Cardiac Conduction and the Cardiac Cycle

The conduction system generates the beat. Recall that cardiac muscle has automaticity — it can generate its own rhythm. This comes from specialized cells forming the conduction system:

1. Sinoatrial (SA) node (in the right atrium): the pacemaker, which spontaneously fires impulses that set the heart rate. The impulse spreads across both atria, causing them to contract.
2. Atrioventricular (AV) node (between atria and ventricles): briefly delays the impulse. This pause lets the atria finish emptying into the ventricles before the ventricles contract.
3. AV bundle (bundle of His) and bundle branches: carry the impulse down through the wall between the ventricles.
4. Purkinje fibers: spread the impulse rapidly through the ventricle walls, so the ventricles contract from the bottom up, pushing blood efficiently into the arteries.

Because the SA node sets the pace on its own, the heart beats without conscious control; the autonomic nervous system only speeds it up (sympathetic) or slows it down (parasympathetic/vagus).

The cardiac cycle. One heartbeat is one cardiac cycle, alternating between contraction and relaxation:

• Systole: the chambers contract and eject blood. During ventricular systole, the ventricles squeeze blood into the pulmonary artery and aorta (the AV valves are closed — "lub").
• Diastole: the chambers relax and fill with blood. During ventricular diastole, blood flows in from the atria (the semilunar valves are closed — "dup").

Blood pressure readings reflect this cycle: the top number (systolic) is the pressure during ventricular contraction; the bottom (diastolic) is during relaxation.

The ECG. An electrocardiogram records the heart's electrical activity from the skin surface. Its classic waves correspond to conduction events: the P wave (atrial depolarization → atrial contraction), the QRS complex (ventricular depolarization → ventricular contraction), and the T wave (ventricular repolarization → relaxation). Changes in these waves help diagnose arrhythmias, heart attacks, and conduction problems.

How it works

One heartbeat, electrically and mechanically:

SA node fires → atria depolarize (P wave) → atria contract (fill ventricles)
   → AV node delays → signal down AV bundle/branches → Purkinje fibers
   → ventricles depolarize (QRS) → ventricles contract (systole, eject blood)
   → ventricles repolarize (T wave) → relax (diastole, fill)
   → SA node fires again

Comparisons

Common confusions

• SA vs AV node. SA = pacemaker (starts the beat); AV = delay (times the ventricles).
• Systole vs diastole. Systole = contraction/ejection; diastole = relaxation/filling.
• P/QRS/T waves. P = atria fire; QRS = ventricles fire; T = ventricles reset.
• The heart sets its own rhythm — nerves only adjust the rate, not create the beat.

Memory aids

• SA node = "the Starting Amp (pacemaker/drummer)."
• "Systole = Squeeze; Diastole = Dilate/fill."
• ECG: "P before QRS before T" = atria, then ventricles, then reset.

Quick review

• The conduction system: SA node (pacemaker, fires atria) → AV node (delay) → AV bundle/branches → Purkinje fibers (ventricles contract). The heart has autorhythmicity; nerves only adjust rate.
• The cardiac cycle alternates systole (contract/eject) and diastole (relax/fill); these set the systolic/diastolic blood pressure.
• The ECG records electrical activity: P (atrial), QRS (ventricular), T (repolarization) — used to detect arrhythmias and heart attacks.