Final answer:
The ability of cardiac muscle cells to contract without neural stimulus is called automaticity or autorhythmicity, unique to the heart's pacemaker cells, enabling the heart to maintain a steady rhythm.
Step-by-step explanation:
The ability of cardiac muscle cells to contract spontaneously without a stimulus from a nerve source is called automaticity. This unique characteristic is also known as autorhythmicity, which describes the cardiac muscle's capacity to initiate its own electrical impulse that triggers the mechanical contraction to pump blood at a fixed pace without nervous or endocrine control. Cardiac muscle shares some characteristics with both skeletal muscle and smooth muscle, but autorhythmicity is a property unique to cardiac tissue. Unlike skeletal muscle, cardiac muscle contracts regularly without neural stimulation due to the presence of specialized pacemaker cells that possess automaticity.