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| Nervous Regulation |
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| The heart is regulated to a large extent through the central nervous system. The heart nerve branches from the vagus nerve from the medulla oblongata and the sympathetic nerve fibres from the spinal cord. |
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| The vagus nerves are cardiac inhibitory nerves. When the vagus nerve is stimulated, the activity of the heart is inhibited. The inhibitory action of the vagus nerve is brought about as follows. |
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| a) The heart rate is slowed down |
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| b) The conductivity of the bundle of His is reduced. |
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| c) The force of contraction is diminished. |
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| d) The duration of systole is diminished but that of the diastole is increased. |
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| e) Excitability of the heart is also reduced. |
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| The sympathetic nerves are accelerator nerves. These fibres stimulate the SA node, AV node and also the muscles of the auricles and the ventricles Stimulation of the sympathetic nerves causes |
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| a) Increase in frequency of heart rate |
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| b) Increase in force of contraction |
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| c) Increases excitability and irritability of the heart |
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| d) Increases the conductivity of the cardiac muscle and bundle of His |
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| thermal regulation |
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| Adrenaline secreted by the adrenal medulla of adrenal gland accelerates the rate of heart beat during emergency conditions. Noradrenaline increases the heart beat during normal condition. Thyroxine and sex hormones also influence heart rate. |
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| Temperature |
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| The rate of heart beat is affected by variation in temperature. |
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| During increased body activity the CO2 content of blood raises. It combines with water to form carbonic acid in the blood. This lowers the pH, which increases the heart beat. |
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