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Effects of the autonomic nervous system on the heart

The effects of the sympathetic nervous system on the heart are to increase heart rate (chronotropic effect), contractility (positive inotropic effect) and increase the speed of conduction through the AV node. These effects are achieved through the activation of ß receptors. There are at least 2 types of ß receptors, ß1 and ß2. In normal hearts, the population of ß1 receptors is usually larger than that of the ß2 receptors. In some pathological conditions e.g. heart failure, there is a relative increase in the population of ß2 receptors.

The parasympathetic effects on the heart are mediated via acetylcholine receptors located on the SA and AV nodes. The vagus nerve supplies the heart with parasympathetic fibres. There is a constant vagal tone slowing the heart rate. When this tone increases e.g. emotional stress and pain, the heart rate can drop. When this tone is blocked e.g. by administering atropine, the heart rate increases. In addition to a negative chronotropic effect, the parasympathetic systems reduces the speed of conduction through the AV node. There are no other significant effects of the parasympathetic nervous system on the heart.

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