cardiac conduction system

Cardiac Conduction System

Consists of the SA node, internodal tracts/pathways (including Bachmann's bundle), AV node, bundle of His, bundle branches, and Purkinje fibers; composed of specialized cardiomyocytes with almost no contractile filaments

Action Potential

The electrical signal generated by voltage-dependent changes in membrane potential due to ion channel opening/closing; differs in ion basis between conduction system cells and working cardiomyocytes

Depolarization

A shift of the membrane potential toward a more positive value, triggering contraction in working cardiomyocytes; travels from the SA node through the conduction system to all cardiomyocytes

Repolarization

The return of the membrane potential toward its resting negative value after depolarization, caused mainly by K+ efflux through K+ channels

Resting Membrane Potential

The stable membrane potential during Phase 4 in non-pacemaker cells (e.g., -85 to -90 mV in Purkinje/ventricular cells), maintained by high extracellular Na+/Ca2+ and high intracellular K+

Voltage-gated Ion Channel

A channel that opens or closes in response to changes in membrane voltage (e.g., L-type Ca2+ channels, fast Na+ channels, K+ channels), underlying the phases of the cardiac action potential

Current

The flow of ions (Na+, Ca2+, K+) across the cardiomyocyte membrane through channels, producing the different phases of the action potential (e.g., INa, ICa, IK, Ito, IKr, IKs, IK1)

Sinus (SA) Node

The pacemaker of the heart; spontaneously depolarizes (Phases 4, 0, 3 only) and sets the heart rate; has the highest intrinsic firing rate (60-100 bpm in humans)

Spontaneous Depolarization

The gradual, automatic depolarization during Phase 4 in SA/AV nodal cells, caused by "funny" Na+ channels slowly bringing the membrane to threshold; determines heart rate and is influenced by sympathetic (increases slope) and parasympathetic (decreases slope) input

AV Node

Node that allows sequential atrial then ventricular contraction; has the slowest conductive properties, delaying the impulse ~120ms, due to fewer gap junctions and small fiber size, allowing atria to empty into ventricles before ventricular contraction

His-Purkinje System

Includes the Bundle of His, bundle branches, and Purkinje fibers; allows rapid spread of depolarization for near-simultaneous contraction of the ventricular chambers; has the lowest intrinsic pacemaker rate (25-40 bpm)

Specialized Cardiomyocytes

Cells of the conduction system (SA node, AV node, His-Purkinje) with almost no contractile filaments and increased gap junctions (except AV node) for rapid impulse transmission; can spontaneously generate action potentials

Working Myocytes

Contractile cardiomyocytes of the atria and ventricles that respond to depolarization from the conduction system by contracting; their action potential includes Phases 0,1,2,3,4 with a prolonged Phase 2 plateau

Excitation-Contraction Coupling (E-C Coupling)

The coupling of electrical activity ("excitation") in specialized cardiomyocytes with mechanical contraction in working cardiomyocytes; the entire atria or ventricles excite and contract simultaneously, alternating between atria and ventricles to complete the cardiac cycle

Gap Junction

Channels allowing ions to pass easily between adjacent cardiomyocytes, enabling rapid electrical conduction; increased in most of the conduction system but reduced in the AV node to slow conduction

Intercalated Disc

The structure connecting cardiomyocytes end-to-end, containing gap junctions, that couples the conduction system to working myocytes to pass the electrical signal and trigger contraction

ECG (EKG)

Electrocardiogram; records voltage differences between electrodes on the body surface produced by the heart's depolarization/repolarization (the compound action potential of the heart), plotted over time

ECG Waveforms (P wave, PQ/PR interval, QRS, T wave)

P wave = atrial depolarization; PR interval = time from atrial depolarization to ventricular depolarization (reflects AV nodal delay); QRS complex = ventricular depolarization; T wave = ventricular repolarization

Mean Electrical Vector

The net direction of electrical current flow in the heart at a given moment; a vector traveling toward the exploring electrode produces a positive ECG deflection, while one traveling away produces a negative deflection