These cells undergo automaticity and are responsible for the heart rate. The action potential for cardiac pacemaker cells (SA node, AV node, and bundle of His/Purkinje fibers) is unique to the AP of general cardiac myocytes. Phase 4: Resting potential at -90 mV with minor depolarization from -90 mV to -70 mV the passive outflow of potassium In summary, the 5 phases of the cardiac myocyte are as follows: At the resting potential, the cell has only the original potassium channel slowly leaking potassium out of the cell, and we are returned to phase 4 of the action potential. This causes rapid repolarization, dropping the cell's voltage to -90 mV and closing the currently open potassium channel. With phase 3, the voltage-gated calcium channels close, leaving the outward-flowing potassium channels as the only open channels. This creates a net charge balance in the cell, creating a plateau. ![]() In phase 2, the plateau phase, potassium channels open out of the cell, and voltage-gated calcium channels open into the cell. ![]() Upon reaching peak voltage, voltage-gated potassium channels open and move potassium out of the cell, decreasing voltage again. These voltage-gated sodium channels close very quickly upon depolarizing the cell. These ions depolarize the voltage just enough to open these voltage-gated (VG) sodium channels, further depolarizing the cell to approximately +50 mV. The initial depolarization from -90 mV to -70 mV is caused by positively charged sodium and calcium ions entering the cell through gap junctions from neighboring cells. In the next phase, phase 0, sodium channels open at approximately -70 mV. This set of potassium channels is passively open and consistently outflows potassium during phase 4. To begin, phase 4 is at resting potential and consists of the first set of open potassium channels, with positively charged potassium flowing out of the cell, keeping the voltage low at approximately -90 mV. There are multiple types of potassium channels involved in the cardiac myocyte AP. The ultimate conclusion of a completed AP is the contraction of the cardiac myocyte. Repolarization will be represented by the return of the cell's voltage from a positive value to its resting potential of -90 mV. ![]() Depolarization is the voltage change from the resting potential of -90 mV toward a positive value. The resting potential, or baseline of the AP, is roughly -90 mV and is considered phase 4. The action potential has five distinct phases numbered 0-4. The action potential (AP) in the heart is unique to other action potentials in the body. Please see the article image for a visual representation. It will be broken down into two sections: the action potential, which is unique in the heart to other action potentials in the body, and electrophysiology. The cellular physiology of the heart is complex.
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