells helped hold the brain together. An article1 in Scientific American Mind brought me up to date and demonstrated how woefully ignorant we were at that time. There are different types of glial cells. Astrocytes ferry nutrients and waste and mediate neuronal communication. Oligodendrocytes coat axons with insulating mylein, boosting signal speeds. Microglia fight infection and promote repair.

Previously, the neuron doctrine governed our understanding of the brain. According to the neuron doctrine all information in the nervous system is transmitted by electrical impulses over networks of neurons linked through synaptic connections. Recent research has demonstrated that some bypasses neurons completely, and flows without electricity through networks of glial cells. It has shown the role of glial cells in information processing and learning, as well as in neurological disorders and psychiatric illness.

In contrast to neurons, which communicate serially across chains of synapses, glia broadcast their signals widely throughout the brain, similar to cell phones, In contrast to the rapid communication throughout neural networks, the chemical communication of glia is very slow and spreads like a tidal wave through neural tissue at a pace of seconds or tens of seconds.

New brain imaging techniques have shown that after having engaged in such activities as learning to play a musical instrument, to read, or to juggle, structural changes occur in brain areas that control these cognitive functions. What is remarkable is that changes are seen in regions whee there are no complete neurons. These are “white matter” areas that are formed from bundles of axons coated with myelin, a white electrical insulator. All theories of learning had held that it is solely by strengthening synaptic connections is how learning occurs. As there are few synapses in while matter, clearly something else is happening that involves glial cells.

With respect to neurological and psychological illnesses, glial cells have been found to play a role. Alzheimer’s Disease is one of these illnesses, but the discussion of Alzheimer’s and glial cells will be postponed to a subsequent post. Glial cells account for the mystery of why spinal cord injury results in permanent paralysis. Proteins in the myelin insulation that oligodendrocytes wrap around axons stop injured axons from sprouting and repairing damaged circuits. Chronic pain is the result of microglia do not stop releasing the substances that promote the healing processes after healing is complete. Consequently, sensitivity to pain continues after healing is complete.

It is not surprising that glia play a central role in neurological disease as astrocytes and microglia are first responders to disease. Compulsive behavior, schizophrenia, and depression might all have there roots in the glial cells. Epilepsy is also regarded as a prime-candidate for glial-based therapeutics.

1Fields, D.R. (2011). The Hidden Brain. Scientific American Mind. May/June, 53-59.

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