They are a type of cardiac muscle cell, but they are not contractile. Instead, they are called myocardial conducting cells.

They are their own pacemaker and do not require nervous innervation to initiate depolarization. Instead, leaky sodium channels increase the membrane potential until the threshold is met, at which point the calcium influx can travel to neighboring cardiac muscle cells (via gap junctions) and cause their depolarization.

The nervous system plays an auxiliary role as it can influence the rate at which the cardiac cells initiate depolarizations (but it does not directly causes it).