True about cardiac muscle

During phases 0 to 2 and about half of phase 3 (until the membrane potential reaches approximately -50 mV during repolarization), cardiac muscle cannot be excited again; that is, it is in its absolute refractory period. It remains relatively refractory until phase 4. Therefore, tetanus of the type seen in skeletal muscle cannot occur. The T system in cardiac muscle is located at the Z lines rather than at the A-I junction, where it is located in mammalian skeletal muscle. Along the sides of the muscle fibers next to the disks, the cell membranes of adjacent fibers fuse for considerable distances, forming gap junctions. These junctions provide low-resistance bridges for the spread of excitation from one fiber to another. Plateau (phase 2) is due to a slower but prolonged opening of voltage-gated Ca2+ channels.Reference: Ganong's review of Medical Physiology, 25th ed, pages: 112, 113, 114