87. Electrophysiology of the Heart – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Cardiac electrophysiology explains how electrical impulses are generated, conducted, and coordinated to produce effective cardiac contraction. The heart contains fast-response and slow-response action potentials, each adapted to a specific physiological function. Atrioventricular (AV) nodal delay ensures proper timing between atrial and ventricular contraction, allowing optimal ventricular filling.
• Cardiac electrical activity depends on the movement of sodium (Na⁺), calcium (Ca²⁺), potassium (K⁺), and pacemaker (If) currents.
  • Na⁺ current (INa): Produces rapid depolarization (Phase 0) in atrial muscle, ventricular muscle, and Purkinje fibers.
  • L-type Ca²⁺ current (ICaL): Produces the plateau phase in working myocardium and depolarization in nodal tissue.
  • T-type Ca²⁺ current (ICaT): Contributes to pacemaker depolarization.
  • K⁺ currents: Maintain resting membrane potential and produce repolarization.
  • Pacemaker current (If): Generates spontaneous depolarization in pacemaker cells.
• Fast-response action potentials occur in atrial muscle, ventricular muscle, and Purkinje fibers and consist of five phases:
  • Phase 0: Rapid depolarization due to Na⁺ influx.
  • Phase 1: Initial repolarization due to transient K⁺ efflux.
  • Phase 2: Plateau phase caused by balance between Ca²⁺ influx and K⁺ efflux.
  • Phase 3: Final repolarization due to K⁺ efflux.
  • Phase 4: Stable resting membrane potential maintained mainly by K⁺ conductance.
• Slow-response action potentials occur in the SA node and AV node.
  • They contain three phases (0, 3, and 4); Phases 1 and 2 are absent.
  • Phase 0: Ca²⁺ influx through L-type calcium channels.
  • Phase 3: K⁺ efflux.
  • Phase 4: Pacemaker potential (prepotential) produced by reduced K⁺ efflux, activation of If (funny current), and T-type Ca²⁺ influx. The slope of Phase 4 determines heart rate.
• Pacemaker potential is responsible for the automatic rhythm of the SA node. The If current, carried through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, initiates spontaneous depolarization. Sympathetic stimulation increases the slope of the pacemaker potential and increases heart rate, whereas parasympathetic (vagal) stimulation hyperpolarizes pacemaker cells, decreases the slope, and slows heart rate.
• Cardiac excitation normally begins in the SA node, spreads through the atria to the AV node, then passes through the His bundle, bundle branches, and Purkinje fibers to activate the ventricles. The atria and ventricles are electrically insulated by the fibrous cardiac skeleton, so impulses reach the ventricles only through the AV node.
• AV nodal delay (about 0.1 second) results from slow calcium-dependent conduction, small nodal cells, and fewer gap junctions. It allows complete atrial contraction before ventricular systole and protects the ventricles from excessively rapid atrial impulses, such as in atrial fibrillation. Purkinje fibers conduct impulses most rapidly (≈4 m/s), ensuring synchronous ventricular contraction.