Mechanical Activity of the Heart: Excitation-Contraction Coupling, Cardiac Cycle, and Pathological Alterations

Mechanical Activity of the Heart

Overview of Cardiac Excitation-Contraction Coupling

The heart's ability to contract and pump blood is tightly regulated by a sequence of electrical and mechanical events. Excitation-contraction coupling refers to the process by which an electrical stimulus (action potential) leads to muscle contraction in cardiac contractile cells.

• Excitation-Contraction Coupling: The mechanism by which depolarization of the cardiac cell membrane triggers contraction of the heart muscle.

• Key Steps:

  1. Action potential from an adjacent cell depolarizes the cardiac cell membrane.

  2. Voltage-gated Ca2+ channels open, allowing Ca2+ influx.

  3. Ca2+ entry triggers further Ca2+ release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyR).

  4. Ca2+ binds to troponin, initiating cross-bridge formation between actin and myosin, resulting in contraction.

  5. Relaxation occurs when Ca2+ unbinds from troponin and is pumped back into the SR or out of the cell.

• Formula: $\text{Ca}^{2+}_{\text{influx}} \rightarrow \text{Ca}^{2+}_{\text{SR release}} \rightarrow \text{Troponin binding} \rightarrow \text{Contraction}$

• Example: Cardiac muscle contraction during systole is initiated by Ca2+ influx and subsequent Ca2+ release from the SR.

Electrical and Mechanical Events of the Cardiac Cycle

The cardiac cycle consists of a series of electrical and mechanical events that result in the rhythmic contraction and relaxation of the heart chambers, ensuring effective blood flow.

• Electrical Events: Initiated by the sinoatrial (SA) node, propagated through the atria, AV node, bundle branches, and Purkinje fibers.

• Mechanical Events: Contraction (systole) and relaxation (diastole) of atria and ventricles follow electrical depolarization and repolarization.

• Sequence:

  1. P wave: Atrial depolarization; atrial contraction occurs at the end of the P wave.

  2. P-R segment: Electrical signal slows through AV node and bundle.

  3. QRS complex: Ventricular depolarization; ventricular contraction begins shortly after Q wave and continues through T wave.

  4. T wave: Ventricular repolarization; relaxation follows.

• Key Point: Mechanical events lag behind electrical events; contraction follows depolarization, and relaxation follows repolarization.

• Example: The ECG shows the timing of electrical events, which precede the corresponding mechanical actions of the heart chambers.

Phases of the Cardiac Cycle

The cardiac cycle is divided into distinct phases, each characterized by specific changes in pressure and volume within the heart chambers.

• Ventricular Diastole: Ventricles fill with blood; includes atrial systole.

• Isovolumic Contraction: Ventricles contract with no change in volume; all valves are closed.

• Ventricular Ejection: Semilunar (SL) valves open; blood is ejected into the pulmonary artery and aorta.

• Isovolumic Relaxation: Ventricles relax with no change in volume; all valves are closed.

• Pressure-Volume Relationships:

  • During diastole: Left atrial (LA) pressure > left ventricular (LV) pressure; LA volume decreases, LV volume increases.

  • Isovolumic contraction: LA pressure < LV pressure < aortic pressure; LV volume remains constant.

  • Ventricular ejection: LV pressure > aortic pressure; LV volume decreases.

  • Isovolumic relaxation: LA pressure < LV pressure < aortic pressure; LV volume remains constant.

• Example: The Wiggers diagram illustrates the relationship between electrical, mechanical, and pressure-volume changes throughout the cardiac cycle.

Timing of Electrical vs. Mechanical Events

Electrical events, as recorded by the electrocardiogram (ECG), precede and trigger the mechanical events of the cardiac cycle. Understanding this timing is crucial for interpreting cardiac function and diagnosing abnormalities.

• Depolarization: Precedes contraction.

• Repolarization: Precedes relaxation.

• Clinical Application: Abnormal timing or sequence can indicate cardiac dysfunction.

• Example: A delay in the QRS complex may result in delayed ventricular contraction.

Pathological Alterations in Cardiac Mechanical Activity

Diseases affecting the heart valves or contractile function can disrupt normal mechanical activity, leading to clinical symptoms and detectable changes in heart sounds and blood flow.

• Stenotic Valves: Narrowing of valve opening increases resistance to flow, resulting in increased velocity and turbulent flow (heart murmur when valve should be open).

• Insufficient (Incompetent) Valves: Valve leaflets do not seal completely, causing regurgitation (backward flow) and turbulent flow (heart murmur when valve should be closed).

• Normal vs. Abnormal Murmur Patterns:

  • Systolic murmur: Occurs during ventricular contraction.

  • Diastolic murmur: Occurs during ventricular relaxation.

• Examples:

  • Mitral Stenosis: Narrow mitral valve increases resistance from left atrium to left ventricle during diastole.

  • Mitral Regurgitation: Incompetent mitral valve allows backward flow into left atrium during systole.

Summary Table: Cardiac Cycle Phases and Pressure-Volume Relationships

Phase	Pressure Relationship	Volume Change	Valve Status
Ventricular Diastole	LA > LV	LV volume increases	AV valves open, SL valves closed
Isovolumic Contraction	LV > LA, LV < Aorta	LV volume constant	All valves closed
Ventricular Ejection	LV > Aorta	LV volume decreases	SL valves open, AV valves closed
Isovolumic Relaxation	LV < Aorta, LV > LA	LV volume constant	All valves closed

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.