Nervous Tissue. The unit of nervous tissue is called neuron. It is the longest cell & it doesn t undergo cell division in matured state.

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1 Nervous Tissue The unit of nervous tissue is called It is the longest cell & it doesn t undergo cell division in matured state. It is a special modified tissuess or cells for getting following two basic properties:- 1. Excitability: - This cell maintains a particular potential difference across the membrane. There is a change in potential difference when it is stimulated by any physical, chemical or electrical stimulus & this is called excitability. 2. Conductivity: - The stimulus received by it is transmitted along the length of it in the form of nerve impulse. Structure: - Its size varies from 4 (in cerebellum) to 135 (in ventral horn of spinal cord). A typical neuron consists of two parts:- A. Cell Body or Perikaryon or Soma or, Cyton B. Cytoplasmic Extensions A. Cell body: - It is the irregular in shape and is covered by thin cell membrane. It contains cytoplasm, nucleus, nucleolus, mitochondria, Golgi apparatus, rough endoplasmic reticulum, neurofibrils etc. It also contains Nissl s granules (consists of Ribosomes & RER) which are conical, angular or rhomboidal granules.

2 B. Cytoplasmic Extensions: - From soma one or more projections comes out to perform different function. Those are: - a. Dendrons & Dendrites: - These are small, wide projections come out from soma. Its fine branches are called dendrites. These carry the message towards the cell body so also called afferent process. b. Axon or Nerve fibre: - It is long process comes out from axon hillock. It conducts the nerve impulse away from the soma. It may be branched or unbranched. The branch which remains perpendicular to the axon is called collateral. Its terminal becomes branched & contains a round structure called button which releases two chemicals i.e. acetylcholine & adrenaline called neurotransmitter. Structure of Axon: - The cytoplasm of axon is called axoplasm which contains many mitochondria, rough endoplasmic reticulum & neurofibrils but lacks Nissl s granules, Golgi apparatus & nucleus. The membrane of axon is called axolemma. Axolemma may or may not enveloped by myelin sheath which is secreted by Schwann cells. These cell surrounds the axon & also remains covered by Neurilemma. Myelin or medullary sheath remains constricted at some places at regular interval called Node of Ranvier (1 mm) after the name of French histologist Louis Ranvier. At node of Ranvier only axolemma & neurilemma remains continous. Types of Neuron: - 1. On the basis of structure: - i. Unipolar Neuron: - The neuron in which only one processes come out but it further divides into two branches, one functions as Dendron and other for axon is called unipolar ii. Bipolar Neuron: - The neuronn in which two projections that are dendron & axon come out from a cell body is called bipolar iii. Multipolar Neuron: - The neuron in which more than two projections are originated from cell body is called multipolar {Above shown neuron is multipolar }

3 2. On the basis of myelinated sheath: - i. Myelinated nerve fibre or medullated nerve fibre: - ii. The neuron which remains surrounded by myelinated sheath (formed by a fatty substance called myelin or contains cholesterol, phospholipids & other substances) is called myelinated It carries the nerve impulse faster than non-myelinated Non-myelinated neuron: - The neuron in which myelinated sheath remains absent is called non-myelinated 3. On the basis of function: - i. Sensory Neuron: - The neuron which carries nerve impulse from peripheral nervous system (PNS or sense organ) to central nervous system (CNS) is called sensory neuronn or afferent ii. Motor Neuron: - The neuron which carries nerve impulse form CNS to PNS is called motor iii. Mixed Nerve Fibres: - The nerve fibres which contains both sensory neuron & motor neuron are called mixed nerve fibre. [In case of sensory & motor we can use both neuron or nerve but in case of mixed only nerves.] Neuron & Nerves: - The unit of nervous system or one cell of nervous tissue is called Whereas nerves or nerve fibers are formed by combination of a large number of

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5 A large number of nerve fiber is surrounded by a layer of connective tissue called endoneurium. Such bundles are again surrounded by a layer of connective tissue called perineurium & finally, all these bundles are bind together by epineurium to form nerves. Some modified form of neuron:- Neuroglia Cells: - glial cells. Interneuron space present in CNS is filled with non-nervous cells called neuroglia or In CNS, number of neuroglia cells is ten times or more than that of neurons. Functions of Neuroglia Cells: - I. It fills up the space present between nerves. II. It plays an important role in memory process. III. It provides nourishment to the IV. Some of the cells are modifiedd into microglia which are phagocytic. V. These cells are capable to divide & regenerate so these cells help to repair the injured tissues. [Unfortunately, it is also one of the main cause of brain tumours] Oligodendrocytes: - Some neuroglia cells surround the axon of CNS and forms myelin sheath around it. [Schwann cells are absent in CNS] Ependymal Cells: - These cells are cubical & ciliated epithelial cells which forms the epithelium of the ventricles of brain & spinal cord. Neurosecretory Cells: - The nervous tissue which is capable to secret chemicals is called neurosecretory cells. Example; Neurons of hypothalamus.