Final answer:
A sino-atrial node action potential starts with a prepotential phase due to a slow Na+ influx and lacks a stable resting potential, exhibiting autorhythmicity. Contractile cells have stable resting potentials and an extended plateau phase in their action potentials for complete heart contractions.
Step-by-step explanation:
The sino-atrial node action potential differs from a typical action potential seen in other types of cells due to its unique properties as the heart's pacemaker. The SA node's action potential starts with a prepotential phase which is absent in most other cells. This phase is characterized by a slow influx of Na+ ions, gradually depolarizing the cell membrane until the threshold is reached. Once the threshold is hit, a rapid influx of Ca2+ ions occurs, leading to depolarization, followed by repolarization as K+ exits the cell. The SA node lacks a true resting potential, instead oscillating between depolarization and repolarization phases, a phenomenon describing the node's autorhythmicity.
On the other hand, contractile cells of the heart demonstrate a stable resting potential at approximately -80 mV for atrial cells and -90 mV for ventricular cells. Once stimulated, these cells undergo a similar rapid depolarization and repolarization. However, the contractile cells feature an additional extended plateau phase in their action potential, attributed to a sustained Ca2+ influx, which ensures a longer refractory period allowing for complete contraction. This mechanism is crucial for the heart's function to effectively pump blood.
In summary, while the basic phases of depolarization and repolarization are similar between SA node and contractile cell action potentials, the key differences lie in the prepotential phase, the autorhythmic nature of the SA node, and the extended plateau phase in contractile cells' action potentials.