1) Physiological properties of cardiac muscle

• Excitability: ability to respond to a stimulus by generating an action potential (AP).
• Conductivity: ability to propagate excitation cell-to-cell (via gap junctions) and through specialized conduction tissue.
• Contractility (inotropy): ability to develop force and shorten.
• Automaticity (autorhythmicity): ability of some cardiac cells to depolarize spontaneously.
• Rhythmicity: ability to generate regular repeated impulses.
• Refractoriness: reduced/absent excitability after depolarization (very important in heart).

• Atrial syncytium and ventricular syncytium are electrically connected internally.
• Fibrous skeleton electrically insulates atria from ventricles, so normal AV conduction goes mainly through AV node/His system.

2) Excitability. Action potential of cardiomyocytes

A) Working cardiomyocytes (atrial/ventricular myocytes, Purkinje mostly “fast response”)

• Phase 4 (resting potential): stable, about -85 to -90 mV, mainly IK1 (K+ permeability).
• Phase 0 (rapid depolarization): opening of fast voltage-gated Na+ channels → Na+ influx.
• Phase 1 (initial repolarization): transient outward K+ current (Ito), Na+ channels inactivate.
• Phase 2 (plateau): balance between Ca2+ influx (L-type channels) and K+ efflux.
• Phase 3 (repolarization): Ca2+ channels close, delayed rectifier K+ currents dominate.

B) Excitability changes during AP

• Absolute refractory period (ARP): no new propagated AP possible (phases 0,1,2 and early 3).
• Relative refractory period (RRP): stronger-than-normal stimulus can evoke response (late phase 3).
• Supranormal period: brief interval near end of repolarization when excitability may be slightly increased.

3) Automaticity of the heart

Definition

Nature (ionic basis)

• Spontaneous diastolic depolarization (phase 4) due to:
  • Funny current If (HCN channels, mainly Na+ inward mixed current)
  • reduced K+ outward current
  • T-type and then L-type Ca2+ channel activation
• Threshold reached → AP fires.
• Repolarization mainly by K+ efflux.

AP in conduction system (slow-response tissues: SA/AV nodes)

• Phase 4: spontaneous depolarization (pacemaker potential).
• Phase 0: mainly Ca2+ influx (L-type), slower upstroke than fast Na+-dependent tissue.
• Phase 3: K+ efflux repolarization.
• Phases 1 and 2 are not prominent.

Gradient of automaticity

• SA node: ~60 to 100/min (dominant pacemaker)
• AV node/junction: ~40 to 60/min
• His-Purkinje/ventricular pacemakers: ~20 to 40/min

4) Cardiac cycle and phases

Ventricular events (left heart values commonly cited)

1. Atrial systole (~0.1 s)
   • Completes ventricular filling (“atrial kick”).
   • End-diastolic volume (EDV) reached.
2. Ventricular systole (~0.3 s)
   • Isovolumetric contraction: AV valves close (S1), semilunar closed, pressure rises rapidly, volume constant.
   • Ejection phase:
     • Rapid ejection then reduced ejection.
     • Semilunar valves open when ventricular pressure exceeds aortic/pulmonary pressure.
3. Ventricular diastole (~0.5 s)
   • Isovolumetric relaxation: semilunar valves close (S2), all valves closed, pressure falls rapidly.
   • Ventricular filling:
     • Rapid filling
     • Diastasis (slow filling)
     • Next atrial systole completes filling.

5) Systolic and minute volumes of circulation

• End-diastolic volume (EDV): volume before systole (about 120 to 130 mL).
• End-systolic volume (ESV): volume after systole (about 50 to 60 mL).
• Stroke volume (SV, systolic volume): [ SV = EDV - ESV ] Typical ~70 mL at rest.
• Ejection fraction (EF): [ EF = \frac{SV}{EDV} \times 100% ] Typical ~55 to 70%.
• Cardiac output (CO, minute volume): [ CO = SV \times HR ] At rest: ~5 L/min (about 4 to 6 L/min in adults).

6) Relationship between excitation, contraction, and excitability during cardiac cycle

• Electrical activation (AP) precedes mechanical contraction.
• During plateau (phase 2), Ca2+ entry through L-type channels triggers Ca2+ release from SR (calcium-induced calcium release) → cross-bridge cycling → contraction.
• Relaxation follows Ca2+ reuptake into SR (SERCA) and extrusion (NCX, Ca2+ ATPase).

Phase relationships in one beat

• Early systole: myocardium depolarized; excitability is very low/absent (ARP), contraction developing.
• Mid-systole: strong contraction; still largely refractory.
• Late systole to early diastole: repolarization progresses; excitability gradually returns (RRP).
• Diastole: full excitability restored; chambers relax and fill.

• Because mechanical systole overlaps with electrical refractory period, heart cannot be tetanized like skeletal muscle.
• This guarantees alternating contraction and filling, essential for effective pumping.