Autonomic Regulation of the Heart

The heart's activities are tightly regulated by both the parasympathetic and sympathetic divisions of the autonomic nervous system. The autonomic nervous system exerts its effects on heart rate (chronotropic effects), heart muscle contractility (inotropic effects), and conduction velocity between the atria and the ventricles (dromotropic effects).

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The chronotropic effects of the autonomic nervous system are all about its effects on heart rate (the firing rate of SA nodal cells).

Positive Chronotropic Effects

The sympathetic nervous system causes increases in heart rate via activation of β1 receptors on SA nodal cells ("positive chronotropic effects"). Activated β1 receptors trigger the Gs signaling cascade to achieve their effect.

Negative Chronotropic Effects

The parasympathetic nervous system causes decreases in heart rate via activation of M2 receptors on SA nodal cells ("negative chronotropic effects"). Activated M2 receptors trigger the Gi signaling cascade to achieve their effect.

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The inotropic effects of the autonomic nervous system are all about its effects on the contractility of cardiac myocytes.

Positive Inotropic Effects

The sympathetic nervous system causes increases in contractility via activation of β1 receptors on cardiac myocytes ("positive inotropic effects"). Activated β1 receptors trigger the Gs signaling cascade to achieve their effect.

Negative Inotropic Effects

The parasympathetic nervous system causes decreases in contractility via activation of M2 receptors on cardiac myocytes ("negative inotropic effects"). Activated M2 receptors trigger the Gi signaling cascade to achieve their effect.

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The dromotropic effects of the autonomic nervous system are all about its effects on the conduction velocity, or rate of depolarization, of AV nodal cells.

Positive Dromotropic Effects

The sympathetic nervous system causes increases in conduction velocity (steeper upstroke) via activation of β1 receptors on AV nodal cells ("positive dromotropic effects"). Activated β1 receptors trigger the Gs signaling cascade to achieve their effect.

Negative Dromotropic Effects

The parasympathetic nervous system causes decreases in conduction velocity (less steep upstroke) via activation of M2 receptors on AV nodal cells ("negative dromotropic effects"). Activated M2 receptors trigger the Gi signaling cascade to achieve their effect.

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