What is the duration of the cardiac muscle cell's refractory period?

What is the duration of the cardiac muscle cell's refractory period?

• A. About 250 ms
• B. About 50 ms
• C. About 1 second
• D. About 2 ms

Answer: (A)

The main idea is that the cardiac muscle has a long refractory period because its action potential includes a plateau phase with calcium entry, which keeps the cell in an excited state for a longer time and prevents a second contraction too soon. In the ventricular myocardium, the absolute refractory period lasts about 200–250 milliseconds, often rounded to ~250 ms. This long window ensures the heart muscle cannot tetanize and has time to relax between beats, allowing proper filling of the chambers. If you compare the options, 2 milliseconds is typical for skeletal muscle and would allow rapid re-excitation, which isn’t the case for cardiac muscle. A duration of about 50 milliseconds is too short to cover the plateau and the full refractory period, risking premature contractions. About 1 second is far longer than the cardiac action potential and would disrupt normal heart rhythm. The ~250 ms duration best matches the true length of the cardiac refractory period and aligns with the plateau phase of the cardiac action potential.

The main idea is that the cardiac muscle has a long refractory period because its action potential includes a plateau phase with calcium entry, which keeps the cell in an excited state for a longer time and prevents a second contraction too soon. In the ventricular myocardium, the absolute refractory period lasts about 200–250 milliseconds, often rounded to ~250 ms. This long window ensures the heart muscle cannot tetanize and has time to relax between beats, allowing proper filling of the chambers.

If you compare the options, 2 milliseconds is typical for skeletal muscle and would allow rapid re-excitation, which isn’t the case for cardiac muscle. A duration of about 50 milliseconds is too short to cover the plateau and the full refractory period, risking premature contractions. About 1 second is far longer than the cardiac action potential and would disrupt normal heart rhythm. The ~250 ms duration best matches the true length of the cardiac refractory period and aligns with the plateau phase of the cardiac action potential.