Biological Systems MCAT Practice Exam

What initiates the plateau phase in the cardiac action potential?

1. Influx of sodium ions

2. Efflux of potassium ions

3. Influx of calcium ions

4. Closure of calcium channels

The correct answer is: Influx of calcium ions

The plateau phase in the cardiac action potential is critical for maintaining the prolonged contraction of cardiac muscle, which is essential for effective heart function. This phase is primarily initiated by the influx of calcium ions into the cardiac myocytes. When the cardiac action potential depolarizes and reaches a certain threshold, voltage-gated calcium channels open, allowing calcium ions to flow into the cells. This influx of calcium ions leads to a lasting depolarization, which characterizes the plateau phase. During this plateau, the balance between the influx of calcium and the efflux of potassium results in a sustained depolarization of the membrane potential. The prolonged presence of calcium in the cells not only contributes to the electrical stability of the membrane but also facilitates muscle contraction through calcium-induced calcium release from the sarcoplasmic reticulum. This mechanism is essential for generating a strong, coordinated contraction of the cardiac muscle, which differs from the rapid contractions seen in skeletal muscle. The other processes mentioned, such as the influx of sodium ions and the efflux of potassium ions, relate to earlier stages of the action potential and contribute to the depolarization and repolarization phases but do not initiate the plateau itself. The closure of calcium channels tends to occur later in the action potential, allowing