Cardiac Conduction System:

• SA Node (Sinoatrial Node):
  • Located in the roof of the right atrium.
  • Composed of specialized cardiomyocytes that spontaneously depolarize—known as the heart’s natural pacemaker.
  • Generates an impulse at approximately 80-100 bpm, normally slowed by parasympathetic input to 60-80 bpm during rest.
• Depolarization Spread:
  • From the SA node, the impulse quickly disperses through atrial myocardium via gap junctions (trabeculae carneae) to synchronize atrial contraction.
  • Wave travels through internodal pathways to the AV node in the floor of the right atrium.
• AV Node & Delay:
  • The AV node provides a crucial delay, allowing atria to contract and ventricles to fill before ventricular depolarization begins.
• Ventricular Conduction:
  • From AV node: impulse moves down the Bundle of His (atrioventricular bundle).
  • Divides into left and right bundle branches in the interventricular septum.
  • Spreads via Purkinje fibers to activate the entire ventricular myocardium for synchronous contraction.
• Pacemaker Hierarchy:
  • The SA node sets the rhythm as it depolarizes fastest.
  • If SA node fails, the AV node can act as a secondary pacemaker (60-80 bpm).

If both nodes are damaged, the Purkinje fibers can generate rhythms as low as 20-40 bpm, insufficient for life support.

Nervous Regulation of Heart Rate:

• The medulla oblongata in the brainstem controls heart rate via the cardiac control center.
• It assesses sensory input from:
  • Baroreceptors: Located in the aortic arch and carotid sinus, measure blood pressure.
  • Chemoreceptors: Located in arteries and brain ventricles, detect blood pH, CO₂, and O₂ levels.
• Rest and Digest (Parasympathetic):
  • Vagus nerve (cranial nerve X) releases acetylcholine onto SA node.
  • Acetylcholine opens potassium channels, hyperpolarizing the cells and slowing depolarization, reducing heart rate.
• Fight or Flight (Sympathetic):
  • Sympathetic nerves release epinephrine (or norepinephrine) onto beta-1 adrenergic receptors on the SA node.
  • This accelerates depolarization, increasing heart rate and force of contraction.
• Summary:
  • Heart rate is modulated by a balance between parasympathetic and sympathetic inputs.
  • Rest: Parasympathetic dominates → slower heart rate (~60 bpm).

Exercise/stress: Sympathetic dominates → faster heart rate (~100 bpm or more).

ECG (Electrocardiogram) Overview:

• A non-invasive test that records electrical activity through surface electrodes.
• Waves:
  • P wave: Atrial depolarization.
  • QRS complex: Ventricular depolarization.
  • T wave: Ventricular repolarization.
• Changes or abnormalities in ECG can identify:
  • Damage or ischemia (lack of oxygen).
  • Conduction delays or blocks.
  • Arrhythmias/dysrhythmias.
  • Infarctions (heart attacks).

Arrhythmias/dysrhythmias: Abnormal rhythms, e.g., tachycardia (>100 bpm), bradycardia (<60 bpm), or irregular patterns.

Summary:

The heart’s rhythm is driven by the SA node and propagated through specialized conduction pathways. Heart rate regulation involves neural input from the medulla via sympathetic and parasympathetic nerves, modulating depolarization speed. The ECG provides crucial insights into cardiac health, revealing electrical abnormalities indicative of disease. Understanding this system aids in diagnosing and managing cardiac conditions effectively.