The pounding is a neurological reflex as well as due to a physical effect. When we are startled by a sound, the reflex can be activated within 150 ms, which is faster than acting due to conscious thought. This is because the sound travels from the cochlea through the brainstem to the amygdala, which treats the noise as a potential threat before it has been identified.
The amygdala alerts the hypothalamus, which activates the sympathetic nervous system, which triggers the adrenal glands to release adrenaline into the blood. Epinephrine binds to receptors in cardiac muscle and increases the rate and force of contractions.
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