At the peak of the action potential (AP) in contractile myocytes (heart muscle cells), which occurs during a phase referred to as Phase 2 (the plateau phase), several key physiological phenomena take place:
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Depolarization: The initial rapid depolarization (Phase 0) occurs due to the opening of fast sodium (Na⁺) channels, leading to a rapid influx of sodium ions. This is followed by a transient repolarization as sodium channels inactivate.
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Plateau Phase: During Phase 2, calcium (Ca²⁺) channels (specifically L-type calcium channels) open, allowing a significant influx of calcium ions into the cell. This influx is crucial for triggering muscle contraction as it leads to the release of more calcium from the sarcoplasmic reticulum through a process known as calcium-induced calcium release.
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Maintenance of Membrane Potential: The plateau phase is characterized by a balance between the influx of calcium ions and the efflux of potassium ions (K⁺), which helps to maintain a relatively stable membrane potential during this phase.
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Contraction: The plateau phase corresponds to the mechanical contraction of the heart muscle. The prolonged duration of this phase ensures that contraction occurs before the cell begins to repolarize.
In summary, at the peak of the action potential in contractile myocytes, there is a major influx of calcium ions contributing to muscle contraction, and the cell maintains a depolarized state due to calcium influx and potassium efflux balancing each other. After this plateau, the cell will begin to repolarize, leading to relaxation.