Cardiac physiology 2

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Questions and Answers

What occurs during the isovolumetric relaxation phase of the cardiac cycle?

All heart valves are closed and the ventricles relax. (correct)

The heart muscle contracts to eject blood.

The ventricles are filled with blood.

The atria contract to push blood into the ventricles.

Which ion is primarily responsible for depolarization during the cardiac action potential?

Sodium (Na⁺) (correct)

Chloride (Cl⁻)

Calcium (Ca²⁺)

Potassium (K⁺)

Which statement best describes the sinoatrial (SA) node in the cardiac conduction system?

It controls the relaxation phase of the heart.

It is less active than the atrioventricular (AV) node.

It is responsible for the contraction of the ventricles.

It generates the action potential that drives the heartbeat. (correct)

What is the key role of the cardiac conduction system?

To propagate the cardiac action potential and control heart rhythm. Signup and view all the answers

What occurs during the ventricular action potential's plateau phase?

Balanced influx of calcium and efflux of potassium. Signup and view all the answers

What occurs during Phase 0 of depolarization?

L-type Ca2+ channels open after threshold is reached. Signup and view all the answers

What membrane potential represents the threshold for the L-type Ca2+ channels to open?

-40 mV Signup and view all the answers

What does the decay of pacemaker potential indicate?

The potential will eventually reach the threshold. Signup and view all the answers

Which ion's channels open as the pacemaker potential decays to reach threshold?

Ca2+ Signup and view all the answers

What happens to the membrane potential during the depolarization phase?

It shifts from -40 mV to a more positive value. Signup and view all the answers

What causes the short repolarization during the action potential?

K+ efflux leading to loss of positive charge Signup and view all the answers

Which statement about fast VG K+ channels is correct?

They quickly inactivate after opening. Signup and view all the answers

What is the primary role of VG Na+ channels during the action potential?

To allow Na+ influx, leading to depolarization. Signup and view all the answers

What is observed in phase 2 of the action potential?

Plateau due to a balance of K+ and Ca2+ influx. Signup and view all the answers

At what membrane potential are fast VG K+ channels primarily involved in repolarization?

-40 mV Signup and view all the answers

During which phase of the action potential do fast VG Na+ channels inactivate?

Shortly after reaching +20 mV. Signup and view all the answers

Which ions are primarily responsible for the plateau phase of the action potential?

A balance of Na+ and Ca2+ influx. Signup and view all the answers

What occurs during the inactivation of VG Na+ channels?

Membrane potential becomes more negative. Signup and view all the answers

What is the primary function of the cardiac conduction system?

To deliver electrical excitation throughout the heart Signup and view all the answers

Which statement accurately describes the SA nodal myocytes?

They exhibit rhythmical, self-generated action potentials Signup and view all the answers

What is the intrinsic rate set by the pacemaker cells?

60-100 bpm Signup and view all the answers

Which phase describes the pacemaker potential in the SA nodal action potential?

Phase 4 - Pacemaker potential Signup and view all the answers

What occurs during Phase 0 of the SA nodal action potential?

Membrane depolarisation occurs Signup and view all the answers

Which of the following factors can influence the intrinsic heart rate set by the pacemaker?

Autonomic nervous system activity Signup and view all the answers

How do pacemaker cells ensure a coordinated contraction of the heart muscle?

By generating electrical excitation followed by muscle contraction Signup and view all the answers

Which phase involves the repolarisation of the membrane potential in the SA nodal action potential?

Phase 3 - Repolarisation Signup and view all the answers

What initiates the depolarization phase in the action potential?

Na+ influx through VG Na+ channels Signup and view all the answers

What change occurs to intracellular charge during depolarization?

The intracellular charge becomes positive Signup and view all the answers

Which of the following channels is primarily responsible for repolarization after depolarization occurs?

VG K+ channels Signup and view all the answers

At what voltage level is the membrane most depolarized during the action potential?

+20 mV Signup and view all the answers

What happens to the electrochemical gradient for Na+ during the depolarization phase?

It increases driving force for Na+ influx Signup and view all the answers

What does the term 'threshold' refer to in the context of action potentials?

The minimum potential needed to open Na+ channels Signup and view all the answers

In which phase is 'Initial repolarization' observed?

Immediately after depolarization Signup and view all the answers

What ion primarily causes the rapid depolarization during an action potential?

Na+ Signup and view all the answers

Which condition describes the membrane potential at -80 mV?

Hyperpolarization Signup and view all the answers

What role do L-type Ca2+ channels play in the action potential?

They support calcium influx for contraction Signup and view all the answers

What initiates repolarization in the action potential process?

K+ efflux along the electrochemical gradient Signup and view all the answers

What is the effect of K+ channels remaining open during the action potential?

It results in hyperpolarization Signup and view all the answers

Which channel is primarily responsible for the initial phase of depolarization?

L-type Ca2+ channel Signup and view all the answers

Which channel is activated during hyperpolarization of the membrane potential?

HCN channel Signup and view all the answers

During which phase are L-type Ca2+ channels inactivated?

During repolarization Signup and view all the answers

What is the role of Na+ ions during the action potential?

They cause depolarization by entering the cell. Signup and view all the answers

What is the primary effect of K+ efflux in the context of action potentials?

It contributes to the repolarization phase. Signup and view all the answers

Which channel's activation is crucial for the transition from hyperpolarization back to resting potential?

HCN channel Signup and view all the answers

What happens to the membrane potential when VG K+ channels are delayed in closing?

K+ efflux continues, leading to prolonged hyperpolarization. Signup and view all the answers

What is the significance of T-type Ca2+ channels during the action potential?

They play a role in the early depolarization phase. Signup and view all the answers

Study Notes

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.

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