What ions are involved in a cardiac action potential? Which way does each move across the membrane?
The correct answer and explanation is:
The ions involved in a cardiac action potential are primarily sodium (Na⁺), calcium (Ca²⁺), and potassium (K⁺). Each ion moves across the cardiac cell membrane in specific directions during different phases of the action potential.
- Sodium (Na⁺): During the initial phase of the cardiac action potential, voltage-gated sodium channels open, causing a rapid influx of Na⁺ ions into the cell. This inward movement of positively charged sodium ions depolarizes the cell membrane, causing the membrane potential to become more positive.
- Calcium (Ca²⁺): Shortly after the sodium influx, voltage-gated calcium channels open. Calcium ions move into the cell from the extracellular space, contributing to the plateau phase of the action potential. This inward flow of Ca²⁺ maintains depolarization and prolongs the action potential, which is essential for cardiac muscle contraction.
- Potassium (K⁺): During repolarization, potassium channels open, allowing K⁺ ions to exit the cell. This outward movement of positively charged potassium ions returns the membrane potential toward its resting negative state. Potassium efflux is responsible for repolarizing the cardiac cell and restoring the resting membrane potential.
Explanation:
A cardiac action potential is the electrical signal that initiates the contraction of heart muscle cells. It involves a sequence of ion movements across the cell membrane that change the electrical charge inside the cell relative to the outside.
At rest, the cardiac cell membrane has a negative potential inside, mainly due to potassium ions concentrated inside the cell and sodium ions concentrated outside. When the heart cell is stimulated, sodium channels open first, allowing Na⁺ to rush into the cell, causing rapid depolarization. Next, calcium channels open and Ca²⁺ flows inward, prolonging the depolarized state in a plateau phase unique to cardiac muscle. This plateau is important because it allows sufficient time for the heart muscle to contract and pump blood effectively.
Finally, potassium channels open, and K⁺ leaves the cell, making the inside more negative again. This repolarization phase resets the cell’s electrical state, preparing it for the next action potential. The coordinated movement of these ions ensures the heart beats rhythmically and efficiently, which is essential for proper blood circulation.