Complex sentence: The degree of binding that occurs depends on concentration of calcium ions in the cell; in an intact heart, it is usually the action of the sympathetic nervous system (through catecholamines) that determines the concentration of calcium ions in the cytosol of cardiac muscle cells.
Split into simpler sentences: The degree of binding that occurs depends on concentration of calcium ions in the cell. Within an In vivo intact heart, it is usually the action of the sympathetic nervous system (through catecholamines) that determines the concentration of calcium ions in the cytosol of cardiac muscle cells.