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Preksha Dhyana :
pones the function of digestion and urination to a later time.
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The actions of both divisions of the A.N.S. are mostly antagonistic, e.g. the sympathetic nerves act to speed up and strengthen the heart-beat and increase the blood pressure while the parasympathetic nerves slow down the heart-rate and lower the blood pressure.
In some cases, however, their action is complementary e.g. both divisions are involved in the completion of a sex act in the male. It seems strange that impulses from one division may cause a particular organ to contract, while those from the other to the same organ produce relaxation. Some explanation is found in different neuro-transmitter chemical messengers released by the nerve-endings of the two divisions. Autonomic nerves are intimately involved in numerous important reflexes such as the regulation of blood pressure, evacuation of the bowels and bladder and the sexual reflexes. Thus it is not surprising that damage to the nerves of this system can have far-reaching and pervasive effects on body functions.
NERVE-TISSUE
Nerve-tissue is made up of the neurons, which are the active conducting elements and the neuroglia (glia means glue) or glial cells which support and provide nourishment to the nerve cells. The neuron is the structural and functional unit of the nervous system. It is an electrically charged cell which cannot be replaced like other cells. Neurons vary widely in size. The smallest cells in the cortex are only 1/200 mm. in diameter while the large motor cells in the spinal cord measure 1/8 mm. in diameter.
Structure of Neuron
Neurons are highly specialized in two key areas of function excitability and conductivity. A typical neuron consists of a cell body and two types of protoplasmic extensions: axons and dendrites. Some of the neurons are as much as three feet long. Each neuron has only a single axon, but most of them possess numerous dendrites. If the cell body is damaged, the extensions are unable to conduct impulses and the neuron dies. Moreover, the neuron is
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