Cardiac Muscle Action Potentials – Vocabulary Flashcards

A set of vocabulary flashcards summarizing key terms and currents involved in cardiac muscle action potentials.

Contractile cardiac muscle cells

Myocytes that generate force; their action potential features a plateau and is initiated by fast Na+ influx.

Pacemaker cells

Specialized cardiac cells that spontaneously depolarize via pacemaker currents and use Ca2+ influx, not Na+, for upstroke.

INa (Na+ current)

Fast inward Na+ current responsible for Phase 0 depolarization in contractile cells.

ICa (Ca2+ current)

Inward Ca2+ current that produces the upstroke in pacemaker cells and the plateau in contractile cells while triggering contraction.

IK (K+ current)

Outward K+ current that sets resting membrane potential and mediates repolarization in all cardiac cell types.

If (Pacemaker current)

Mixed Na+/K+ current that gradually depolarizes pacemaker cells toward threshold between beats.

Resting membrane potential (cardiac)

Approximately –90 mV, more negative than in neurons or skeletal muscle because of high K+ permeability.

IK1 (Inward-rectifier K+ current)

K+ leak current active in Phase 4 that stabilizes the –90 mV resting potential in contractile cells.

Phase 0 (Depolarization)

Rapid upstroke produced by voltage-gated Na+ channel opening in contractile myocytes.

Phase 1 (Rapid repolarization)

Brief repolarization caused by transient outward K+ current (Ito) and Ca2+-activated Cl– current.

Ito (Transient outward K+ current)

Early, short-lived K+ efflux that shapes Phase 1 repolarization.

Phase 2 (Plateau)

Period where Ca2+ influx through L-type channels balances K+ efflux, keeping membrane potential nearly constant.

L-type Ca2+ channels

Long-lasting Ca2+ channels that open during Phase 2; slow inactivation prolongs the plateau.

T-type Ca2+ channels

Transient Ca2+ channels with faster inactivation; contribute to depolarization mainly in pacemaker cells.

IKs (Slow delayed rectifier K+ current)

Slowly activating K+ current that aids repolarization during the plateau and Phase 3.

IKr (Rapid delayed rectifier K+ current)

More quickly activating K+ current that accelerates late repolarization.

IKur (Ultra-rapid delayed rectifier K+ current)

Very fast activating K+ current present in some atrial myocytes, contributing to repolarization.

Phase 3 (Repolarization)

Stage where cumulative K+ efflux exceeds Ca2+ influx, returning the membrane to –90 mV.

Absolute refractory period (cardiac)

Interval lasting until the end of Phase 3 when a cardiac cell cannot be re-excited, preventing tetany.

Plateau phase significance

Prolongs the action potential, allows Ca2+ entry for contraction, and prevents premature re-excitation.

ATP-sensitive K+ channel (KATP)

K+ channel opened by low ATP/high ADP during hypoxia; increases K+ conductance to hyperpolarize and protect the heart from arrhythmias.

Hypoxia-induced arrhythmia

Abnormal rhythms arising when low ATP reduces Na+/K+-ATPase activity, depolarizes RMP, and increases excitability.

Tetany (cardiac prevention)

Continuous contraction avoided in the heart because the prolonged refractory period prevents high-frequency stimulation.

Nernst potential for K+

Electrical potential (≈ –90 mV) predicted for K+; closely matches the cardiac resting membrane potential.

Delayed rectifier K+ channels

Family of voltage-gated K+ channels (IKs, IKr, IKur) that open during the plateau and repolarization phases.

Phase 4 (Resting potential)

The stable resting membrane potential in contractile cardiac cells, maintained by IK1IK1​ (inward-rectifier K+ current) at approximately −90extmV−90extmV