Final answer:
Cardiac muscle has a longer refractory period than skeletal muscle due to its action potential plateau from Ca¹² influx, which prevents tetany and ensures effective blood pumping. Cardiac refractory periods last about 250 ms in total, vital for heart function.
Step-by-step explanation:
The reason why cardiac muscle has a much longer refractory period than skeletal muscle is to ensure that the heart muscle contracts effectively and pumps blood efficiently. Cardiac muscle cells have action potentials with a sustained depolarization "plateau" due to Ca¹² influx through voltage-gated calcium channels. This plateau extends the refractory period, which allows the muscle to fully contract and relax before another contraction is initiated. This is critical because it prevents tetany, a state where the muscle remains involuntarily contracted. In cardiac muscles, tetany would be fatal as it would prevent the heart from pumping blood, and thus sustaining life.
The absolute refractory period for cardiac muscle is approximately 200 ms, and the relative refractory period is around 50 ms, totaling about 250 ms. This is in stark contrast to skeletal muscle, which has a much shorter refractory period, allowing for rapid and frequent contractions, suitable for voluntary movements but unsuitable for the heart's function.