The parasympathetic system regulates homeostasis and is the central system responsible for recuperation and restoring the body's energy reserves. It also acts a moderator for cardiac operation.
The parasympathetic system has a direct path to the heart and a fast messenger. It is therefore capable of suddenly slowing down the heart, triggering rapid drops in heart rate. But a withdrawal of the parasympathetic brake triggers a sudden rise in heart rate. The sudden slow-down or speed-up of heart rate belong to changes in parasympathetic activity, that is to say changes in the high frequencies (HF).
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