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13.10
Introduction to neuroglial cells
  You will recall in each and every level of matter there exists a symbiotic co-dependence, between matter- the proton, the neutron, the star, the planets, the cell and the mitochondria and chloroplasts that supply the necessary reactions within cells to produce basic food.  
  Yet when it comes to neurons, little if anything tends to be written about its bonded twin- glial cells. Without healthy glial cells, neurons can not function one second. And it is because of the degradation of glial cells that many neural diseases occur.  
13.10.1 The glue that enables neurons to do what they do  
  Neurons form a minority of the cells in the nervous system. Exceeding them in number by at least 10 to 1 are neuroglial cells, which exist in the nervous systems of invertebrates as well as vertebrates. Neuroglia can be distinguished from neurons by the lack of axons and the presence of only one type of process. In addition, they do not form synapses, and they retain the ability to divide throughout their life span. While neurons and neuroglia lie in close apposition to one another, there are no direct junction specializations, such as gap junctions, between the two types. Gap junctions do exist between neuroglial cells, raising questions about their functions and properties.  
  The term neuroglia means "nerve glue," and these cells were originally thought to be structural supports for neurons. This is still thought to be plausible, but other functions of the neuroglia are now generally accepted. It has long been known that oligodendrocytes and Schwann cells produce the myelin sheath around neuronal axons. Studies show that some constituent of the axonal surface stimulates Schwann cell proliferation and that the type of axon determines whether there is loose or tight myelination of the axon. In tight myelination a glial cell wraps itself like a rolled sheet around a length of axon until the fibre is covered by several layers. Between segments of myelin wrapping are exposed sections called nodes of Ranvier, which are important in the transmission of nerve impulses. Myelinated nerve fibres are found only in vertebrate animals, leading biologists to conclude that they are an adaptation to transmission over relatively long distances.  
13.10.2 The types of neuroglia cells  
  Three main groups of neuroglia cells have been identified: (1) astrocytes, subdivided into fibrous and protoplasmic types; (2) oligodendrocytes, subdivided into interfascicular and perineuronal types; and sometimes (3) microglia.  
  Fibrous astrocytes are prevalent among myelinated nerve fibres in the white matter of the central nervous system. Organelles seen in the somata of neurons are also seen in astrocytes, but they appear to be much sparser. These cells are characterized by the presence of numerous fibrils in their cytoplasm. The main processes exit the cell in a radial direction (hence the name astrocyte, meaning "star-shaped cell"), forming expansions and end feet at the surfaces of vascular capillaries.  
  Unlike fibrous astrocytes, protoplasmic astrocytes occur in the gray matter of the central nervous system. They have fewer fibrils within their cytoplasm, and cytoplasmic organelles are sparse, so that the somata are shaped by surrounding neurons and fibres. The processes of protoplasmic astrocytes also make contact with capillaries.  
  Oligodendrocytes have few cytoplasmic fibrils but a well-developed Golgi apparatus. They can be distinguished from astrocytes by the greater density of both cytoplasm and nucleus, the absence of fibrils and glycogen in the cytoplasm, and large numbers of microtubules in the processes. Interfascicular oligodendrocytes are aligned in rows between the nerve fibres of the white matter of the central nervous system. In gray matter perineuronal oligodendrocytes are located in close approximation with the somata of neurons. In the peripheral nervous system, neuroglia that are equivalent to oligodendrocytes are called Schwann cells.  
  Microglial cells are small, crenate cells with dark cytoplasm and a dark nucleus. It is uncertain whether they are merely damaged neuroglial cells or occur as a separate group in living tissue.  
13.10.3 The function of neuroglia  
  Another well-defined role of neuroglial cells is in repair following injury to the central nervous system. It has been well documented that astrocytes divide after injury to the nervous system and that they occupy the spaces left by injured neurons. The role of oligodendrocytes after injury is not so clear, but evidence suggests that they can proliferate and form myelin sheaths.  
  When neurons of the peripheral nervous system are cut, they undergo a process of degeneration followed by regeneration, the fibres regenerating in such a way that they return to their original target sites. Schwann cells that remain after nerve degeneration apparently mark the route. This route direction is also performed by astrocytes during development of the central nervous system. In the developing cerebral cortex and cerebellum of primates, astrocytes project long processes to certain locations, and neurons migrate along these processes to arrive at their final locations. Thus, neuronal organization is brought about to some extent by the neuroglia.  
  Astrocytes are also believed to have high-affinity uptake systems for neurotransmitters such as glutamate and gamma-aminobutyric acid (GABA). This function is important in the modulation of synaptic transmission. Uptake systems tend to terminate neurotransmitter action at the synapses and perhaps also act as storage systems for the neurotransmitters when they are needed. For instance, when motor nerves are cut, the nerve terminals degenerate and their original sites are occupied by Schwann cells. It has been found not only that electrical signals can be recorded on muscle cell receptors in the absence of any form of stimulation but also that currents applied to the Schwann cells evoke neurotransmitter release. Apparently, the synthesis of neuro-transmitters by neurons also requires the presence of neuroglial cells in the vicinity.  
  In the past it was thought that neuroglia were not electrically excitable, but it has been shown that neuroglial cells in vitro have voltage-sensitive properties similar to those of excitable neurons. If electrical activity similar to that occurring in neurons were generated in neuroglial cells in vivo, the implications for glial-neuronal interaction would be enormous. Such proof is not available, however.  
  Finally, the environment surrounding neurons in the brain consists of a network of very narrow extra cellular clefts. In 1907 the Italian biologist Emilio Lugaro suggested that neuroglial cells exchange substances with the extra cellular fluid and in this way exert control on the neuronal environment. It has since been shown that glucose, amino acids, and ions--all of which influence neuronal function--are exchanged between the extra cellular space and neuroglial cells. After high levels of neuronal activity, for instance, neuroglial cells can take up and spatially buffer potassium ions and thus maintain normal neuronal function.  
     
     
 
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