Cardiac physiology

What is membrane potential?

The electrical potential difference between the inside and outside of a cell.

What is the resting membrane potential of cardiomyocytes?

−90 mV.

What is the resting membrane potential of neurons?

−70 mV.

What is the resting membrane potential of most mammalian cells?

−50 mV.

Do SA node cells have a stable resting potential?

No

What ions primarily generate membrane potentials?

K⁺, Na+ and Ca2+

What is depolarization?

A decrease in membrane negativity (becoming more positive).

What is repolarization?

Return of membrane potential to its negative resting value.

What is hyperpolarization?

Membrane potential becomes more negative than resting.

What are cardiomyocytes?

Striated

What are intercalated discs?

Specialized junctions enabling mechanical and electrical coupling between cardiomyocytes.

What are gap junction

🧬 Structure

• Made of connexons, which are hexameric rings of connexin proteins.

• Two connexons (one from each cell) align to form a continuous channel between cells.

⚡ Function

❤ Why They Matter in the Heart

• Gap junctions in intercalated discs allow action potentials to spread rapidly between cardiomyocytes.

• This ensures the heart contracts as a functional syncytium — coordinated, unified pumping.

What is the function of gap junctions?

To synchronize depolarization and contraction across the myocardium.

• Allow ions and small molecules to move directly between cells.

• Enable electrical coupling

• Support metabolic and signalling coordination across cell networks.

What is the SA node?

The heart’s natural pacemaker initiating electrical impulses.

Excites the right atrium, then left atrium via the Bachmann’s bundle

What is the AV node?

A conduction node that delays impulses and passes them to the bundle of His.

What is the bundle of His?

A conduction pathway transmitting impulses to the ventricles.

What are Purkinje fibres?

Fast‑conducting fibres spreading depolarization through ventricular myocardium.

What is automaticity?

The ability of pacemaker cells (autorhythmic cells) to depolarize spontaneously.

What channels generate the pacemaker potential?

If (funny) channels

What are If channels?

Hyperpolarization‑activated cyclic nucleotide‑gated channels allowing Na⁺ influx.

What triggers rapid depolarization in SA node cells?

Opening of L‑type Ca²⁺ channels at −40 mV.

What repolarizes SA node cells?

Opening of delayed rectifier K⁺ channels.

What is the resting potential of contractile cardiomyocytes?

−90 mV due to inward rectifier K⁺ channels.

What triggers phase 0 in contractile cells?

Opening of fast voltage‑gated Na⁺ channels.

What is phase 1 of the cardiac action potential?

Early repolarization via transient K⁺ efflux.

What is phase 2 of the cardiac action potential?

Plateau phase from Ca²⁺ influx (L‑type channels) balanced by K⁺ efflux.

What is phase 3 of the cardiac action potential?

Repolarization due to K⁺ efflux and Ca²⁺ channel closure.

What is phase 4 of the cardiac action potential?

Resting membrane potential maintained by K⁺ leak channels.

Why is the cardiac refractory period long?

Due to the plateau phase from slow Ca²⁺ channels

What is a sarcomere?

The contractile unit of muscle containing actin and myosin.

What proteins make up thin filaments?

Actin

What does troponin C do?

Binds Ca²⁺ to initiate contraction.

What does troponin I do?

Inhibits actin‑myosin ATPase in resting muscle.

What does troponin T do?

Anchors the troponin complex to tropomyosin.

What is calcium‑induced calcium release (CICR)?

Ca²⁺ entering via L‑type channels triggers larger Ca²⁺ release from the SR via ryanodine receptors.

What are T‑tubules?

Sarcolemmal invaginations bringing L‑type Ca²⁺ channels close to SR ryanodine receptors.

What is SERCA?

Sarcoplasmic reticulum Ca²⁺ ATPase that pumps Ca²⁺ back into the SR during relaxation.

What ends contraction?

Ca²⁺ dissociation from troponin C and reuptake by SERCA.

What neurotransmitter increases heart rate?

Norepinephrine (sympathetic).

What receptor does norepinephrine act on?

β₁‑adrenergic receptors.

How does sympathetic stimulation affect pacemaker cells?

Increases If channel activity and speeds depolarization.

What neurotransmitter decreases heart rate?

Acetylcholine (parasympathetic).

What receptor does acetylcholine act on?

Muscarinic (M₂) receptors.

What is chronotropy?

Change in heart rate.

What determines cardiac output?

Heart rate × stroke volume.

What determines stroke volume?

Preload

What is preload?

End‑diastolic volume/stretch of ventricular muscle.

What is afterload?

Resistance the ventricle must overcome to eject blood.

What is contractility?

Strength of contraction independent of preload.