Which factor shortens ERP by sympathetic activation?

Which factor shortens ERP by sympathetic activation?

• A. Sympathetic activation
• B. Parasympathetic activation
• C. Ischemia
• D. Exercise

Answer: (A)

ERP is the period after an action potential during which the tissue cannot be re-excited. When sympathetic activation kicks in, ERP shortens because the fight-or-flight drive speeds up repolarization. Norepinephrine acting on β-adrenergic receptors raises cAMP and activates PKA, which enhances repolarizing potassium currents (IKs, and to a degree IKr) and also increases calcium handling that together shorten the action potential duration. With a shorter action potential, the refractory period ends sooner, so the tissue can respond again more quickly. This effect is part of what happens during exercise or stress, where sympathetic tone is high. In contrast, parasympathetic activation tends to slow conduction and can prolong refractoriness in some tissues, and ischemia tends to alter refractoriness in a way that can be destabilizing rather than simply shortening it.

ERP is the period after an action potential during which the tissue cannot be re-excited. When sympathetic activation kicks in, ERP shortens because the fight-or-flight drive speeds up repolarization. Norepinephrine acting on β-adrenergic receptors raises cAMP and activates PKA, which enhances repolarizing potassium currents (IKs, and to a degree IKr) and also increases calcium handling that together shorten the action potential duration. With a shorter action potential, the refractory period ends sooner, so the tissue can respond again more quickly. This effect is part of what happens during exercise or stress, where sympathetic tone is high. In contrast, parasympathetic activation tends to slow conduction and can prolong refractoriness in some tissues, and ischemia tends to alter refractoriness in a way that can be destabilizing rather than simply shortening it.