BIO 304 · Human Anatomy & Physiology
Cardiac Conduction System
Cardiovascular System · Module 11
A reference for the Cardiac Conduction video. The heart generates its own rhythm. A network of specialized myocardial cells fires impulses from atria to ventricles in a precise sequence that an ECG can read from outside.
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- Trace the conduction pathway from SA node to ventricular myocardium.
- Compare pacemaker action potentials with contractile action potentials.
- Identify the P wave, QRS complex, T wave, and intervals on a normal ECG.
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Conduction Pathway
Specialized conducting cells
- SA noderight atrium near SVC; intrinsic rate 60-100; primary pacemaker
- Internodal pathwayscarry signal across atria to AV node
- AV nodeinteratrial septum; intrinsic rate 40-60; delays signal ~0.1s
- Bundle of His (AV bundle)penetrates into interventricular septum
- Right & left bundle branchesdown either side of septum
- Purkinje fibersspread through ventricular myocardium; intrinsic rate 20-40; fastest conduction
Why the AV delay matters
- ~100 ms pauseallows atrial systole to finish before ventricular contraction begins
- Bottom-up contractionPurkinje delivers signal to apex first; contraction pushes blood up toward valves
Pacemaker action potential
- No true resting potentialmembrane drifts upward toward threshold
- Funny current (If)slow Na+ leak in; "funny" because it opens at hyperpolarization
- Threshold reachedL-type Ca²⁺ channels open; depolarization upstroke (Ca²⁺, not Na+)
- RepolarizationK+ channels open
- Auto-rhythmicitycycle repeats; pacemaker rate controlled by ANS
Action Potentials & ECG
Contractile cell AP
- Phase 0 (upstroke)fast Na+ channels open; rapid depolarization
- Phase 1brief K+ efflux; small dip
- Phase 2 (plateau)Ca²⁺ influx balances K+ efflux; sustained depolarization — this is unique to cardiac muscle
- Phase 3Ca²⁺ channels close; K+ continues out; repolarization
- Phase 4resting potential restored; ready to fire again
Why the plateau matters
- Long refractory periodprevents tetanus — the heart can't sustain contraction
- Allows proper fillingeach beat is followed by relaxation and refilling
ECG basics
- P waveatrial depolarization
- QRS complexventricular depolarization (also masks atrial repolarization)
- T waveventricular repolarization
- PR intervalstart of P to start of QRS; AV node delay; ~0.12-0.20 s
- QRS durationnormally < 0.12 s; widened in bundle branch block
- QT intervalstart of QRS to end of T; ventricular electrical cycle; longer = arrhythmia risk
Autonomic control
- Sympatheticnorepinephrine on beta-1 receptors → speeds SA, increases contractility
- Parasympathetic (vagus)ACh on muscarinic → slows SA and AV nodes
- Resting toneparasympathetic dominates; intrinsic SA rate without ANS is ~100/min
When conduction fails
- AV block (1st degree)PR interval > 0.20 s; signal slow but reaches
- AV block (2nd degree)some atrial beats not conducted
- AV block (3rd degree, complete)no atrial-to-ventricular conduction; ventricles use junctional or ventricular pacemaker
- Bundle branch blockone ventricle activates late; widens QRS
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