Cardio Structure, Pacemaker Activity and the Electrocardiogram

By: Asfand Gul Kasi

Structure of the Heart

The heart is a muscular organ roughly the size of a person’s fist, located in the chest with its pointed end, called the apex, directed downward. Functionally, the heart acts as two pumps working in series. The right side of the heart pumps blood to the lungs for oxygenation, while the left side pumps oxygenated blood to the rest of the body through the systemic circulation.

Each side of the heart contains two chambers:

• Atria (upper chambers) receive blood
• Ventricles (lower chambers) pump blood out

Thus, the heart consists of four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Blood flows in a unidirectional pathway from atria to ventricles. The right and left sides are separated by muscular walls called septa, which allow electrical communication within each side while maintaining physical separation between chambers.

Importantly, the atria and ventricles are electrically isolated by a connective tissue structure called the cardiac skeleton. This separation ensures that electrical signals pass through a controlled pathway instead of spreading randomly.

Electrical Conduction System of the Heart

The heart beats because of a specialized electrical conduction system composed of modified cardiac muscle cells. These cells do not produce force like typical muscle cells; instead, they generate and transmit electrical signals that coordinate contraction.

The conduction pathway follows a precise sequence:

1. Sinoatrial (SA) Node
   Located in the right atrium, this is the natural pacemaker of the heart. It generates electrical impulses at a rate of 60–100 beats per minute.
2. Atrioventricular (AV) Node
   Located between the atria and ventricles, the AV node acts as a gateway. It briefly delays the signal, allowing the atria to contract and fill the ventricles before ventricular contraction begins.
3. Bundle of His
   This pathway carries the electrical signal into the ventricular septum.
4. Right and Left Bundle Branches
   The signal splits and travels down both sides of the septum.
5. Purkinje Fibers
   These fibers spread throughout the ventricles, causing a rapid and coordinated ventricular contraction.

This conduction system ensures that atria contract first, followed by ventricles, producing efficient pumping of blood.

Pacemaker Potential and Electrical Activity

Pacemaker cells possess a unique electrical property known as an unstable resting membrane potential. Unlike typical cardiac muscle cells, pacemaker cells gradually depolarize on their own until they reach a threshold that triggers an action potential.

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The pacemaker cycle consists of three main phases:

• Phase 4 Slow depolarization
  Driven by sodium “funny channels” and calcium channels that slowly raise the membrane potential.
• Phase 0 Depolarization
  Voltage-gated calcium channels open, allowing calcium ions to enter the cell.
• Phase 3 Repolarization
  Potassium channels open, causing potassium to leave the cell and restoring the resting potential.

This repeating cycle creates the automatic rhythmic heartbeat.

Autonomic Regulation of Heart Rate

Although the heart has its own pacemaker, the autonomic nervous system adjusts the heart rate depending on the body’s needs.

Sympathetic Nervous System

• Increases heart rate
• Shortens the pacemaker cycle
• Activated during exercise or stress
• Acts through beta-1 adrenergic receptors

This produces tachycardia, defined as a heart rate above 100 beats per minute.

Parasympathetic Nervous System

• Slows heart rate
• Hyperpolarizes pacemaker cells
• Dominant during rest and sleep
• Mediated by the vagus nerve through muscarinic receptors

A heart rate below 60 beats per minute is called bradycardia. In trained athletes, bradycardia can be normal due to increased parasympathetic tone.

Electrocardiogram (ECG)

Electrocardiogram (ECG) and Electrical Activity of the Heart

The electrocardiogram (ECG) records the electrical activity of the entire heart using electrodes placed on the body surface. It represents the summation of electrical signals from millions of cardiac cells.

Key ECG components include:

• P Wave Atrial depolarization
• PR Interval Time between atrial and ventricular activation
• QRS Complex Ventricular depolarization
• ST Segment Plateau phase of ventricular action potential
• T Wave Ventricular repolarization
• R–R Interval Time between heartbeats (heart rate)

Changes in these waves or intervals help physicians diagnose problems such as arrhythmias, conduction blocks, or myocardial damage.

Clinical Importance of the Conduction System

Disruptions in the conduction pathway can lead to abnormal rhythms known as arrhythmias. For example, a complete AV block occurs when electrical signals from the atria cannot reach the ventricles. In such cases, a slower pacemaker within the Bundle of His or Purkinje fibers takes over, producing a very slow heart rate.

Electrocardiograms are therefore essential diagnostic tools in cardiology, allowing doctors to detect electrical abnormalities and guide treatment.

The End

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