Nervous Tissue

Last Updated on Thursday, 16 December 2010 01:13 Written by Sandesh Sunday, 15 August 2010 04:53

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General Characteristics:

The property of irritability and conductivity has developed to highest degree in the nervous tissue. It is a complex network of interconnected cells whose function is to transmit and sometimes store information.

Origin: Nervous tissue is originated from ectoderm

Nervous tissue consists of Neurons or nerve cells, nerve fibers and neuroglial cells.

• Neurons are the structural and functional unit of nervous system. They are sensitive to changes and have property of transmitting the electrical impulses from cell to cell.
• Nerve fibers are the basic parts of neurons. They take part in the transmission of nerve impulses.
• Neuroglial cells have the property to multiply but do not possess nervous function. They form non-excitable, supporting and protective component of the nervous tissue.

Location: Nervous tissue is widely distributed innervating every part of the body that responds to stimuli.

Functions:

• senses change within and outside the body
• interprets changes
• respond to interpretation by initiating muscular contractions or glandular secretions
• through sensation, integration and response, the nervous tissue represents the body’s most rapid means of maintaining homeostasis.

Structure of Neuron:

Neuron is the structural and functional unit of nervous system. It consists of three distinct portions, cell body, dendrites and axon.

1. Cell Body: It is also known as cyton (perikaryon or soma) and is about 3-100 micrometer in diameter. It is irregularly shaped or oval and contains a granular cytoplasm called neuroplasm surrounded by cell membrane. The cytoplasm contains a large well defined nucleus in the center of the cell body. Other cell organelles such as mitochondria, golgi complex, lysosomes, ribosomes and numerous microtubules are also present. Besides these, the cytoplasm also contains prominent Nissl granules and neurofibrils. Nissl granules are rich in RNA and are associated wit protein synthesis. They are absent in axon.

2. Dendrites: From the margin of cyton many branched processes called dendrites are given off. They are highly branched thick extensions of the cytoplasm of the cell body. They contain mitochondria, Nissl bodies, neurofibrils and other cytoplasmic organelles. Dendrites conduct impulse towards the cyton and hence they are afferent in function.

3. Axon or nerve fibres: Axons are actually processes of cyton and not independent constituents. Each axon is a single, long, thin process of uniform thickness. It branches away from the cyton and ends into terminal branches called telodendria. The telodendria again terminate into knob-like structures called end bulbs, axon terminals or terminal buttons. They contain synaptic vesicles which release certain chemicals called neurotransmitters which help in synaptic transmission of the nerve impulses. The plasma membrane of axon is called axolemma and the cytoplasm is called axoplasm. The axoplasm contains abundant neurofibrils and mitochondria. Axon conduct impulse away from cyton, hence they are efferent in function.

A majority of axons are enveloped by one or two sheath-like investments. An axon may be surrounded by multilayered white segmented covering called myelin sheath or medullary sheath. Axons with such covering are known as medullated while those without sheath are called non-medullated. Myelinated nerve fiber appears white and comprises white matter of brain and spinal cord. Nonmyleinated nervous fiber appears white and comprises grey matter of in brain and spinal cord. Both types are axons are further surrounded by a neurilemma made up of Schwann cells. This sheath is not continuous but is interrupted by constrictions known as nodes of Ranvier. The axon together with its sheath is callednerve fiber.

The physiological junction between the axonic ends of one neuron with the dendrites of other neuron is called synapse. A gap called synaptic gap exists between the axonic and dendritic ends.

Classification of neuron:

On the basis of number of processes:

• Unipolar neuron- with one single process extending from cyton, found in all invertebrates and lower vertebrates.
• Bipolar- two processes extending from cyton, each arising from either end, found in retina of eye, the inner ear and nose
• Multipolar- several dendrites and a single axon

On the basis of function:

• Afferent neuron or sensory- conduct impulses from site of stimulus reception to CNS (central nervous system)
• Efferent neuron or motor- conduct impulse form the CNS to the site of action

Difference between myelinated and non-myelinated nerve tissue: