PHYSIOLOGIC PROPERTIES OF THE HEART ♥️

1. AUTORHYTHMICITY (Chronotropism)

1.1. - generate action potentials spontaneously or “self-excitation”

1.2. - Contributed by SA node, AV node, His-Purkinje System

1.2.1. SA node Action Potentials

1.2.1.1. Phase 0 - upstroke; caused by an increase in Ca2+ conductance

1.2.1.2. Phase 3 - repolarization; caused by an increase in K+ conductance

1.2.1.3. Phase 4 - depolarization; caused by an increase Na+ conductance

1.2.2. Phases 1 and 2 are absent in Nodal Action Potential

2. CONDUCTIVITY (Dromotropism)

2.1. - action potentials are propagated within the cardiac tissue

2.2. SA NODE - primary pacemaker;

2.2.1. INTERATRIAL PATHWAYS

2.2.1.1. Anterior interatrial tract of Bachman

2.2.1.2. Middle interatrial tract of Wenckebach

2.2.1.3. Posterior interatrial tract of Thorel

2.2.2. INTERNODAL PATHWAYS

2.2.2.1. Anterior internodal pathways

2.2.2.2. Middle internodal pathways

2.2.2.3. Posterior internodal pathways

2.3. - when SA node drives the heart, latent pacemakers are supressed;

2.3.1. - latent pacemakers have an opportunity to drive the heart only if SA node is supressed

2.4. AV node and His-Purkinje System - latent pacemakers

3. EXCITABILITY (Bathmotropism)

3.1. - Refractory periods

3.2. - capacity of myocardial cells to generate action potentials in response to inward, depolarizing current

3.3. - when Na+ channels are closed and unavailable, inward depolarizing current cannot flow

4. CONTRACTILITY (Inotropism)

4.1. Frank-Starling mechanism

4.2. Phase 0 (Depolarization) - fast sodium channels open

4.3. Phase 1 (Initial Repolarization) - fast sodium channels close

4.4. Phase 3 (Rapid Repolarization) - calcium channels close and slow potassium channels open

4.5. Phase 2 (Plateau) - calcium channels open and fast potassium channels close

4.6. Phase 4 (Resting Membrane Potential) - averages about -80 to -90 millivolts

5. GRANADOS, CRYSTAL GALE R. REFERENCE: Module 6.1 Functional Anatomy of the Heart and Physiologic Properties of the Heart (handout)