NERVOUS TISSUE
Author: Dr. R. Menaka
...................................................................................................
The nervous tissue is specialized to receive stimuli from the environment to transform them into nerve impulses and to transmit them to the nerve centre, from where appropriate response is transmitted to another organ or part of the body which reacts to the response of the original stimulus. Nucleus is pale staining and vesicular but the nuclear membrane stains dark with haematoxylin- chromatin is fine and dispersed. Nucleolus is single larger very prominent and may be basophilic or acidophilic. In sympathetic ganglia, the nucleus is eccentric in position and cells are often binucleate. The cytoplasm contains mitochondria, golgi apparatus chromophil substance or Nissl bodies and neurofibrils. The neurofibrillae are fine cytoplasmic fibrils which form bundles and interlace in the cell body but are parallel in axon and dendrites, demonstrate by special techniques neuroplasm- is the undifferentiated part of the cytoplasm which surrounds and separate the neurofibrillae. It is also called interfibrillar substances. The chromophil substance also called as Nissal bodies are characteristics of nerve cell bodies. They form irregular clumps or granules in the neuroplasm between the bundles of neurofibrils. They are basophilic. They are seen in the dendrites also but not in the axons and axon hillock. Larger in motor neurons then in sensory neurons.
E/m studies reveal that Nissal bodies are composed of dense network of rough surface endoplasmic reticulum which RNA granules on the outer surface of the double membranes. Their presence indicates high metabolic activity in synthesizing neurotransmitter protein, essential for replenishment for cytoplsmic material in the axons. Following repeated stimulation during degeneration and regenerations after an injury to an axon or in other pathlogical conditions. Nissal materials undergoes an apparent reduction or disappearance (chromotolysis).
Nerve cells amy also show some inclusions or pigments under certain conditions. Two kinds of pigments are seen. Lipofuchsin and melaniniL lipofuchsin is yellowish or brown insoluble in usual fat solvents and appear in the form of granules dispersed throughout the cell. This pigment is not present in the new born birth appear in increasing amounts with advancing age.
Melanin appears in the form of the brown granules in nerve cells of the olfcatory bulb, in the floor of the IV ventricles, the substantia nigra of midbrain etc. Its significant is not known.
Nerve fibers are formed by the axon and its sheath. All peripheral fibers consists of the axon covered with a thin protoplasmic sheath called neurolemma or sheath of Schwann. Another sheath myelin or medullary sheath seen in some nerve fibers is next to the axis cylinder and neurolemma. Those without myelin sheath are amyelinated or non-medullated nerve fiber.