Cardiac Muscle Tissue and the Cardiac Cycle: Structure, Function, and Electrical Activity

course outline module 4 part 2

Cardiac Muscle Tissue and the Cardiac Conduction System

Structural and Functional Characteristics of Cardiac Muscle Tissue

Cardiac muscle tissue is specialized for continuous, rhythmic contraction and is found only in the heart. Its unique structure supports its function in pumping blood throughout the body.

• Striated Appearance: Cardiac muscle fibers have a striated appearance due to the organized arrangement of actin and myosin filaments, similar to skeletal muscle.

• Branched Cells: Cardiac muscle cells (cardiomyocytes) are short, branched, and interconnected, forming a functional network.

• Intercalated Discs: Specialized junctions called intercalated discs connect adjacent cells, containing gap junctions and desmosomes. These structures allow rapid electrical communication and strong mechanical attachment between cells.

• Involuntary Control: Cardiac muscle contracts involuntarily, regulated by the autonomic nervous system and intrinsic conduction system.

• Single Central Nucleus: Most cardiac muscle cells contain a single, centrally located nucleus.

Example: The intercalated discs enable the heart to contract as a coordinated unit, ensuring efficient blood ejection.

The Cardiac Conduction System

The cardiac conduction system is a network of specialized cardiac muscle cells responsible for initiating and distributing electrical impulses that stimulate heart contraction.

• Sinoatrial (SA) Node: Located in the right atrium; acts as the heart's natural pacemaker by generating spontaneous action potentials.

• Atrioventricular (AV) Node: Receives impulses from the SA node and delays transmission 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 toward the apex of the heart.

• Purkinje Fibers: Distribute the impulse throughout the ventricular myocardium, triggering coordinated ventricular contraction.

Additional info: The conduction system ensures the heart beats in a coordinated and efficient manner.

Action Potentials in Cardiac Contractile Fibers

Phases of the Cardiac Action Potential

Cardiac contractile fibers generate action potentials with distinct phases, allowing for prolonged contraction and effective pumping.

• Phase 0 (Depolarization): Rapid influx of Na+ ions through voltage-gated sodium channels causes the membrane potential to become positive.

• Phase 1 (Initial Repolarization): Brief outflow of K+ ions begins repolarization.

• Phase 2 (Plateau): Influx of Ca2+ ions through voltage-gated calcium channels balances K+ outflow, creating a plateau phase unique to cardiac muscle.

• Phase 3 (Repolarization): Closure of Ca2+ channels and continued K+ outflow restore the resting membrane potential.

• Phase 4 (Resting Potential): The cell returns to its resting state, ready for the next action potential.

Equation:

$\text{Resting Membrane Potential} \approx -90\ \text{mV}$

Example: The plateau phase prevents tetanus in cardiac muscle, ensuring rhythmic contractions.

Electrical Events of a Normal Electrocardiogram (ECG)

Components of the ECG

An electrocardiogram (ECG or EKG) records the electrical activity of the heart and is used to assess cardiac function.

• P Wave: Represents atrial depolarization (contraction).

• QRS Complex: Represents ventricular depolarization (contraction) and atrial repolarization (relaxation, masked by QRS).

• T Wave: Represents ventricular repolarization (relaxation).

Example: A normal ECG shows a regular pattern of P waves, QRS complexes, and T waves, indicating normal cardiac rhythm.

Pressure and Volume Changes During the Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle consists of a series of pressure and volume changes that occur during one heartbeat, divided into systole (contraction) and diastole (relaxation).

• Atrial Systole: Atria contract, increasing atrial pressure and pushing blood into the ventricles.

• Ventricular Systole: Ventricles contract, causing ventricular pressure to rise above atrial pressure, closing the AV valves and opening the semilunar valves to eject blood into the arteries.

• Ventricular Diastole: Ventricles relax, ventricular pressure falls, semilunar valves close, and AV valves open to allow ventricular filling.

Equation:

$\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$

Example: During ventricular systole, the left ventricular pressure rises sharply, causing the aortic valve to open and blood to be ejected into the aorta.

Timing of Heart Sounds, ECG Waves, and Pressure Changes

Relationship Between Heart Sounds, ECG, and Cardiac Cycle

Heart sounds are produced by the closing of heart valves and are closely related to the electrical and mechanical events of the cardiac cycle.

• First Heart Sound (S1): Occurs with closure of the AV valves (mitral and tricuspid) at the beginning of ventricular systole; follows the QRS complex.

• Second Heart Sound (S2): Occurs with closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole; follows the T wave.

• ECG Correlation: The P wave precedes atrial contraction, the QRS complex precedes ventricular contraction, and the T wave precedes ventricular relaxation.

Example: The "lub-dub" sounds correspond to S1 and S2, which can be timed with the ECG and pressure changes during the cardiac cycle.