Final answer:
The spontaneously developing local potential that generates action potentials in the SA node is called prepotential depolarization.
Step-by-step explanation:
The spontaneously developing local potential that generates action potentials in the SA node is called the prepotential depolarization. This mechanism is responsible for the autorhythmic property of cardiac muscle. The prepotential is due to a slow influx of sodium ions until the threshold is reached, followed by a rapid depolarization and repolarization.