Cardiac Muscle and the Cardiac Cycle: Structure, Function, and Electrophysiology

Cardiac Muscle Structure and Function

Structure of a Cardiac Muscle Cell

Cardiac muscle cells, or cardiomyocytes, are specialized for continuous rhythmic contraction and are structurally distinct from skeletal and smooth muscle cells.

• Mitochondria: Cardiac muscle cells contain numerous mitochondria, reflecting their high energy demand for sustained contraction.

• T-tubules: Invaginations of the sarcolemma (cell membrane) that help transmit action potentials deep into the cell, ensuring coordinated contraction.

• Intercalated Discs: Specialized junctions between adjacent cardiac cells containing desmosomes (which provide mechanical strength and prevent cells from separating during contraction) and gap junctions (which allow direct electrical communication between cells, enabling synchronized contraction).

• Striations: Cardiac muscle cells are striated due to the organized arrangement of actin and myosin filaments.

Example: The presence of gap junctions allows the heart to function as a functional syncytium, where action potentials rapidly propagate from cell to cell.

Energy Demand and ATP Production in Cardiac Muscle

Cardiac muscle cells have a high energy demand due to their continuous activity.

• High/Low Energy Demand: The heart requires a constant supply of ATP to maintain contraction and ion gradients.

• Molecules Used for ATP Production: Cardiac cells primarily use fatty acids and glucose as substrates for ATP production via aerobic respiration. They can also utilize lactate and ketone bodies.

• ATP Production: Most ATP is generated in mitochondria through oxidative phosphorylation.

Example: During increased physical activity, the heart increases its uptake of fatty acids and glucose to meet energy demands.

Cardiac Electrophysiology

Autorhythmicity and Pacemaker Cells

Autorhythmicity refers to the heart's ability to generate its own rhythmic electrical impulses without external stimulation.

• Strongest Autorhythmic Center: The sinoatrial (SA) node is the primary pacemaker of the heart, setting the pace for the entire myocardium.

• Overpowering Other Centers: The SA node has the highest rate of spontaneous depolarization, which suppresses other potential pacemakers (such as the AV node and Purkinje fibers) through a process called overdrive suppression.

• Channel Type Facilitating Autorhythmicity: Funny channels (If channels, or HCN channels) allow a slow influx of Na+ ions, contributing to the pacemaker potential and spontaneous depolarization.

Example: If the SA node fails, the AV node can take over as a secondary pacemaker, but at a slower rate.

Electrical Conduction System of the Heart

The heart's electrical conduction system ensures coordinated contraction of the atria and ventricles.

• Order of Conduction:

  1. Sinoatrial (SA) node

  2. Atrial muscle fibers

  3. Atrioventricular (AV) node

  4. Bundle of His (AV bundle)

  5. Right and left bundle branches

  6. Purkinje fibers

• Importance of AV Node Delay: The delay at the AV node allows the atria to complete contraction and empty blood into the ventricles before ventricular contraction begins.

Example: The AV node delay is critical for efficient cardiac output and prevents simultaneous contraction of atria and ventricles.

Action Potential of Contractile Cardiomyocytes

The action potential in contractile cardiac muscle cells is essential for coordinated contraction and is distinct from that of pacemaker cells.

• Phases of Action Potential:

  1. Rapid depolarization (Na+ influx)

  2. Initial repolarization (K+ efflux)

  3. Plateau phase (Ca2+ influx through L-type calcium channels balances K+ efflux)

  4. Repolarization (K+ efflux continues, Ca2+ channels close)

  5. Resting potential

• Source of Calcium: Calcium enters the cell from the extracellular fluid (ECF) through voltage-gated calcium channels and triggers further release of Ca2+ from the sarcoplasmic reticulum (SR) via ryanodine receptors (CICR: calcium-induced calcium release).

Example: The plateau phase prolongs the action potential, preventing tetanus in cardiac muscle.

Electrocardiogram (ECG) and Cardiac Cycle

ECG Waves: P wave, QRS Complex, and T wave

The ECG records the electrical activity of the heart and is used to assess cardiac function.

• P wave: Represents atrial depolarization (contraction of the atria).

• QRS complex: Represents ventricular depolarization (contraction of the ventricles).

• T wave: Represents ventricular repolarization (relaxation of the ventricles).

Example: Abnormalities in the QRS complex can indicate ventricular conduction defects.

The Cardiac Cycle

The cardiac cycle describes the sequence of mechanical and electrical events that repeat with every heartbeat.

• Phases of the Cardiac Cycle:

  1. Atrial systole: Atria contract, AV valves open, blood flows into ventricles.

  2. Isovolumetric contraction: Ventricles contract, all valves closed, pressure rises.

  3. Ventricular ejection: Semilunar valves open, blood ejected into aorta and pulmonary artery.

  4. Isovolumetric relaxation: Ventricles relax, all valves closed, pressure falls.

  5. Ventricular filling: AV valves open, blood flows from atria to ventricles.

• Valve Status:

  • AV valves (tricuspid and mitral) are open during ventricular filling and atrial systole; closed during ventricular contraction.

  • Semilunar valves (aortic and pulmonary) are open during ventricular ejection; closed during ventricular filling and isovolumetric phases.

Example: The first heart sound (S1) is produced by closure of the AV valves at the beginning of ventricular systole.