Cardiac Physiology: Electrical Activity and the Cardiac Cycle

Cardiac Physiology

Introduction

Cardiac physiology focuses on the mechanisms that regulate the function of the heart, including its electrical activity, contraction, and the resulting blood flow. Understanding these processes is essential for comprehending how the heart maintains circulation and responds to physiological demands.

Electrical Activity of the Heart and the Electrocardiogram

Electrical Events of the Heart

• Autorhythmicity: The heart is autorhythmic, meaning it can generate action potentials without external stimulation. This property ensures the heart beats in a coordinated and rhythmic manner.

• Cell Types:

  • Pacemaker (autorhythmic) cells: Specialized cells that initiate and regulate the heartbeat.

  • Contractile cells: Cells responsible for the contraction and relaxation of heart muscle.

• Functional Syncytium: Cardiac muscle cells are connected by gap junctions, allowing the heart to function as a coordinated unit.

• Intrinsic Cardiac Conduction System: A network of autorhythmic cells that initiate and distribute electrical impulses to coordinate heart rhythm.

Cardiac Cells – Autorhythmic Cells

Autorhythmic cells are responsible for initiating the electrical impulses that set the pace of the heartbeat. Their unique action potentials differ from those of contractile cells.

• Pacemaker Potential: The membrane potential of these cells slowly depolarizes due to the opening of Na+ channels and closing of K+ channels, never remaining at a flat line.

• Threshold: When the pacemaker potential reaches threshold, an action potential is triggered.

• Action Potential Phases:

  1. Pacemaker Potential: Slow depolarization due to Na+ influx and reduced K+ efflux.

  2. Depolarization: Rapid influx of Ca2+ through voltage-gated Ca2+ channels.

  3. Repolarization: Ca2+ channels close and K+ channels open, returning the membrane potential to its negative value.

Example: The sinoatrial (SA) node contains pacemaker cells that set the pace for the entire heart.

Pacemaker and Action Potentials of Typical Cardiac Pacemaker Cells

• Phase 1: Pacemaker Potential – Slow depolarization due to Na+ influx and K+ channel closure.

• Phase 2: Depolarization – Once threshold is reached, Ca2+ channels open, causing rapid depolarization.

• Phase 3: Repolarization – Ca2+ channels close, K+ channels open, and the cell returns to resting potential.

Key Formula:

$\text{Heart Rate (HR)} = \frac{60}{\text{R-R interval (seconds)}}$

Additional info: The R-R interval is measured on an ECG and represents the time between two consecutive R waves, corresponding to one cardiac cycle.

Intrinsic Cardiac Conduction System

The intrinsic conduction system ensures the heart beats in a coordinated manner by transmitting electrical impulses through a specific pathway.

• Sinoatrial (SA) Node: Located in the right atrium; acts as the primary pacemaker, generating impulses at 60–100 beats per minute.

• Atrioventricular (AV) Node: Receives impulses from the SA node and delays them briefly to allow atrial contraction before ventricular contraction.

• Bundle of His (AV Bundle): Conducts impulses from the AV node to the ventricles.

• Right and Left Bundle Branches: Carry impulses through the interventricular septum.

• Purkinje Fibers: Distribute impulses throughout the ventricles, causing coordinated contraction.

Example: Damage to the SA node can result in slower heart rates as the AV node or other pacemaker sites take over.

Summary Table: Cardiac Conduction System Components

Component	Location	Function
SA Node	Right atrium	Primary pacemaker; initiates heartbeat
AV Node	Interatrial septum	Delays impulse; allows atrial contraction
Bundle of His	Interventricular septum	Conducts impulse to ventricles
Bundle Branches	Interventricular septum	Transmit impulse to Purkinje fibers
Purkinje Fibers	Ventricular walls	Distribute impulse for coordinated contraction

Additional info: The conduction system ensures the atria contract before the ventricles, optimizing blood flow and cardiac efficiency.