Cardiac physiology 2

Cardiac Physiology II: Electrical Activity of the Heart

• Topic: Cardiac electrical activity of the heart
• Learning Outcomes: Describe phases of ventricular and sinoatrial nodal action potentials, explain how ventricular action potentials trigger cardiac myocyte contraction, describe how the heart generates its own action potential, explain propagation of the cardiac action potential, and how it's controlled by the cardiac conduction system.
• Last Lecture Recap: Covered the phases of the cardiac cycle (rapid filling, diastole, ejection and isovolumetric contraction and relaxation) and how the heart coordinates these phases for rhythmic contraction.

Basic Concepts of Cell Excitability

• Polarization: The difference in charge across the cell membrane. A polarized cell (at rest) has a more negative intracellular environment.
• Depolarization: Intracellular environment becomes less negative (more positive).
• Repolarization: The return of the membrane back to its original polarization.
• Voltage: The difference in positive charge between the membrane's inner and outer sides. A voltage of -80 mV indicates a lower positive charge inside.
• Current: Flow of charged particles (ions). Influx refers to ions moving into the cell; efflux means ions moving out.

Important Ion Channels and Transporters in the Heart

• Slides depict ion channels and transporters illustrated with diagrams showing their location (extracellular or intracellular) and actions.
• Types of channels/transporters: (e.g., voltage-gated channels, L-type Ca2+ channels, Na+/K+ ATPase, Na+/Ca2+ excitatory)
• Function: Ions move in and out of the cell according their concentration gradients via these channels/transporters, which are critical to maintaining appropriate ionic gradients for proper heart function.

The Big Picture

• Action Potential Sequence: Sinoatrial (SA) node action potential → Cardiac conduction system → Ventricular action potential → Ca2+-induced Ca2+ release → Myocyte contraction.

The Cardiac (Ventricular) Action Potential

• The cardiac action potential is graphed, showing phases 0 to 4, each with specific time periods and voltages or millivolts.

Phase 4 – "Resting Membrane Potential"

• At rest, the voltage-gated potassium (K+) channels are open, allowing K+ to leave the cell. This contributes to the resting membrane potential.

Phase 0 – "Rapid Depolarization"

• Upon reaching threshold, voltage-gated sodium (Na+) channels open, leading to rapid Na+ influx, resulting in depolarization.

Phase 1 – "Initial Repolarization"

• Voltage-gated sodium (Na+) channels quickly inactivate. Fast voltage-gated potassium (K+) channels open triggering a short repolarization period.

Phase 2 – "Plateau"

• Voltage-gated calcium (Ca2+) channels open, resulting in a plateau phase. This phase is important for prolonged contraction.

Phase 3 – "Repolarization"

• Voltage-gated calcium (Ca2+) channels close and voltage-gated potassium (K+) channels remain open, leading to repolarization. The absolute refractory period is also mentioned.

Summary of Ions in Action Potentials

• Different ion channels play various roles in the phases of the cardiac action potential.

Ultrastructure of Cardiac Muscle

• Cardiac muscle shows intercalated discs that connect individual cardiac cells—composed of desmosomes (mechanical attachment) and gap junctions (electrical junctions) for rapid ion passage between adjacent cells.

Cardiac Muscle Forms a "Functional Syncytium"

• The action potential spreads rapidly through the cardiac muscle cells due to the gap junctions, enabling coordinated contraction of the myocardium.

The Big Picture So Far

• Action potentials travel through the cardiac conduction system to the ventricles.

Where Does the Wave of Excitation Begin?

• Excitation begins in the SA (sinoatrial) node, which acts as the heart's pacemaker.

Electrical Events in the Cardiac Cycle: Atrial Depolarization

• The SA node initiates the action potential. This depolarization spreads across both atria, leading to atrial contraction. The action potential then moves to the AV node, where there's a delay. This delay allows ventricles to fill with blood.

Electrical Events in the Cardiac Cycle: Ventricular Depolarization

• The depolarization wave travels from the AV node through the bundle of His and its branches to the Purkinje fibers, eventually leading to ventricular contraction.

Electrical Events in the Cardiac Cycle: Ventricular Repolarization

• Ventricular repolarization happens after contraction and leads to ventricular relaxation, with the wave traveling from the endocardium (inner surface) to the epicardium (outer surface).

Introduction to the Electrocardiogram (ECG)

• This is a pre-work for ECG practical in the coming week.

Further Resources

• Links to websites for videos related to this lecture
• Further readings include textbooks like Guyton & Hall and Herring & Paterson, as well as online resources.