The sympathetic nervous system increases heart rate primarily by releasing hormones (specifically catecholamines like epinephrine and norepinephrine) that act on the heart.
Here's a more detailed breakdown:
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The Sympathetic Nervous System (SNS) Activation: When the body perceives stress, exertion, or danger (the "fight or flight" response), the SNS is activated.
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Nerve Signals and Hormone Release: The SNS sends nerve signals to the adrenal glands. These glands then release catecholamines, primarily epinephrine (adrenaline) and norepinephrine (noradrenaline), into the bloodstream.
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Action on the Heart: Epinephrine and norepinephrine bind to specific receptors (beta-adrenergic receptors, particularly β1 receptors) on the cells of the heart.
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Increased Heart Rate (Chronotropy): Binding to these receptors triggers a cascade of intracellular events that ultimately:
- Increase the flow of ions (like sodium and calcium) into the heart's pacemaker cells (sinoatrial node, or SA node).
- Speed up the rate of depolarization in these cells.
- Reduce the duration of repolarization.
- These combined effects lead to a faster rate of SA node firing, which dictates the heart rate.
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Increased Contractility (Inotropy): The catecholamines also increase the force of contraction of the heart muscle (myocardium). This means each heartbeat pumps more blood. This increased contractility is also mediated by the β1 receptors and increased calcium influx into the myocardial cells.
In summary, the sympathetic nervous system elevates heart rate by releasing epinephrine and norepinephrine, which bind to beta-adrenergic receptors in the heart. This, in turn, accelerates the firing rate of the heart's pacemaker cells and increases the force of contraction.
