Nervous Tissue

Nervous tissue, i.e., tissue forming the nervous system; possesses two basic qualities—irritability and conductivity; irritations received by it are processed and transformed into nervous excitation or nerve impulse, which is then transmitted to organs that perform work—muscles, glands (see Nervous system, physiology). The elements of N. tissue are nerve cells and nerve fibers (see), which constitute the morphological unit—the neuron. Between nerve cells and fibers are located cells and fibers belonging to neuroglia (see).-N. tissue is very rich in blood vessels, and together with them, connective tissue also enters into it. Already in unicellular organisms, there are limited areas, especially sensitive ones, which react to irritation; in multicellular organisms, however, special tissue develops that takes upon itself the function of perceiving irritation and conduction. This tissue develops from ectoderm. Then follows the centralization of nervous tissue, i.e., it all collects together and forms the central nervous system, and within the nervous tissue itself, differentiation is observed: cells gather in groups, forming gray matter, while fibers go in bundles, forming white matter (see Head brain). The structure of nervous tissue gradually becomes more complex and reaches maximum complexity in mammals, particularly in humans. When N. tissue becomes diseased, all elements included in it can be affected; various pathological processes are observed in it: disorders of blood circulation, processes of a regressive nature, tumors, inflammations, developmental defects. Some of these processes diffusely affect the entire N. tissue, for example, in inflammations in nerve elements, degenerative changes are observed, exudative processes on the part of the vascular apparatus, and proliferative changes on the part of glial and connective tissue; these changes can combine in various ways, sometimes the process affects only one particular element, and changes in it secondarily lead to changes in others. Loss of substance in N. tissue after any process is compensated for by either 1) proliferation of glial elements, or 2) growth of connective tissue located near blood vessels (scarring), or 3) development of a cyst in place of destroyed N. tissue. Nerve cells do not regenerate, whereas nerve fibers in the peripheral nervous system are capable of regeneration, of revival (see Nerve fibers, regeneration). Methods for staining N. tissue vary depending on which of its elements one wants to study; some methods are used for studying myelin fibers, others—cells and their processes, etc. For staining myelin fibers, the following methods are most commonly used: 1. Weigert's method (see Weigert staining methods). 2. Kulchitsky's method: staining celloidin preparations for 12-24 hours in hematoxylin [10% alcoholic solution of hematoxylin 10.0 (prepared in advance), concentrated acetic acid 1.0-2.0, Aquae dest. 100.0]; then sections without rinsing are transferred to a solution [water saturated with lithium (Lithium carbonicum), 100.0; 1% red blood salt 10.0], where differentiation occurs; myelin remains stained blue, while gray matter takes on a grayish-yellow color; sections are rinsed in water, dehydrated, etc.; xylol, balsam. Preliminary fixation in Müller's fluid. 3. March's method; very thin and small pieces (2-3 cm in length) are placed for 8-15 days in Müller's fluid, and then transferred for 8-12 days to a mixture of 2 parts Müller's fluid and 1 part of 1% osmic acid. The amount of fluid should be abundant, and the pieces should be suspended in such a way that they are washed on all sides. After processing with the mixture, rinse in running water for 24 hours; rapid dehydration with alcohol and embedding in celloidin; cut into 20-30 µ, sections are dehydrated, mounted in balsam; no additional staining is required. Myelin of unchanged nerve fibers takes on a very pale grayish color; regenerating fibers, even if they are in the very initial stages of change, are stained by osmium in an intense black color (the fibers appear as black dots). The method is very demonstrative, allows on serial sections to trace even very thin regenerating bundles over long distances, but it is applicable only in very fresh cases of regeneration (no more than 4-5 weeks) and requires careful handling; in case of failure, it cannot be corrected. 4. Bush's method—a modification of the previous method: fixation in formalin, and then for 8 to 15 days the preparations (also better suspended) are placed in the following mixture: 1% Acidi osmici 1.0, Natrii jodici 3.0 and dest. water 300.0; pieces are washed, dehydrated, embedded in celloidin, cut and studied without additional staining; with this method, pieces of larger size can be stained; its disadvantage is that it causes precipitates. 5. Stölzner's method. A quick method for staining myelin fibers. Sections fixed in formalin are placed for 10 min. or more (up to 24 hours) in a 50% solution of Ferri sesquichlorati; quickly rinsed and transferred to Weigert's hematoxylin for 15 min. (can be left for several hours), rinsed in water and differentiated in Weigert's mordant or in a weak solution of Ferri sesquichlorati, again rinsed in water, dehydrated and mounted in balsam. For staining ganglion cells, there are also various methods. For studying the finest structure of ganglion nerve cells, staining with methylene blue, thionin (see Nissl method), 1% Kresylviolet—staining for 30 min., rinsing in water, differentiation in alcohols of ascending concentration; xylol, balsam.-For silver impregnation of the finest endocellular network of fibrils of the axis cylinders, methods of Bielschowsky, Golgi (see), Ramon y Cajal are used. For isolated staining of neuroglia, very fresh material is necessary; the following methods have been proposed: Weigert's (see Weigert staining methods), Snesarev's (see Snesarev method), Mallory [fixation in Müller's or Weigert's fluid (for neuroglia) with formalin]; rinsing in water. Paraffin or frozen sections are placed for 2 min. in water acidified with acetic acid (1 drop per 10 g of water), then for 2 minutes in a highly diluted Mallory's hematoxylin (hematoxylin 1.75 parts, crystalline carbolic acid 5 parts, 10% phosphomolybdic acid 10 parts and dest. water 200 parts). Rinsing in water, dehydration; xylol, balsam.-For a general overview of changes, staining according to van Gieson (hematoxylin + picrofuchsin), hematoxylin, eosin, nile blue, safranin is recommended.-In addition to those mentioned above, there is a whole series of various methods for staining histological preparations of the nervous system.