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Glial cellsThe Root of Thought Andrew Koob FT Science Press INTRODUCTION: This book draws attention to the importance of glial cells and particularly astrocytes in the functioning of the brain. This is an attempt to reverse more than a century of research neglect, since Cajal asserted the overriding primacy of the neuron in brain function. The material here has no immediate connection to consciousness studies, except to remind us that many approaches to consciousness work with an over simplified and unenquiring model of brain function. The relative neglect of the glia goes back more than a century to Cajal's promotion of the idea that these cells were just a form of buffering between neurons. This idea became entrenched, before it came to be realised that glia were also capable of electrical signaling. Glial cells include Schwann cells, epthelial cells, oligodendricytes, microglia, astrocytes, Moller cells and ependymal cells. Schwann cells are responsible for the mylenisation of axons that allow the rapid transmission of electrical signals. The Schwann cells release the fatty myelin from their cell body, and can swirl it around a group of axons. Oligodendrocytes can also perform this function. Ependymal cells, endothelial cells, tancytes and microglia are involved with the blood brain barrier or other protective functions. Neurons are seen to be physically dependent on astrocytes, rather than the other way round. Neurons in a petri dish die without astrocytes, but astrocytes can survive by themselves. The presence of astrocytes is also necessary for the formation of new synapses. In the hippocampus, which is involved in the formation of memories, about 80% of large synaptic contacts are surrounded by astrocytes. The glial cell that has attracted most attention in recent years is the astrocyte. This is the most abundant type of cell in the human cortex. The proportion of glial cells in the brain rises in relation to the intelligence of an animal, and the proportion of astrocytes in the cortex is also greater in more intelligent animals. Synaptic strengthening is seen as the most likely basis of learning, and it is suggested that astrocytes are involved in this process. The author also suggests that astrocytes might be involved in the processing of sensory inputs, and be capable of influencing motor action. Astrocytes are linked to blood vessels, and appear to control the oxygenation of neurons that is required to pump ion channels. Astrocytes also produce proteins in some areas of the brain, and through this can influence the release of hormones into the bloodstream. Glial cells have been discovered to have a role in the brain's information system. The glial communicate amongst themselves by means of electrical waves triggered by the influx of calcium ions. The glia also receive signals from and send signals to neurons. Neurotransmitters from neurons stimulate astrocytes, and cause an influx of calcium ions. Astrocytes have receptors for every type of neurotransmitter produced by neurons, and these can set off calcium signaling waves within astrocytes. Further to this, it has now been demonstrated that these waves can influence the firing of neurons. A recent study shows that glutamate that was previously thought to be only involved with neurons can also be released from astrocytes, and can cause signaling in neurons. Further, the neurotransmitters, glutamate, aspartamate and GABA cause changes in the electrical potential of astrocytes. Glutamate released from neurons can set of a wave of calcium in astrocytes. Working in the other direction, glutamate released by astrocytes might be taken from the extracellular space by neurons. Astrocyte receptors correspond to the type of neuron that they are near. An astrocyte in the cortex will have glutamate receptors, while an astrocyte in the basal ganglia will have dopamine receptors. Neurons require extracellular calcium to set off synaptic signaling, and some of this extracellular calcium may derive from neighbouring astrocytes. Calcium ions are seen as crucial to the signaling systems of glial cells. In astrocytes, signaling derives from internal calcium stores. The astrocytes store calcium in three organelles, the endoplasmic reticulum, the Golgi complex and mitochondria. Astrocyte signaling is based on calcium released from storage in the endoplasmic reticulum and mitochondria, and then then spreads to other astrocytes through gap junctions. Astrocytes thus form a net work of calcium signaling. The development of a wave of calcium signals is determined by the pattern of previous calcium signaling in the affected astrocytes. Also, a neurotransmitter acting on an astrocyte can set off a calcium wave spreading from one astrocyte to the next. Astrocytes are known to form gap junctions with one another. The gap junction determines what can pass from cell to cell. A single astrocyte in the cortex will connect with 50 to 100 astrocytes via gap junctions. Oligodendrocytes that are involved in myelinisation also form gap junctions with astrocytes. Gap junctions can also exist between astrocytes and neurons. It is suggested that astrocytes may turn out to be synchronised, when they are active, in the same way that active neurons become synchronised at particular frequencies. |
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