Action Potential Morphology Across Cardiac Cell Types

Understanding how action potential shapes vary among cardiac cell types and their physiological significance.

Action Potential Morphology Across Cardiac Cell Types refers to the distinctive shapes, phases, and durations of action potentials generated by various specialized cardiac cells, reflecting their unique electrophysiological properties and functional roles within the heart. These differences arise from variations in ion channel expression, ionic currents, and membrane properties, which together determine the timing and pattern of electrical excitation and contraction in cardiac tissue.

General Overview of Cardiac Action Potentials

Cardiac action potentials consist of five phases (0 to 4), characterized by specific ionic movements across the cell membrane:

Phase 4: Resting membrane potential, maintained primarily by inward rectifier potassium currents (I_K1).
Phase 0: Rapid depolarization due to fast sodium (Na⁺) influx via voltage-gated sodium channels.
Phase 1: Initial repolarization caused by transient outward potassium current (I_to).
Phase 2: Plateau phase dominated by a balance between inward L-type calcium current (I_Ca,L) and outward potassium currents.
Phase 3: Final repolarization driven by delayed rectifier potassium currents (I_Kr and I_Ks).

The morphology of these phases varies significantly among different cardiac cell types, underpinning their specific electrophysiological functions.

Cardiac Cell Types and Their Action Potential Morphologies

1. Ventricular Myocytes

Resting Membrane Potential: Approximately −85 to −90 mV.
Phase 0: Very rapid upstroke due to abundant fast Na⁺ channels, resulting in a steep slope.
Phase 1: Pronounced notch caused by a prominent transient outward potassium current (I_to), creating a sharp initial repolarization.
Phase 2: Distinct plateau phase lasting 200–300 ms, sustained by a balance of inward Ca²⁺ and outward K⁺ currents; critical for effective contraction and refractory period.
Phase 3: Gradual repolarization mediated by delayed rectifier K⁺ currents.
Phase 4: Stable resting potential with minimal spontaneous depolarization.

The long plateau phase ensures coordinated contraction and prevents premature excitation.

2. Atrial Myocytes

Resting Membrane Potential: Slightly less negative (~−80 to −85 mV).
Phase 0: Rapid depolarization similar to ventricular myocytes but with slightly lower amplitude.
Phase 1: More prominent transient outward K⁺ current than ventricles, causing a sharper notch.
Phase 2: Shorter and less pronounced plateau phase than ventricular cells, reflecting reduced L-type Ca²⁺ current.
Phase 3: Faster repolarization due to stronger delayed rectifier and ultra-rapid delayed rectifier potassium currents (I_Kur).
Phase 4: Stable resting potential.

The shorter action potential duration facilitates higher atrial rates and rapid conduction.

3. Purkinje Fibers

Resting Membrane Potential: Similar to ventricular myocytes (~−90 mV).
Phase 0: Very rapid upstroke, sometimes faster than ventricular cells, ensuring rapid conduction.
Phase 1: Less prominent notch compared to ventricular myocytes.
Phase 2: Plateau phase present but slightly shorter and less pronounced than ventricular action potentials.
Phase 3: Repolarization is slower, contributing to long refractory periods.
Phase 4: Stable resting potential with minimal automaticity.

Purkinje fibers serve as specialized conduction pathways with action potentials optimized for rapid impulse propagation and prolonged refractory periods to prevent re-entry arrhythmias.

4. Sinoatrial (SA) Node Cells

Resting Membrane Potential: Less negative (~−60 mV) and unstable, lacking a true resting potential.
Phase 4: Slow spontaneous depolarization (pacemaker potential) driven by "funny" current (I_f), T-type Ca²⁺ current, and decreasing K⁺ conductance.
Phase 0: Slow upstroke primarily mediated by L-type Ca²⁺ channels rather than Na⁺ channels.
Phase 3: Repolarization via increased K⁺ current.
No distinct Phase 1 or 2: The action potential lacks a plateau phase, resulting in a rounded shape.

The gradual depolarization during phase 4 enables automaticity, making SA node cells the primary pacemaker of the heart.

5. Atrioventricular (AV) Node Cells

Similar to SA node cells in morphology but with slower upstroke velocity and longer action potential duration.
Phase 4 depolarization is present but slower than SA node.
The slow conduction velocity through AV nodal cells supports delay between atrial and ventricular contraction.

Comparative Summary of Key Electrophysiological Features

Cardiac Cell Type	Resting Membrane Potential (mV)	Phase 0 Upstroke Velocity	Phase 1 Notch	Plateau Phase (Phase 2)	Phase 4 Stability & Automaticity
Ventricular Myocytes	−85 to −90	Very fast (Na⁺ channels)	Prominent	Long and pronounced	Stable, no automaticity
Atrial Myocytes	−80 to −85	Fast	More prominent	Shorter and less distinct	Stable, no automaticity
Purkinje Fibers	−90	Very fast	Less prominent	Present but shorter	Stable, no automaticity
SA Node Cells	~−60 (unstable)	Slow (Ca²⁺ channels)	Absent	Absent	Unstable; spontaneous depolarization
AV Node Cells	~−60 (unstable)	Very slow (Ca²⁺ channels)	Absent	Absent or minimal	Unstable; slower spontaneous depolarization

Ionic Basis for Morphological Differences

Sodium Channels (I_Na): Abundant in atrial, ventricular, and Purkinje cells, causing rapid phase 0 depolarization. Scarce or absent in nodal cells.
Calcium Channels (I_Ca,L and I_Ca,T): Predominant in nodal cells, responsible for slower depolarization.
Transient Outward Potassium Current (I_to): Prominent in atrial and ventricular cells, shaping phase 1 notch.
Delayed Rectifier Potassium Currents (I_Kr, I_Ks): Mediate repolarization and influence action potential duration.
Inward Rectifier Potassium Current (I_K1): Maintains resting membrane potential, reduced or absent in nodal cells.
Funny Current (I_f): Present in nodal cells, responsible for pacemaker activity.

Functional Implications

Ventricular and Atrial Cells: Long plateau phases ensure effective excitation-contraction coupling and refractory periods that prevent premature reactivation.
Purkinje Fibers: Fast conduction and long refractory periods facilitate synchronized ventricular contraction.
SA and AV Nodes: Automaticity and slow conduction regulate heart rate and timing of ventricular activation.
Variations in morphology underpin the heart’s ability to maintain rhythmic, coordinated contractions and respond to physiological demands.

Visual Representation of Typical Cardiac Action Potentials

Mathematical Representation of Action Potential Duration (APD)

Action potential duration, especially at 90% repolarization (APD90), is an important parameter reflecting the time course of the action potential.

APD_{90} = t_{90\% repolarization} - t_{depolarization\ threshold}

where

t_{depolarization\ threshold} is the time when membrane potential crosses the depolarization threshold in phase 0,
t_{90% repolarization} is the time when the membrane potential has returned to 90% of its repolarization from peak.

Typical APD90 values are:

Ventricular myocytes: 200–300 ms
Atrial myocytes: 100–150 ms
Purkinje fibers: 200–300 ms
SA node cells: shorter and less defined due to lack of plateau

Summary

The morphology of cardiac action potentials varies greatly across different cardiac cell types due to their distinct ion channel makeup and functional roles within the heart. Ventricular and atrial myocytes exhibit fast upstrokes and plateau phases critical for contraction, Purkinje fibers show rapid conduction properties, while nodal cells demonstrate slow, spontaneous depolarizations enabling pacemaker activity. Understanding these differences is fundamental for interpreting cardiac electrophysiology, arrhythmogenesis, and targeted therapeutic interventions.