<unk>  represents the innate ability of the heart muscle (cardiac muscle or myocardium) to contract. The ability to produce changes in force during contraction result from incremental degrees of binding between different types of tissue, that is, between filaments of myosin (thick) and actin (thin) tissue. The degree of binding depends upon the concentration of calcium ions in the cell. Within an in vivo intact heart, the action/response of the sympathetic nervous system is driven by precisely timed releases of a catecholamine, which is a process that determines the concentration of calcium ions in the cytosol of cardiac muscle cells. The factors causing an increase in<unk>  work by causing an increase in intracellular calcium ions (Ca++) during contraction.
Myocardial contractility
Myogenic tone
Vagal tone
Cardiac cycle
Cardiac rhythmicity
Cardiac excitation-contraction coupling
Muscle tone
Skeletal-muscle pump