What happens to the heart rate during sympathetic nervous system activation?

During sympathetic nervous system activation, the heart rate increases due to the release of catecholamines, primarily adrenaline (epinephrine) and norepinephrine. These neurotransmitters bind to beta-adrenergic receptors in the heart, leading to an increase in heart rate, known as positive chronotropic effect. This response is part of the "fight or flight" reaction, preparing the body for quick action by enhancing blood flow to muscles and vital organs.

The increase in heart rate is crucial for supporting heightened physical and mental activities during stress or emergencies. The sympathetic nervous system effectively prepares the body to respond to perceived threats by increasing cardiac output, which is essential for supplying oxygen and nutrients to tissues that need them most in those situations.