GQ: Heart and Cardiac Cycle

Review on the anatomy of the heart – remember that different anatomical designs of the heart have evolved in animals, which translate to variations in their functions.

The heart varies across animals in chamber structure. Fishes have one atrium and one ventricle, amphibians have two atria and one ventricle, reptiles and birds have two atria with a partially separated ventricle, and mammals have two atria and two fully separated ventricles. These differences affect how efficiently blood is separated and circulated, with mammals having the most efficient system.

What are the similarities and differences in the structure of the cardiomyocytes with that of the skeletal myofibers? What are cardiac syncytia and what function do they serve?

Both cardiac muscle cells and skeletal myofibers are striated. Cardiac cells are branching and arranged in series, while skeletal fibers are long and arranged in parallel. Cardiac cells are connected by intercalated discs with desmosomes and gap junctions, allowing electrical connection, while skeletal fibers are not connected. Cardiac syncytia are functional units formed by interconnected cells through gap junctions, allowing rapid electrical transmission and synchronized contraction.

What are the component structures of the electrical conductance system of the heart?

The system includes the sinoatrial (SA) node, Bachmann’s bundle, atrioventricular (AV) node, bundle of His, left and right bundle branches, and Purkinje fibers.

Why is the sinoatrial node considered as the pacemaker of the heart? What role does the atrioventricular node play? Why is the delay necessary?

The SA node is the pacemaker because it generates action potentials spontaneously. The AV node receives and transmits signals to the ventricles. The delay ensures the atria contract first, allowing complete ventricular filling before ventricular contraction.

How does the branching of the Purkinje fibers relate to ventricular contraction?

The extensive branching distributes electrical signals rapidly throughout the ventricles, allowing coordinated and efficient contraction.

How different are the action potentials in cardiac muscle fibers from those in skeletal muscles?

Cardiac muscle action potentials have a plateau phase and longer duration, about 15 times longer than skeletal muscle. The resting membrane potential is around -80 mV. Skeletal muscle lacks a plateau and has shorter action potentials.

How can cardiac muscle AP have plateau and how are they rhythmic?

The plateau occurs due to slow calcium channel opening that delays repolarization. Rhythmicity results from continuous ion conductance changes that allow regular generation of action potentials.

How is the SA node able to spontaneously generate 80 action potentials per minute? What is the role of funny current? How are pacemaker and contractile APs different?

The SA node generates action potentials due to increased sodium conductance and decreased potassium conductance causing gradual depolarization. Funny currents allow this slow depolarization. Pacemaker cells show gradual depolarization, while contractile cells show rapid depolarization with a plateau phase.

Describe the transmission of action potential in cardiac myocytes and explain the plateau. Why does its duration vary?

Action potentials spread through gap junctions. The plateau occurs due to slow calcium channel opening delaying repolarization. Duration varies because different cardiac cells have different ion channel properties.

Explain how an AP leads to contraction in cardiac myocytes and how it differs from skeletal muscle.

Calcium enters from extracellular fluid through slow channels and triggers contraction, also stimulating calcium release from the sarcoplasmic reticulum. In skeletal muscle, calcium mainly comes from the sarcoplasmic reticulum and there is a latent period, while cardiac muscle has no latent period and depends more on extracellular calcium.

Does cardiac muscle have a refractory period? Can it have frequency summation?

Cardiac muscle has a long refractory period, preventing continuous stimulation. It does not exhibit frequency summation.

Describe the stages of the cardiac cycle.

The cycle includes ventricular diastole (filling), atrial systole (final filling), isovolumetric contraction, ventricular ejection, and isovolumetric relaxation.

Discuss pressure and volume changes during the cardiac cycle.

During diastole, ventricular pressure is low and volume increases. During isovolumetric contraction, pressure rises with constant volume. During ejection, pressure rises and volume decreases. During relaxation, pressure decreases while volume remains constant before filling resumes.

How are EDV, ESV, stroke volume, and ejection fraction related?

Stroke volume = EDV − ESV. Ejection fraction = stroke volume divided by EDV. Example: 80 mL / 130 mL ≈ 60%.

Interpret the ECG and relate P, QRS, and T waves.

P wave represents atrial depolarization, QRS complex represents ventricular depolarization, and T wave represents ventricular repolarization.

How would ECG look in arrhythmia? What can it detect? Can it be used in animals?

Tachycardia shows shorter intervals, bradycardia shows longer intervals, and abnormal waves indicate conduction problems. ECG detects arrhythmias and conduction abnormalities and can also be used in animals.

What is the normal heart sound and how is it produced?

The “lub-dub” sound is produced by valve closures. “Lub” is closure of atrioventricular valves, and “dub” is closure of semilunar valves.

What are heart murmurs and what do they indicate?

Heart murmurs are abnormal sounds caused by turbulent blood flow, indicating valve stenosis or regurgitation.

What is cardiac work output? Distinguish its types.

Cardiac work output is the energy used to pump blood. External work ejects blood, while potential energy is stored energy not converted to flow.

Explain the pressure-volume work diagram.

It shows the relationship between ventricular pressure and volume. Pressure increases with volume, rises at constant volume during contraction, and volume decreases during ejection. The area under the curve represents work done.

What are the three main factors affecting cardiac work output?

Contractility, preload, and afterload.

How does heart rate affect cardiac work output? What regulates it?

Cardiac output equals stroke volume times heart rate. Heart rate increases with sympathetic stimulation, hormones, stress, and exercise, and decreases with parasympathetic activity.

What is the Frank-Starling reflex?

Increased stretching of cardiac muscle leads to stronger contraction, improving cardiac output.

What is the role of the nervous system on cardiac function?

The autonomic nervous system regulates heart rate. Parasympathetic slows it, while sympathetic increases rate and force.

How are hearts of athletes different from non-athletes?

Athletes have larger, more efficient hearts with lower resting heart rate and higher stroke volume. These adaptations improve circulation and may reverse after retirement.

Why is it difficult to find a good model organism to study human heart functions?

Although animals share similar heart functions, differences in structure, chamber organization, and physiology limit direct comparison to humans.