The focus of this book is the normal biology of glia. However, glial cells are involved in almost every type of brain pathology. This is particularly true for microglia, which function primarily in pathology. In this final section of the book, therefore, we introduce some of the important aspects of the role of glia in pathology.
Insults to the nervous system trigger specific reactions of glial cells, generally known as a reactive gliosis, which include the reaction of astroglia (reactive astrogliosis), microglia (activation of microglia) and oligodendroglial and Schwann cells (Wallerian degeneration and demyelination). All these reactions of glial cells are of critical importance for the progress of neural pathology. In the most general terms, one has to remember that astroglial cells can outlive neurones; moreover, the astroglia very often are activated in the presence of dying or already dead neurones; conversely, neurones cannot survive without astrocytes. Similarly, in demyelinating diseases axons cannot function properly when myelinating cells malfunction. Finally, the CNS as a whole has no protection against infection except microglia, and failure of the latter to perform leads to irreparable CNS damage. Beside these, rather profound morphological and biochemical reactions, astroglial cells may be involved in relatively rapid pathological processes, as represented by spreading depression.
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