Glial-Neuron Connections

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Biology 202

2006 First Web Paper

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Glial-Neuron Connections

Student Researcher

Advancements in the field of neuroscience within the past decade call the traditional models of brain structure and function into question. With an increased desire to uncover the origin of behavior, many neurobiologists have recently become interested in understanding how brain structure correlates to observable behavior. However, one troublesome observation has repeatedly surfaced. It appears that the brain has the interesting ability to change its structure based on environmental factors. This observation has an array of implications for the previously accepted notion that the structure of the brain directly influences and controls behavior. This observation shakes the foundations of neurobiology, as it calls into question one of its major tenants: that structure unequivocally equals function. What structure within the brain allows for this continual variance? How is this mechanism regulated? What inspires such structural change within one organism?

Brain plasticity is the term given to describing the phenomenon that the nervous system can change structurally, and perhaps functionally, in response to external stimuli. This broad term encompasses many changes that take place among both neurons and glial cells, the two classes of cells present in the nervous system. While much of neurobiology focuses on the interrelationship between sensory neurons, interneurons, and motor neurons, the significance of the glia, the cells that support these communication pathways, often gets neglected. Interestingly, it appears that many of the changes that were once attributed to neurons only are actually taking place in the glia (1). However, due to the enormous increase in research concerning brain plasticity within the past few decades, the role of these vital cells is coming to light.

Perhaps the main reason why glial cells have been written off for such a long time has been their inability to propagate an action potential or transmit any kind of chemical signal. In fact, of the five most common glial cells, only two are involved in any kind of support that isn't physical or housekeeping in nature (2). However, one glial cell, the astrocyte, appears to play a much more active role in the regulation of intercellular communication through the tight regulation of blood flow to each neuron (3). This regulation, also known as the blood-brain barrier (3), is so significant because neuronal communication depends directly on ions and hormones present in the circulatory system. The significance of the role that these astrocytes play can be clearly seen in a Stanford study concerning the ability and efficiency of the functioning of neurons and glia when separate versus when allowed to interact. The findings of the study indicate that while both cells perform their respective functions without each other, they perform then much more efficiently when allowed to interact (4). Because of the wide range of functions that the glial cells must perform in order to keep the nervous system in good shape, it's no wonder that they outnumber neurons by as much ten to one (5).

Due to the dependent nature of neuron and glial cells, much of the research originally aimed at uncovering the mechanisms behind brain plasticity have shed light on the role of glia within the brain. While brain plasticity usually refers to the change of location of the synapses between neurons, glial cells are also largely affected by environmental stimuli. William Greenough led the research in this field in the 1970s when he observed that not only can synapses have the capacity to change well after the developmental period of the brain, but that various glia also have such a capacity (1). Building on Greenough's work, Bryan Kolb and Ian Whishaw's experiments concerning brain plasticity in rats as a result of the level of daily stimulus reveals many more dimensions of the neuron-glia interdependence (1). When the visual field of the rats were stimulated for a period of sixty days, the researchers reported at 20% increase in dendritic fields, which correlates with a substantial increase in glial cells and associated blood supplies (1). The researchers repeated this experiment while manipulating the source of variation from visual stimulus to physical stimulus, to aural stimulus, and each time observed similar findings: that the glial cells of the brain play an important role in maintaining the integrity of the newly formed synapses that result from environmental stimuli (1). Given the close association between the astrocytes and the neurons of the nervous system, it makes perfect sense that as new synapses form, and thus as information gets rerouted within the brain, that the supportive elements of the brain must follow suit to keep up with the ever changing needs of the nervous system.

The research presented thus far indicates that perhaps, even though the importance of the role of glial cells has surfaced much more than in the past, that they are still an essential, yet underrated group of cells within the nervous system. In particular, it seems quite plausible that an important facet of the structure-function argument is lacking without the inclusion of the "higher end" glial cells, such as astrocytes and radial glia. The remarkable functions that these cells provide, be it from the astrocyte's regulation of what essential materials get to the neurons, or the radial glia's guiding function, raise the question, which is higher in function, neuron or glia? In our current classification system of the brain, should glia belong to a box of their own, or should they be included, side by side, with the neurons whose function is so dependent on their presence? Furthermore, considering the importance of the glia's role, and because glia, unlike neurons, are able to divide, what kind of a role could stem cell research someday play in the prevention and treatment of various devastating diseases that result from damaged to the glia, such as Multiple Sclerosis and Alzheimer's Disease? Ultimately, the most important question raised by the observations made concerning the role of glia in the nervous system is a simple one. What is our discussion lacking by not incorporating these vital cells as a focus similar to that of the neuron?

1)"Brain Plasticity and Behavior"

2)"Glia: The Forgotten Brain Cell"

3)Campbell, Neil A. et at. Biology. New York: Pearson, 2005

4)"Lowly Glia Strengthen Brain Connection"


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