cardiac action potential phases

what are the phases (0-4) of ventricular muscle action potential

• Phase 0: Rapid depolarization (Na⁺ influx)

• Phase 1: Partial repolarization (Na⁺ inactivation)

• Phase 2: Plateau (Ca²⁺ influx balanced by K⁺ efflux)

• Phase 3: Repolarization (Ca²⁺ inactivation, K⁺ permeability ↑)

• Phase 4: Resting potential (no pacemaker potential)

why do atrial action potentials have shorter duration than ventricular ones

because of stronger repolarizing K⁺ currents

what is “funny” current (If)

mixed Na⁺/K⁺ inward current responsible for spontaneous depolarization in pacemaker cells

how does parasympathetic input affect SA/AV node

ACh activates M₂ receptors → opens K+channel GIRK (G-protein–coupled inward rectifier K⁺) channels → hyperpolarisation → slows conduction → slows pacemaker rate

how does sympathetic input affect cardiac rhythm

noradrenaline activates β₁-adrenoceptors (via GPCR) → ↑ cAMP → enhances pacemaker and Ca²⁺ currents → faster HR and stronger contraction

which parts of the heart do parasympathetic system innervate

mainly innervates SA and AV nodes

which parts of the heart do sympathetic system innervate

all parts, particularly ventricular muscle