28. Physiological characteristics of the working myocardium. Excitation and contraction. Refractory periods. Extrasystoles, flutter and fibrillation. Myocardial metabolism. Functional morphology and physiological characteristics of the conduction system of the heart. Automaticity. Cardiac rhythm. Abnormalities of conductivity.

28. Physiological Characteristics of the Working Myocardium

1. Working Myocardium: Structure and Function

• The myocardium is the middle layer of the heart wall.

• Composed of striated cardiac muscle.

  • Right ventricle: 2 muscle layers.

  • Left ventricle: 3 muscle layers (to withstand higher systemic pressure).

• Two key cell types:

  • Cardiomyocytes: responsible for contraction.

  • Pacemaker (conductive) cells: generate and transmit electrical impulses.

2. Excitation and Contraction

• Excitability: ability of myocardium to generate an action potential (AP) in response to a stimulus.

• Contractility: ability to convert chemical energy (ATP) into mechanical force for contraction.

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3. Refractory Periods

Absolute Refractory Period

• No new AP can be generated, regardless of stimulus strength.

• Duration: ~250 ms.

• Due to inactivated Na⁺ channels.

• Prevents tetanus (sustained muscle contraction); allows ventricles to refill with blood.

Relative Refractory Period

• AP can be generated if the stimulus is stronger than usual.

• Some Na⁺ channels recover.

• Occurs during the late plateau and repolarization.

• Duration: ~50 ms.

• Maintains proper cardiac rhythm and ventricular filling.

4. Extrasystoles, Flutter, and Fibrillation

• Extrasystoles:

  • Premature contraction.

  • Occurs before full ventricular filling → weaker contraction (Frank-Starling law).

• Atrial Flutter:

  • Rapid, regular atrial contractions: 250–350 bpm.

• Atrial Fibrillation:

  • Very rapid, irregular atrial activity: 300–600 bpm.

  • Ventricular rate is slower and irregular → reduced cardiac output.

5. Myocardial Metabolism

• Requires ATP for both contraction and relaxation.

• Energy demands increase with:

  • ↑ Heart rate

  • ↑ Contractility

• Primarily aerobic metabolism (dependent on oxygen supply).

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6. Functional Morphology and Physiological Characteristics of the Conduction System

1. Sinoatrial Node (SAN) – Primary Pacemaker

• Highest excitability; unstable resting membrane potential.

• Conduction velocity: 300–500 mm/s.

• Initiates each heartbeat.

2. Atrioventricular Node (AVN) – Secondary Pacemaker

• Delays conduction to allow ventricular filling.

• Conduction velocity: ~50 mm/s.

• Continuous with Bundle of His.

3. Bundle of His

• Tertiary pacemaker.

• Splits into right and left bundle branches.

• Conduction velocity: 1000–2000 mm/s.

4. Purkinje Fibers

• Extend from bundle branches into ventricular myocardium.

• Conduction velocity: 2000–4000 mm/s.

• Ensures rapid and synchronized ventricular contraction.

7. Automaticity

• cardiac rhythm

• Automaticity: Ability to spontaneously depolarize and generate APs.

• Found in:

  • SAN, AVN, Bundle of His, Purkinje fibers, cardiomyocytes (to a lesser extent).

• Calcium ions (Ca²⁺) essential for depolarization and contraction.

• Ectopic pacemakers: abnormal sites generating APs when SAN is suppressed or overridden.

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8. Cardiac Rhythm

• Normal rhythm: Determined by SAN (60–90 bpm).

• If AVN takes over: 40–60 bpm.

• Types:

  • Sinus rhythm: Regular spacing between waves.

  • Bradycardia: <60 bpm.

  • Tachycardia: >100 bpm.

9. Abnormalities of Conductivity

By Severity

• First-degree block:

  • Slowed conduction.

  • Prolonged PR interval.

• Second-degree block:

  • Mobitz Type I (Wenckebach):

    • PR interval progressively lengthens until a beat is dropped.

  • Mobitz Type II:

    • Constant PR intervals with intermittent dropped QRS complexes.

  • 2:1 block:

    • Only one QRS complex follows every two P waves.

• Third-degree block:

  • No conduction between atria and ventricles.

  • Atria and ventricles beat independently.

  • QRS complexes are widened and disassociated from P waves.

By Location

• SA Node Block

• AV Node Block

• Intraventricular Block

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