Final answer:
The SA node action potential is missing the plateau phase and the rapid repolarization phase.
Step-by-step explanation:
The SA node (sinoatrial node) is the natural pacemaker of the heart, responsible for initiating the electrical signals that regulate the heartbeat. The SA node generates action potentials which are then transmitted throughout the heart, causing the atria to contract. However, there are two things that the SA node action potential is missing. First, the SA node action potential is missing the plateau phase, which is a characteristic feature of action potentials in other cardiac muscle cells. Second, the SA node action potential is missing the rapid repolarization phase, which allows the cardiac muscle cells to reset and prepares them for the next contraction.