The neurons are surrounded by neuroglia (or glia) cells. These differ from the neurons in that they do not have electrically excitable membranes. They comprise nearly half the brain volume and function to separate and support the neurons. There are two main types of neuroglial cells, termed the macroglia and microglia. The macroglia are divided into the astrocytes, oligodendrocytes and ependymal cells. The astrocytes are characterized by long narrow cellular processes which give them a star-like structure; through their feet-like endings they can make contact with both the capillaries and neurons. It has been suggested that the astrocytes play a role in conducting nutrients from the blood to the neurons. Other roles include the removal by active transport of released neurotransmitters (particularly the inhibitory transmitter gamma-aminobutyric acid-GABA), the provision of precursors for transmitter synthesis (e.g. glutamine for the synthesis of GABA) and the buffering of the neuron against excessive concentrations of potassium ions formed following depolarization. Thus the astrocytes appear to play a critical role both in terms of physical protection of the neurons and in providing a metabolic buffer to ensure homeostasis. The oligodendrocytes occur in both grey and white matter. In white matter they form the insulating myelin sheath around the axon, whereas in grey matter they probably provide myelin for axons coursing through the grey matter. In the peripheral nervous system, these functions are fulfilled by the Schwann cells. The myelin is formed by the outgrowth of the plasma membrane of the oligodendrocyte which is wrapped several times around the axon, thereby excluding extracellular fluid from between the layers of the plasma membrane and thereby generating a highly insulating coat. This insulating coat, which is interrupted at the nodes of Ranvier, is important for the efficient transmission of the electrical impulses down the axon.
The ependymal cells line the inner surfaces of the ventricles and, together with the neuroglial cells, appear to be involved in the exchange of material with the surrounding cerebrospinal fluid. The microglia can be considered as the macrophage cells of the brain whose function is to remove cell debris by a process of phagocytosis following neuronal damage. There is also evidence that the macroglia are involved in localized inflammatory processes within the brain and may play an important role in the cause of neurodegenerative diseases such as Alzheimer's disease (see Chapter 14). Damage to brain tissue is associated with the proliferation of neuroglia. This is termed gliosis and is associated with an increase in the number of macroglia and microglia. Scar tissue is frequently associated with gliosis.
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