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| Nervous Tissue |
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| It is a highly specialised animal tissue that exhibits two unique properties irritability (capacity to respond to the stimulus) and conductivity (capacity to transfer the response from one region to another). Nervous tissue is of ectodermal origin. |
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| The nervous tissue is composed of two types of cells called neurons and neuroglial cells. Neurons are also known as nerve cells. They represent the structural and functional units of the nervous tissue. Neuroglial cells are unspecialised supporting cells. |
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| A neuron is a highly specialised cell having a cell body called cyton or perikaryon and a long process called axon. The shape of the cyton varies with the number and arrangement of the processes arising from the cell. Accordingly, three types of neurons are recognised. |
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 Unipolar neuron where, the cyton is more or less spherical and has a single process that bifurcates. Such neurons are found in the ganglia in brain and spinal cord. |
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 Bipolar neuron where, the cyton is spindle shaped and has two processes, one at each end. Such neurons are found in the sense organs like eye and ear. |
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 Multipolar neuron here, the cyton has several processes, one of which is long and forms the axon. Such neurons are found in the central nervous system. |
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| The multipolar neuron represents the most common type of neuron. It has a more or less star shaped cell body or cyton that is bound by a plasma membrane. It encloses a granular cytoplasm, commonly called as neuroplasm. It is characterized by the presence of a prominent nucleus and a number of granules called Nissl's bodies which are rich in RNA and ribosomes. |
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| The cyton shows two kinds of cytoplasmic processes, dendrons and axon. Dendrons (also called dendrites) are highly branched thick extensions of the cytoplasm containing Nissl's bodies. Their function is to conduct impulses towards the cyton. The second type of projection, called axon (or axis cylinder) is a long, thin process that is highly specialised to conduct the impulses away from the cyton. It usually arises from the cyton as a small conical elevation called axon hillock. |
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| The axon terminates branching into fine filaments called telodendrons (or axon terminals). The cytoplasm of the axon, called axoplasm lacks Nissl's bodies. The axon is usually surrounded by a white, phospho-lipid covering called myelin sheath or medullary sheath. Axons with such a covering are called as myelinated or medullated and those without are called as non-myelinated or non-medullated. The function of myelin sheath is to increase the speed of conduction of the impulse. There is another cellular covering surrounding the axon, called neurilemma (or sheath of Schwaan) containing flattened cells called Schwaan cells. It is thrown into constrictions called nodes of Ranvier. |
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| The telodendrons have at their distal ends, fine bulb like structures called synaptic end bulbs or boutons terminaux. They contain tiny synaptic vesicles, where specific chemical substances called neurotransmitters are stored. |
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| Structure of multipolar neuron |
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