Electrophysiology of heart

1. Explain the properties of cardiac muscle:

Cardiac muscle, also known as myocardium, is a unique type of muscle tissue found only in the heart. It has several distinct properties:

• Involuntary contractions: Cardiac muscle contracts without conscious effort, enabling the heart to continuously pump blood throughout the body.
• Autorhythmicity: Cardiac muscle cells have the ability to spontaneously generate and propagate electrical impulses, which initiate the coordinated contraction of the heart.
• Striated appearance: Similar to skeletal muscle, cardiac muscle has a striated appearance due to the arrangement of sarcomeres, which contain actin and myosin filaments.
• Intercalated discs: These specialized structures connect adjacent cardiac muscle cells, allowing for efficient electrical and mechanical communication between cells.
• Branched structure: Cardiac muscle cells are branched, which aids in the efficient propagation of electrical signals and the synchronization of contractions.
• Long refractory period: The refractory period of cardiac muscle is longer than that of skeletal muscle, which helps prevent the occurrence of sustained contractions (tetany) and allows the heart chambers to refill with blood between contractions.

2. State the two functional populations of cardiac cells:

There are two main functional populations of cardiac cells:

• Contractile cells: These cells make up the majority of the cardiac muscle tissue and are responsible for the mechanical contractions of the heart. They are activated by electrical impulses generated by pacemaker cells.
• Pacemaker (autorhythmic) cells: These specialized cells are responsible for initiating and propagating electrical impulses throughout the heart. They establish the heart’s rhythmic contractions and are primarily located in the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, and Purkinje fibers.

3. Describe the ionic basis of action potentials of SA node and AV node, atrial and ventricular myocytes:

• SA and AV nodes: The action potentials of pacemaker cells in the SA and AV nodes are characterized by slow diastolic depolarization, which is primarily due to the opening of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. The influx of sodium and calcium ions through these channels results in a slow depolarization until the threshold is reached. At the threshold, voltage-gated calcium channels open, allowing calcium influx, which further depolarizes the cell. Finally, potassium channels open, leading to potassium efflux and repolarization of the membrane potential.

• Atrial and ventricular myocytes: The action potentials of contractile cardiac myocytes have five distinct phases:
  • Phase 0: Rapid depolarization occurs due to the opening of voltage-gated sodium channels, resulting in a rapid influx of sodium ions.
  • Phase 1: Early repolarization occurs as sodium channels close and transient outward potassium channels open, leading to a brief efflux of potassium ions.
  • Phase 2: The plateau phase is characterized by the balance between the influx of calcium ions through L-type calcium channels and the efflux of potassium ions through slow delayed rectifier potassium channels.
  • Phase 3: Late repolarization occurs as calcium channels close and slow delayed rectifier potassium channels continue to conduct potassium ions out of the cell, leading to membrane repolarization.
  • Phase 4: The resting membrane potential is maintained by the continuous activity of ion pumps and exchangers, primarily the Na+/K+ ATPase pump and the Na+/Ca2+ exchanger.

4. Learning Outcome:

• Define excitation-contraction coupling.
• Outline the process of excitation-contraction coupling in cardiac muscle.

Excitation-contraction coupling refers to the process by which electrical signals are converted to mechanical force in muscle cells. In cardiac muscle, this process begins with the depolarization of the cell membrane, which leads to the opening of voltage-gated calcium channels. Calcium ions then enter the cell and bind to the regulatory protein troponin, which causes a conformational change in the thin filament, allowing myosin to bind to actin and generate force.

5. Learning Outcome:

• Describe the conducting system of the heart.
• Explain the following: i. Rapid cell to cell conduction ii. The hierarchy of pacemakers iii. Significance of AV delay iv. Significance of AV bundle and define “sinus rhythm.”

The conducting system of the heart consists of specialized cardiac muscle cells that generate and propagate electrical impulses throughout the heart. Rapid cell to cell conduction is achieved through gap junctions, which allow electrical signals to pass quickly from one cell to another. The hierarchy of pacemakers refers to the fact that the sinoatrial (SA) node, located in the right atrium, is the primary pacemaker of the heart, but other pacemaker cells can take over if the SA node fails. The significance of AV delay is that it allows time for the atria to contract and fill the ventricles with blood before the ventricles contract. The AV bundle, also known as the bundle of His, is a specialized bundle of conducting fibers that transmits the electrical signal from the atria to the ventricles. “Sinus rhythm” refers to the normal rhythm of the heart, which is generated by the SA node.

6. Learning Outcome:

• Trace the spread of cardiac excitation.

Cardiac excitation begins with the depolarization of the SA node, which spreads through the atria, causing them to contract. The electrical signal then passes through the AV node, which delays the signal to allow time for the ventricles to fill with blood. From the AV node, the signal travels down the bundle of His and its branches, which rapidly conduct the signal to the ventricles, causing them to contract. The signal then spreads throughout the ventricles via Purkinje fibers, which rapidly transmit the signal to the individual cardiac muscle cells, causing them to contract in a coordinated manner.

After the cardiac excitation spreads through the Purkinje fibers, it triggers the contraction of individual cardiac muscle cells, which leads to the mechanical pumping of blood out of the heart. The spread of cardiac excitation is a precisely timed and coordinated process that ensures effective blood flow through the heart and the body. Any disruption to this process, such as damage to the conducting system or abnormal rhythms, can lead to impaired cardiac function and potentially life-threatening complications. Therefore, the accurate tracing and interpretation of the spread of cardiac excitation is essential for the diagnosis and management of cardiovascular diseases.