Final answer:
Pacemaker cells enable cardiac muscle cells to contract spontaneously due to their unique ability for autorhythmicity, leading to self-initiated depolarization and synchronized cardiac contractions.
Step-by-step explanation:
Pacemaker cells possess an ability known as autorhythmicity, which enables them to depolarize to threshold and initiate action potentials independently. These cells are part of the heart's conduction system and are interconnected via gap junctions, allowing them to synchronize contractions throughout the cardiac muscle. While the heart rate is modulated by the nervous and endocrine systems, the inherent characteristic that allows cardiac muscle cells to contract on their own is the presence of pacemaker cells that can establish an intrinsic rhythm.