FAQ on Cardiac Conduction

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Properties of Cardiac Muscle:

• Shorter in length and less circular in transverse section.

• Exhibit branching.

• Ends of cardiac muscle cells connect with one another through intercalated discs, which contain desmosomes and gap junctions which allow muscle action potentials to conduct throughout muscle fibers.

• Mitochondria are numerous and larger.

• Smaller SR and hence smaller supply of calcium ions.

• Cardiac muscle fibers are autorhythmic in nature and beat automatically.

Describe the location of the Sinoatrial and Atrioventricular nodes:

The SA node is located in the right atrial wall just inferior to the opening of the superior vena cava. The AV node is located between the septum between the two atria, just inferior to the opening of the coronary sinus.

Describe normal conduction sequence of propagation through the heart:

• Excitation begins in the SA node, the cells depolarize causing a pacemaker potential, which causes the action potential.

• Action potential reaches the AV node.

• It then travels from the atria to the ventricles through the AV bundle (Bundle of His).

• It travels to the right and left of the lower heart.

• When it reaches the Purkinje fibers, it travels throughout the myocardium.

What is the normal “pacemaker” of the heart? Why is it called the pacemaker?

SA node is the pacemaker of the heart. It is called a pacemaker because it is here that cells first begin to depolarize and this eventually causes all the muscles of the heart to work in unison.

Describe the speed of the action potential as it travels through the heart’s conduction system. Why is this variability of speed important?

The speed of propagation is extremely fast in the ventricular region, but before that (from SA to AV nodes), it is very slow. This variability of speed is important to make up for the variability of spontaneous depolarization. Even though action potentials travel slower through the SA and AV nodes, the rate of depolarization in this region is up to 100 beats per minute, much faster than the rate of depolarization in the ventricular region. This ensures that all muscle fibers depolarize in unison being led by the SA node. In other words, variation in conduction speeds through the heart ensures a coordinated stimulation, which allows the muscle in the atria to contract first, followed by contraction of the muscle in ventricle , thus making the heart an effective pump.

How is the impulse transmitted from conduction cells to contractile cells?

This is done through gap junctions that connect the cells.

What is the normal resting membrane potential of the cardiac cell? How this is potential maintained? How does this compare with other cells in the body?

The normal resting potential of cardiac cells is -90mV. This is maintained by help of voltage-gated fast sodium channels, voltage-gated slow calcium channels, and voltage-gated potassium channels. Comparison to skeletal muscle: Relatively short refractory period in skeletal muscle cells compared to heart cells. Thus, tetanus (maintained contraction) cannot occur in cardiac cells.

What occurs in the membrane during rapid depolarization of a cardiac muscle cell?

The voltage-gated fast sodium channels open resulting in the inflow of positively charged sodium, causing the cytosol charge to go up, causing rapid depolarization.

What occurs in the membrane during the plateau phase of depolarization?

• Voltage-gated slow calcium channels are open.

• Calcium ions are slowly moving in and maintaining the depolarization phase.

• Some potassium channels are close.

How does the duration of depolarization differ between a cardiac and a skeletal muscle cell?

Skeletal muscle cell has a shorter refractory period compared to a cardiac muscle cell.

What occurs in the membrane during repolarization?

• Voltage-gated potassium channels open causing potassium outflow, resulting in decrease in cytosol charge.

• Calcium channels also close.

Explain what is meant by the refractory period. What is its utility in cardiac muscle?

Refractory period in cells is the time it takes to begin the second contraction cells, after repolarization has occurred. Cardiac cells have a particularly long refractory period so that tetanus (maintained contraction) cannot occur in cardiac cells.

How is depolarization different from contraction?

Depolarization is making cytosol charge more positive by the inflow of calcium and sodium ions. Contraction is caused when calcium ions binding to troponin, which moves the troponin-tropomyosin complex showing the myosin binding sites on actin.

How does the mechanism of myofibril contraction differ between cardiac and skeletal muscle?

Cardiac cells have intercalated discs that help he action potential to travel from one cell to neighboring cells. These discs contain desmosomes and gap junctions. In response, the cardiac cells remains contracted for a longer time compared to a skeletal muscle cell.

What is an "ectopic beat"? Where does it come from?

An ectopic beat is an extra heart beat that follows a normal heart beat. This beat is normal and many humans have them. They occur when the impulse is generated somewhere outside of he natural pacemaker of the heart.

Additional Reading:

Basic Cardiology

1. Electrical Activity of the Heart
2. Heart Muscle Mechanics
3. Heart Sounds and Murmurs
4. Additional FAQ on Heart Sounds and Murmurs
5. Cardiac Conduction Diagram
6. Blood Pressures in Cardiac Chambers
7. What is Pulsus Paradoxus?
8. FAQ on Heart Murmurs and Mechanisms of Turbulent Flow
9. Notes on Fetal Circulation
10. FAQ on Ischemic Myocardial Infarction
11. FAQ on Electrocardiograms / ECG / EKG
12. FAQ on Cardiac Conduction
13. The Heart as a Pump, the Cardiac cycle and Cardiac Output
14. What are the most common causes of aortic stenosis?
15. What is Pulseless Electrical Activity?
16. Causes and Complications of Arteriovenous Fistulas
17. CHADS2 Score for Atrial Fibrillation Stroke Risk
18. How to Reduce Blood Pressure without Medications?
19. Types of Shock
20. Locations of Heart Murmurs on Chest Wall
21. Types of Heart Blocks

Electrocardiogram (EKG/ECG) Topics

1. EKG Chest Leads
2. EKG Limb Leads
3. Quick 12-Lead ECG/EKG Format

Cardiology Videos

1. Video of Cardiology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Thorax Anatomy
2. Vascular Disorders
3. Heart Disorders
4. Histology of the Cardiovascular System
5. Jugular Venous Distention Workup
6. ER Chest Pain Workup
7. Cardiac Examination for Internal Medicine
8. FAQ on Blood Pressure
9. FAQ on principles of fluid and flow dynamics of Blood

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