Ch.18 pt.2: Electrical Events and Conduction System of the Heart

The Cardiovascular System: Electrical Events of the Heart

Overview of Cardiac Electrical Activity

The heart is capable of depolarizing and contracting without nervous system stimulation, though its rhythm can be modified by the autonomic nervous system. The coordinated heartbeat is primarily a function of specialized cellular structures and pathways.

• Gap Junctions: Allow direct electrical communication between cardiac muscle cells, enabling synchronized contraction.

• Intrinsic Cardiac Conduction System: A network of noncontractile (autorhythmic) cells that initiate and distribute impulses to coordinate depolarization and contraction of the heart.

Intrinsic Cardiac Conduction System

Pacemaker Cells and Action Potentials

Pacemaker cells are specialized cardiac cells with unstable resting membrane potentials, known as pacemaker potentials or prepotentials. These cells are responsible for initiating the heart's electrical activity.

• Pacemaker Potential: Slow depolarization due to closing of K+ channels and opening of slow Na+ channels, causing the cell interior to become more positive.

• Depolarization: At threshold (around -40 mV), Ca2+ channels open, allowing a rapid influx of Ca2+ and leading to the rising phase of the action potential.

• Repolarization: Ca2+ channels close and K+ channels open, allowing K+ efflux and returning the cell to a more negative membrane potential.

Equation (for reference):

• Resting membrane potential changes: $\text{Pacemaker potential} \rightarrow \text{Depolarization} \rightarrow \text{Repolarization}$

Sequence of Excitation in the Heart

The cardiac conduction system ensures the orderly spread of electrical impulses through the heart, resulting in coordinated contraction. The sequence is as follows (total time ~0.22 seconds):

1. Sinoatrial (SA) node

2. Atrioventricular (AV) node

3. Atrioventricular (AV) bundle (Bundle of His)

4. Right and left bundle branches

5. Subendocardial conducting network (Purkinje fibers)

Key Components of the Conduction System

• Sinoatrial (SA) Node: Located in the right atrial wall; acts as the heart's pacemaker. Depolarizes faster than the rest of the myocardium, generating impulses at about 75 times per minute (sinus rhythm). The inherent rate is about 100 times per minute, but is tempered by extrinsic factors. Impulse spreads across the atria to the AV node.

• Atrioventricular (AV) Node: Located in the inferior interatrial septum. Delays impulses by approximately 0.1 second due to smaller fiber diameter and fewer gap junctions, allowing atrial contraction to complete before ventricular contraction. Inherent rate is about 50 times per minute in the absence of SA node input.

• Atrioventricular (AV) Bundle (Bundle of His): Located in the superior interventricular septum. It is the only electrical connection between the atria and ventricles, as they are not connected via gap junctions.

• Right and Left Bundle Branches: Two pathways in the interventricular septum that carry impulses toward the apex of the heart.

• Subendocardial Conducting Network (Purkinje Fibers): Complete the pathway through the interventricular septum into the apex and ventricular walls. More elaborate on the left side of the heart. The AV bundle and Purkinje fibers depolarize about 30 times per minute in the absence of AV node input. Ventricular contraction follows immediately from the apex toward the atria. The entire process from SA node initiation to complete contraction takes about 0.22 seconds.

Example: Pacemaker and Action Potentials

The action potential of a typical cardiac pacemaker cell consists of three phases: pacemaker potential, depolarization, and repolarization. This cycle ensures the rhythmic and automatic initiation of heartbeats.

Additional info: The above notes are based on textbook lecture slides and are suitable for college-level Anatomy & Physiology students studying the cardiovascular system, specifically the electrical events and conduction system of the heart.