Final answer:
Cardiac muscle cells cannot undergo summation and tetanic contractions due to their long refractory periods prevented by the thorough regulation of calcium ions. This ensures the heart muscles fully relax between contractions, which is essential for the heart's pumping function and effective blood circulation.
Step-by-step explanation:
The mechanism that prevents myocardial cells from undergoing summation and tetanic contractions is due to the unique properties of cardiac muscle tissue's excitation-contraction coupling and refractory periods. Unlike skeletal muscle, the cardiac muscle has a long refractory period due to its electrical pattern of rapid depolarization followed by a plateau phase and repolarization. This refractory period is long enough to prevent any new action potential from being generated before the heart muscle has adequately relaxed and pumped blood, thus preventing tetany and allowing the chambers of the heart to fill with blood appropriately.
Calcium ions play a vital role in cardiac muscle physiology, not just for contraction but also in ensuring this prolonged plateau phase and absolute refractory period. The cardiac muscle cells primarily undergo twitch-type contractions with these long refractory periods followed by brief relaxation periods, which is essential for the heart's function as a pump. The influx of calcium ions through slow calcium channels and the release from the sarcoplasmic reticulum together provide the necessary calcium for contraction without leading to an overlapping of contractions that would result in tetany, which is incompatible with life.
Overall, the lack of summating ability in the cardiac muscle is critical for maintaining a rhythmic and efficient heartbeat, ensuring that each contraction is complete and the heart chambers can fill for the next cycle, thus supporting effective blood circulation throughout the body.