chapter 12

Learning Objectives

• Describe the electrophysiology of the heart, including action potential and phases 1 through 4, as well as their clinical implications.

• Describe the properties of cardiac muscle: automaticity, excitability, conductivity, and contractility, emphasizing how these properties differentiate cardiac muscle from skeletal muscle.

• Explain the refractory periods of the heart, including their importance in preventing arrhythmias and ensuring effective pumping action.

• Identify the major components of the heart's conductive system and their specific roles in the electrical conduction pathway.

• Describe the cardiac effects of the sympathetic and parasympathetic nervous systems, and how they interact to maintain homeostasis in response to various physiological demands.

Electrophysiology of the Heart

Polarized State

• The heart muscle cells (myocytes) maintain a polarized state where there is a higher concentration of positive ions (primarily sodium and calcium) outside the cell compared to the inside. This results in a deficiency of positive cations within the cell, which contributes to its negative charge, allowing for electrical excitability.

Resting Membrane Potential (RMP)

• The electrical difference generated by electrolytes inside and outside the cell creates the potential force of RMP, which is vital for cardiac function.

• Measured in millivolts (mV); myocardial cells typically have an RMP of approximately -90 mV, indicating a highly negative internal potential relative to the outside.

Action Potentials

• The action potential of cardiac cells is comprised of 4 distinct phases (0, 1, 2, 3, 4) that reflect changes in ion permeability and concentrations across the cell membrane, influencing the electrical activity of the heart.

• Action potentials vary structurally between pacemaker and nonpacemaker myocardial cells; pacemaker cells (like those in the SA node) exhibit spontaneous depolarization.

Phases of Action Potential

1. Phase 0: Rapid depolarization occurs due to a surge of sodium ions (Na+) flowing into the cell, making the inside of the cell more positive.

2. Phase 1: Initial repolarization begins as potassium ions (K+) exit the cell, reducing the positive charge inside the cell.

3. Phase 2: The plateau phase occurs as calcium ions (Ca²+) enter the cell, balancing the efflux of potassium, which is crucial for prolonged systolic contraction and preventing immediate re-excitation.

4. Phase 3: A rapid repolarization phase happens as potassium rapidly exits the cell, restoring the negative internal charge.

5. Phase 4: The cell returns to a resting state (RMP), where it is polarized and prepared for the next action potential, ensuring rhythmic heart contractions.

Conductive System of the Heart

Overview

• The heart's electrical cycle initiates with the Sinoatrial (SA) node, which is responsible for setting the rhythm of the heart (the primary pacemaker). It generates electrical impulses that coordinate contractions of the heart chambers.

Pathway of Electrical Impulses

• In the Right Atrium: The electrical impulse travels through specialized conduction pathways: the anterior, middle, and posterior internodal tracts, efficiently reaching the atrioventricular (AV) junction, where atrial contraction is propagated.

• Bachmann's Bundle: This structure conducts impulses directly to the left atrium, making sure both atria contract simultaneously, which is critical for efficient blood flow into the ventricles.

• AV Junction: Located behind the tricuspid valve, it serves as a gatekeeper, relaying impulses to the ventricles via the Bundle of His (sometimes referred to as the AV bundle), ensuring proper ventricular contraction timing.

• Bundle of His: This structure divides into left and right bundle branches, coursing through the interventricular septum to facilitate coordinated contraction of the ventricles.

• Purkinje Fibers: Situated at the apex of the heart, these fibers rapidly conduct impulses throughout the ventricular myocardium, enabling swift and synchronized depolarization, occurring in approximately 0.22 seconds.

Refractory Periods

• These periods represent the heart muscle's response to previous electrical stimuli, playing a critical role in preventing excessive firing and maintaining effective cardiac function.

• Absolute Refractory Period: During phases 0 through the early half of phase 3, cells are completely unresponsive to any new stimulus; this is crucial in preventing tetany (sustained contraction).

• Relative Refractory Period: This occurs during the latter half of phase 3; while some cells may respond to a strong stimulus, the risk of inducing an abnormal rhythm increases.

• Nonrefractory Period: Coinciding with phase 4, when cells are fully polarized and ready to respond to normal stimuli, allowing for the next heartbeat.

Autonomic Nervous System and Cardiac Response

• Sympathetic Stimulation: Increases heart rate through the release of catecholamines (norepinephrine), enhancing cardiac output to meet physiological demands (e.g., during exercise or stress).

• Sympathetic Block: Leads to decreased heart rate and reduced myocardial contractility, which is important in circumstances where lower heart activity is beneficial (e.g., rest).

• Parasympathetic Stimulation: Primarily via the vagus nerve decreases heart rate and promotes digestion and recovery. It decreases the pace of electrical impulses, allowing for a more efficient heart rate at rest.

• Parasympathetic Block: Results in an increased heart rate by reducing inhibitory signals, thus facilitating a more active state.

Summary

Understanding the electrophysiology of the heart is vital for recognizing how electrical impulses facilitate cardiac function and the role of the autonomic nervous system in regulating heart rate and contractility. Each phase of the action potential highlights crucial changes in ion flow, which are foundational to cardiac excitability and contractility, and understanding these mechanisms is key to addressing various cardiac conditions effectively.