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How do sympathetic nerves increase heart rate?
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human cardiovascular system
The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.
The sympathetic nervous system increases the heart rate by acting on the sinoatrial (SA) node, which serves as the heart's natural pacemaker.1
When sympathetic nerves innervating the heart are activated, or when epinephrine and norepinephrine are released from the adrenal gland, they exert a direct effect on the myocardial cells of the SA node. Specifically, this process:1
- Decreases the resting potential of the SA node myocytes.1
- Increases the rate of diastolic depolarization, which is the spontaneous upward drift of the membrane potential toward the critical threshold required to trigger an action potential.1
By accelerating the frequency at which these cells reach the threshold for firing, the sympathetic nervous system effectively increases the heart rate. This mechanism is a key component of the fight-or-flight response, preparing the body for physical exertion or acute stress.123