Conduction System of the Heart

Study Notes

The conduction system is a network of specialized cardiac muscle fibers called autorhythmic fibers.
Autorhythmic fibers repeatedly generate action potentials that trigger heart contractions.
They make up about 1% of cardiac muscle fibers.
These fibers conduct the electrical signal through the heart, ensuring that the chambers contract in a coordinated manner.

The action potential originates in the SA node and spreads through the entire atrial muscle mass and eventually to the A-V node.
Conduction velocity in atrial muscle is about 0.3 m/sec, but is faster (1 m/sec) in specialized bands of atrial fibers, including the anterior interatrial band (Bachman’s bundle), which passes through the anterior atrial walls to the left atrium.
Three other small bands, called anterior, middle, and posterior internodal pathways, curve through the atrial walls and terminate in the A-V node.
Rapid conduction in these bands is due to the presence of specialized conduction fibers, similar to the rapidly conducting Purkinje fibers of the ventricles.

The impulse reaches the A-V node about 0.03 seconds after originating in the sinus node.
There's a delay of 0.09 seconds in the A-V node itself before the impulse enters the penetrating portion of the A-V bundle.
Another delay of 0.04 seconds occurs in the penetrating A-V bundle.
The total delay in the A-V nodal and A-V bundle system is about 0.13 seconds, resulting in a total delay of 0.16 seconds before the excitatory signal reaches the ventricular muscle.

The atrial conductive system is organized to prevent the cardiac impulse from reaching the ventricles too rapidly.
This delay allows time for the atria to empty their blood into the ventricles before ventricular contraction begins.
The A-V node and its adjacent conductive fibers are primarily responsible for delaying transmission into the ventricles.

The A-V bundle prevents action potentials from traveling backward from the ventricles to the atria.
This prevents re-entry of cardiac impulses and ensures only forward conduction from the atria to the ventricles.
Atrial and ventricular muscle are separated by a fibrous barrier, which acts as an insulator, preventing impulse passage except through the A-V bundle.
In rare cases, an abnormal muscle bridge or accessory pathway may penetrate the fibrous barrier, allowing re-entry of impulses and leading to cardiac arrhythmias.

After penetrating the fibrous tissue, the A-V bundle passes downward in the ventricular septum and divides into left and right bundle branches on either side of the septum.
Each branch spreads downward toward the apex of the respective ventricle and further divides into smaller branches.
These branches course sidewise around each ventricular chamber, returning toward the base of the heart.
The ends of the Purkinje fibers penetrate about one-third of the way into the muscle mass and become continuous with the cardiac muscle fibers.

The slow conduction in the transitional, nodal, and penetrating A-V bundle fibers is mainly caused by diminished numbers of sodium channels and a slower rate of sodium permeability.
This slow conduction is important for allowing time for the atria to empty their blood into the ventricles before ventricular contraction.