Cardiac muscle and conduction system

This chapter covers the cardiac muscle’s characteristics, the three states each cardiac cell cycles through for every impulse, and the heart’s conduction system.

Cardiac muscle

There are two types of cardiac cells: electrical or conduction system cells and myocardial cells. The electrical cells are found in the conduction system. They have three unique characteristics:

• Automaticity: The cells create and discharge the electrical impulse.
• Excitability: The cells respond to the electrical impulse.
• Conductivity: The cells transmit electrical impulses to other cells.

The myocardial cells are found in the myocardium. They can shorten and lengthen their fibers for contraction (called contractility). The fibers made from the myocardial cells can contract for prolonged periods of time without fatigue and use less adenosine triphosphate (ATP) than other muscles. These features are beneficial for the heart.

Cardiac muscle fibers are electrically linked, forming one unit. A myocardial cell forms a strong, electrical connection to neighboring cells through special junctions called intercalated discs, which allow the cell-to-cell communication needed for coordinated contraction - the fibers act as one unit instead of contracting a little at a time. This “one unit” approach is important because the atrial chambers need to contract together, and the ventricles need to contract at about the same time.

Conduction system structures

As previously stated, the electrical cells make up the conduction system of the heart. These cells are found throughout the myocardium. They can respond to and transmit electrical impulses to neighboring cells. The conduction system is composed of five structures:

• Sinoatrial (SA) node: Located in the posterior superior wall of the right atrium, the SA node is called the “pacemaker of the heart.” This is because the electrical cells in the SA node generate the impulse that starts the heartbeat. When the SA node discharges the impulse, it travels in many directions through the heart muscle. The impulse also moves quickly across special bands of tissue called internodal tracts. The Bachmann bundle, a specialized internodal tract, takes the impulse to the left atrium. Other internodal tracts take the impulse quickly to the AV node. The impulse moving through the atrial chambers triggers the chambers to contract.
• Atrioventricular (AV) node: The AV node is located at the base of the interatrial septum. When the impulse reaches the AV node, it moves very slowly through it.
• Bundle of His (also called the atrioventricular [AV] bundle): The bundle of His is located in the upper interventricular septum. When the impulse leaves the AV node, it moves to the bundle of His.
• Right and left bundle branches: The bundle branches are located in the lower interventricular septum. After the impulse passes through the bundle of His, it enters the right and left bundle branches. The right bundle branch brings the impulse to the right ventricle. The left bundle branch brings the impulse to the left ventricle.
• Purkinje fibers: The bundle branches split into many Purkinje fibers. Purkinje fibers transmit the impulse quickly and efficiently to the ventricular myocardial cells. This causes the ventricular chambers to contract.

Intrinsic pacemaker rates

Every structure in the conduction system can generate an impulse on its own, and each one does so at its own rate. The fastest one sets the heart rate, which is why the SA node is the pacemaker of the heart. If the SA node fails, or its impulse is blocked on the way down, the next structure below takes over at its own slower rate. The heart keeps beating; it just beats more slowly.

Pacemaker site	Intrinsic rate (beats/min)
SA node	60-100
AV node (junctional)	40-60
Ventricles (bundle branches and Purkinje fibers)	20-40

The cardiac muscle is different from other muscles in the body. The heart is controlled by the autonomic nervous system and the heart’s own conduction system. When the heart rate needs to change to meet the demands of the body, the autonomic nervous system automatically kicks in.

States of the cardiac cell cycle

The cardiac cells cycle through three states or steps in the same sequence for each impulse:

1. Polarized state: Before the impulse hits the cells, they are in a polarized state. There is no electrical activity during the polarized state. Think of this as the “waiting” stage.
2. Depolarized state: When the impulse hits the cells, the cells’ charges change. This is due to the movement of the ions (e.g., sodium, potassium, and calcium) across the cells’ membrane. The change of the cells’ charges allows the impulse to move through the cell, causing action potential (also called depolarization). Electrical activity can be recorded on an electrocardiogram (ECG) when the cells are in the depolarized state. (ECGs are discussed in Chapter 26.)
3. Repolarized state: After the impulse passes over the cells, the ions move back to their original location. This causes the cells’ charge to change. This recovery phase is called the repolarized state. Electrical activity (less than the depolarized state) can also be recorded on an ECG during the repolarized state.

When you are discussing the three states and the chambers affected, the chamber (atrial, ventricular) comes before the state (e.g., atrial polarization, ventricular depolarization).

Conduction system and blood flow

The conduction system is the electrical system in the heart, and its activity is what’s recorded on the ECG. Electrical impulses from the conduction system normally cause the chambers of the heart to contract, moving blood through the heart. However, the ECG only records electrical activity - it doesn’t confirm that a contraction actually happened. Electrical impulses can be present without producing an effective mechanical contraction or blood flow, so the electrical and mechanical events, though normally linked, aren’t the same thing. This mechanical action, when it occurs, can be seen with echocardiography (ECHO), an imaging test that shows the heart’s chambers and valves moving in real time.