Tissue and Other Levels of Organization: Nervous Tissues, Neurons, Neuroglia Cells and Transmission of Nerve Impulse (For CBSE, ICSE, IAS, NET, NRA 2022)

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Nervous tissue consists of two main types of cells: neurons and neuroglia cells. Nerve cells or neurons transmit nerve impulses that move information around the body and Neuroglia cells are also known simply as glia and have various functions in support of nerve cells but do not transmit nerve impulses themselves.

Neurons (Nerve Cells)

Neurons are the functional units of nervous system. Nervous tissues constitute the brain, spinal cord, nerves and the sensory cells and sense organs. A neuron consists of two distinct regions.

  • Cell body: contains a nucleus and majority of cells other organelles embedded in a mass of cytoplasm. In the cytoplasm are embedded numerous cell organelles like mitochondria, Golgi apparatus, RER and neurofibrils. The cytoplasm also contains prominent conical angular or rhomboidal granules called Nissl՚s granules,
  • Cytoplasmic process: two kinds of cytoplasmic extensions-dendrons or dendrites and axon, dendrons are small, wide and break up into fine terminal branches. Dendrons are the processes which conduct nerve impulses towards the cell body. Axons or nerve fibers are long that may be several meters long. The terminal end fibers have fine branches with swollen ends and are neurosecretory. They release the neurosecretions such as acetylcholine and noradrenalin. The acetyl choline and noradrenalin are called neurotransmitters. Each axon is filled with cytoplasm called axoplasm that is continuous with the cytoplasm of the cell body. The axon is covered by the membrane called axolemma. Like any other cells of the body, it has the main cell body called cyton from which project out a varying number of processes – one of which is usually very long. The cyton also contains dark granules called Nissl bodies. These are made of RNA and Protein. There are two types of nerve fibers or axon depending on the presence or absence of myelin sheath: medullated nerve fiber and non-medullated nerve fiber.
Image Showing a- Neuron Structure with Label and B- L. S. Thr …
Image Showing Nerve Fibres; A-Medullated Nerve Fibre and B- …

Neuroglia Cells

Neuroglia cells are ten times more numerous than neurons and are found packed around the neurons throughout the central nervous system. The different kinds of neuroglia cells are classified; ependymal cells, macroglia, oligodendrocytes and microglia.

  • Function: Neuroglia cells support neurons mechanically by filling up the majority of interneuron space and also nourish the cytoplasmic extensions of neurons.

Transmission of Nerve Impulse

The branching dendrites receive the stimulus and transmit through the cyton to the axon, which finally transmits it through its variously branched end into either a muscle or to a gland. The axon constitutes the nerve fiber. The nerve fiber may or may not be covered by an extra sheath called medullary sheath secreted by sheath cells. It is made of myelin a lipid like substance.

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