Final answer:
Contractile cardiac muscle cells have a long refractory period that allows the muscles to relax completely before contracting again, ensuring that the heart beats effectively. This refractory period is part of the muscles' electrical pattern and is crucial for heart function.
Step-by-step explanation:
Contractile cardiac muscle cells have a characteristic electrical pattern that includes a rapid depolarization, followed by a plateau phase and then repolarization. This pattern facilitates a long refractory period, which ensures the muscle relaxes completely before the next contraction can occur. This refractory period is crucial because it allows the heart to fill with blood after each contraction and prevents the possibility of tetanic contractions, which is when the muscle would lock in a contracted state, impeding the heart's ability to pump effectively. During an action potential, the influx of calcium ions (Ca++) from the sarcoplasmic reticulum (SR) and through voltage-gated calcium channels in the sarcolemma trigger the contraction. As the calcium ions are removed and tropomyosin re-shields the actin-binding sites, the muscle relaxes. This whole process is coordinated by intercalated discs that connect cardiac muscle cells and are composed of desmosomes and gap junctions, providing both mechanical support and electrical connectivity between cells.