Cardiac Conduction System

Ensures rhythmic contraction of the heart.

• Atria contract first, ventricles second (allows blood to move properly).

• Coordinates pumping: atria → ventricles → out through pulmonary artery or aorta.

Automaticity

• Heart generates its own rhythm (does not require external nerve input).

• Specialized cardiac cells = nodal cells (not neurons, not typical muscle).

• Pacemaker ability → can beat even outside the body (short time).

Key Components

1. SA Node (Sinoatrial Node)

   • Location: right atrium, superior/posterior corner.

   • Rhythm: 60–80 bpm → sets sinus rhythm.

   • Spontaneously depolarizes (pacemaker).

   • Sends depolarization wave across atria → atrial contraction.

2. Fibrous Cardiac Tissue

   • Separates atria from ventricles.

   • Prevents direct conduction to ventricles.

   • Forces signals to funnel through the AV node.

3. AV Node (Atrioventricular Node)

   • Location: base of right atrium near septum.

   • Rhythm: 40–60 bpm (slower).

   • Delays conduction ~0.1–0.16 sec → allows atrial emptying before ventricles contract.

4. AV Bundle (Bundle of His)

   • Pathway through septum.

   • Splits into right and left bundle branches.

5. Purkinje Fibers

   • Spread throughout ventricles, especially apex → base direction.

   • Rhythm: 15–40 bpm.

   • Very fast conduction (1.5–4 m/s).

   • Ensures ventricles contract from bottom up (like squeezing toothpaste).

Hierarchy of Pacemakers

• SA node (primary pacemaker, fastest).

• If SA fails → AV node takes over (slower).

• If AV fails → Purkinje fibers take over (too slow for adequate output).

Autonomic Nervous System Regulation

• Sympathetic Nervous System (SNS) – fight/flight

  • Neurotransmitter: norepinephrine (noradrenaline).

  • Acts on SA/AV nodes + ventricular muscle.

  • Effects:

    • ↑ HR (by ↑ Na⁺ & Ca²⁺ influx into nodal cells).

    • ↑ Contractility (by ↑ Ca²⁺ influx into myocytes).

• Parasympathetic Nervous System (PNS) – rest/digest

  • Via vagus nerve.

  • Neurotransmitter: acetylcholine (ACh).

  • Acts only on SA & AV nodes (not muscle).

  • Effects:

    • ↓ HR (opens K⁺ channels → hyperpolarization → slows depolarization).

Clinical Relevance

• Basis for ECG/EKG interpretation.

• Explains arrhythmias if conduction pathway fails.

• Pacemaker hierarchy important for diagnosing conduction block/failure.

• Explains HR control during exercise, stress, or rest.