Essay on the Important Components of Nervous Tissue
It is a tissue of ectodermal origin. All cells exhibit electrical properties but main properties are excitability (= ability to initiate nerve impulse in response to stimuli i.e. changes outside and inside) and conductivity (= ability to transmit a nerve impulse i.e. potential change in membrane of nerve impulse).
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Cells of nervous tissue are nerve cells (neurons) and supporting cells inside the CNS is called neuroglia and in the peripheral nervous system are called Schwann cells.
Supporting cells are non conducting cells that are in intimate physical contact with neurons. They provide physical support, electric insulation and metabolic exchanged with the vascular system. Components of nervous tissue: neuron and supporting cells
I. Neurons:
A neuron consists of a large cell body – cyton (perikaryon) or soma, thin protoplasmic processes-dendrites and a long process-axon.
The soma contains abundant cytoplasmic granules and a large nucleus with prominent nucleolus. Mitochondria are in abundance. Nissl bodies are modified ER and ribosomes. They help in protein synthesis. Dendrites are extensions of cell body. Dendrites contain both nissl granules and neurofibrils.
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Axon is a longer process and branches distally into many fine filaments called telodendria. The part of the axon which joins the cell body is called axon hillock. The plasma membrane of axon is called axolemma and the cytoplasm, axoplasm.
Axon’s hillock is the most sensitive part of neuron. Axon contains neuroplasm with neurofibrils and mitochondria. Nissl’s granules, golgi bodies, ribosomes and oil globules are absent. An axon may produce lateral branches called collaterals.
Axon and its collaterals ultimately end in a group of fine branches called terminal arborisations.
Terminal arborisations may be connected with muscles, glands, skin and other structures for transmission of impulses. At place, terminal arborisations of a neuron form synapse with telodendria of another neuron.
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This is axo-dendritic synapse. Other types of synapses are axo-axonic, axo-somatic and dendro-dendritic. There is no organic union in the region of synapse. Transmission occurs across a synapse through a neuro-transmitter chemical like acetylcholine.
Direction of nerve impulse is usually axo-dendritic (dendrite —> axon —> dendrite). It is reversed in axo-axonic and dendro-dendritic synapses.
A nerve fibre is an extended axon. It is covered by neurilemma which is made up of Schwann cells. Neurilemma is not present inside central nervous system instead neuroglia cells- oligodendrocytes, form covering of axon, producing myelin sheath.
Some nerve fibres are surrounded by a lipid rich insulating layei-myelin sheath. Such fibres are called myelinated or medullated nerve fibres. Myelin sheath is interrupted at some places; these are called nodes of Ranvier.
Nerve fibres without myelin sheath are called non-myelinated nerve fibres and they do not possess nodes of Ranvier. Speed of conduction of nerve impulse is faster in myelinated fibres.
On the basis of function neurons can be differentiated as
Sensory (afferent nerves):
They bring sensory impulses from sense organs to central nervous system.
Motor (efferent nerves):
They carry out motor impulses from central nervous system to the effectors (muscles and glands).
Mixed (Interneurons/connector):
They are present in the central nervous system and occur between the sensory and motor neurons for distant transmission of impulse.
On the basis of structure neurons can be differentiated as
Unipolar:
When one axon extends from the cell body. E.g. – embryo and then divides into two long branches. E.g. – sensory neurons
Bipolar neurons:
When one axon and one dendrite come out from the soma. E.g. – Retina
Multipolar neurons:
When one axon and two or more dendrites extend from the soma. E.g. – Motor neurons and interneurons.
Pseudounipolar neurons:
When one axon extends from the cell body and then divides into two. E.g. – dorsal root ganglion.
Non-polar neurons:
When all processes of the soma are equal and nerve impulse can be conducted in any direction. E.g. – neurons of coelenterates.
II. Supporting cells (for neurons and their fibres):
(i) Neuroglia is special connective tissue cells, found in brain and spinal cord. They are divided into 2 major categories – macroglia and microglia.
Macroglia (larger) are of two types – astrocytes and oligodendrocytes.
(a) Astrocytes are star shaped that give off a number of process. It provides physical and metabolic support for nerve cells. They may be divided into fibrous astrocytes seen in white matter of CNS and protoplasmic astrocytes seen in grey matter of CNS.
(b) Oligodendrocytes (myelin secreting cells) occur in both white and grey matter. They have a few fine processes.
Microglias are smallest cells having short and fine processes. They are more numerous in grey matter. The> are phagocytic in nature. In the adult CNS, they are present in small number but proliferate and become actively phagocytic in disease and injury.
(ii) Ependymal cells forms a columnar epithelium that lines the ventricles (cavities of brain) and central canal of spinal cord.
Their free surface bears microvilli (helps in absorption of cerebrospinal fluid) and cilia (whose movement contributes to the flow of cerebrospinal fluid). These cells possess one or more long processes towards opposite side which penetrate the nervous tissue.
(iii) Neurosecretory cells function as endocrine cells. They release chemicals from their axons into blood, instead of into the synapses.