The sympathetic nervous system, via sympathetic nerves, is primarily responsible for increasing heart rate.
While the question asks about a single nerve, it's important to understand that the increase in heart rate is a result of the overall activity of the sympathetic nervous system, which involves a network of nerves.
Here's a breakdown:
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Autonomic Nervous System: The autonomic nervous system controls involuntary functions like heart rate, digestion, and breathing. It has two main branches:
- Sympathetic Nervous System (SNS): Often referred to as the "fight or flight" system. When activated, it releases neurotransmitters like norepinephrine (noradrenaline) that increase heart rate, blood pressure, and respiration.
- Parasympathetic Nervous System (PNS): Often referred to as the "rest and digest" system. It releases acetylcholine, which slows heart rate and promotes relaxation. The vagus nerve is the primary component of the parasympathetic nervous system affecting the heart.
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Sympathetic Nerves and the Heart: Sympathetic nerves reach the heart and release norepinephrine. Norepinephrine binds to receptors (primarily beta-1 adrenergic receptors) on heart cells. This binding increases the rate at which the sinoatrial (SA) node (the heart's natural pacemaker) fires, leading to a faster heart rate. It also increases the force of heart muscle contraction (contractility), further increasing cardiac output.
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In summary: While there isn't one single "nerve" solely responsible, the sympathetic nerves of the sympathetic nervous system are what directly cause the increase in heart rate through the release of norepinephrine. The effect is mediated through a network of nerves and ganglia, rather than a single, isolated nerve.
