hard · MCAT bio-biochem
A cardiac myocyte is stimulated with a second, suprathreshold electrical impulse delivered 150 ms after the first action potential begins, during the plateau phase of the cardiac action potential.
What is the physiological consequence, and which ion channel property explains it?
- No second action potential is generated, because voltage-gated Na^+ channels remain inactivated throughout the long absolute refractory period.
- A second action potential fires normally because cardiac Na^+ channels recover from inactivation almost immediately after the upstroke, quite unlike skeletal muscle.
- A second action potential fires with reduced amplitude, because Ca^2+ channels rather than Na^+ channels are what is truly responsible for the refractory period here.
- Tetanic summation occurs, producing a sustained contraction, because the plateau phase does not constitute a true refractory period.
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