In its resting state, the heart cell has low levels of calcium. When it is stimulated to contract, calcium floods into a heart cell, and it instigates contraction. And calcium’s entry is a self-feeding loop: the entry of calcium releases more calcium from the heart cell, resulting in a sharp, steep spike in calcium levels. The interconnections between the cells—those “junctions” that were identified in the 1950s—carry the ionic message from cell to cell.
