Cardiac Conduction System and Pacemaker Cell Physiology

Cardiac Conduction System

Intrinsic Cardiac Conduction System

The intrinsic cardiac conduction system is a network of specialized cardiac muscle cells responsible for initiating and distributing electrical impulses throughout the heart. This system ensures that the heart beats in a coordinated and rhythmic manner, allowing efficient pumping of blood.

• Definition: The intrinsic conduction system consists of autorhythmic cells that generate and conduct action potentials independently of the nervous system.

• Main Components: The system includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His (AV bundle), right and left bundle branches, and Purkinje fibers.

• Function: Coordinates the timing of atrial and ventricular contractions to maintain effective cardiac output.

Pacemaker Cells

Pacemaker cells are specialized cardiac muscle cells that have the ability to spontaneously generate action potentials. They are essential for initiating the heartbeat and setting the pace for cardiac rhythm.

• Location:

  • SA Node: Located in the right atrium near the entrance of the superior vena cava; primary pacemaker of the heart.

  • AV Node: Located at the junction of the atria and ventricles; secondary pacemaker.

  • Other Sites: Bundle of His, bundle branches, and Purkinje fibers can act as pacemakers if higher centers fail.

• Function: Generate and propagate electrical impulses that trigger cardiac muscle contraction.

• SA Node: Sets the normal heart rate (sinus rhythm) at approximately 60–100 beats per minute in adults.

Electrophysiology of Pacemaker Cells

Depolarization and Repolarization in Pacemaker Cells

Pacemaker cells exhibit unique action potentials that differ from those of contractile cardiac muscle cells. Their action potentials are responsible for initiating the heartbeat and controlling its rhythm.

• Depolarization:

  • Triggered when the membrane potential reaches a threshold (about -40 mV).

  • Ion Movement: Rapid influx of Ca2+ ions through voltage-gated calcium channels causes the upstroke of the action potential.

• Repolarization:

  • Occurs as K+ ions exit the cell through voltage-gated potassium channels.

  • This outflow of potassium returns the membrane potential to its resting value.

• Pacemaker Potential (Prepotential):

  • Between action potentials, slow influx of Na+ ions (via "funny" channels) gradually depolarizes the cell toward threshold.

  • This property allows pacemaker cells to spontaneously reach threshold and fire action potentials at regular intervals.

Summary Table: Ion Movements in Pacemaker Cell Action Potentials

Phase	Main Ion Movement	Channels Involved
Pacemaker Potential (Prepotential)	Na+ influx	"Funny" Na+ channels (If)
Depolarization	Ca2+ influx	Voltage-gated Ca2+ channels
Repolarization	K+ efflux	Voltage-gated K+ channels

Example: Sequence of Cardiac Impulse Conduction

• Impulse originates in the SA node → spreads through atria → reaches AV node → travels down bundle of His → through bundle branches → to Purkinje fibers → causes ventricular contraction.

Additional info: The autonomic nervous system can modulate the rate of pacemaker activity, with sympathetic stimulation increasing heart rate and parasympathetic stimulation decreasing it.