Myocardial Excitation-Contraction Coupling and Cardiac Cycle

A comprehensive set of flashcards covering key concepts from myocardial excitation-contraction coupling, cardiac cycles, and the physiological processes of the heart.

What are the steps involved in myocardial excitation-contraction coupling?

1) Action potential enters from adjacent cell, opening voltage-gated Ca2+ channels. 2) Ca2+ induces Ca2+ release through RyR receptors in the SR, creating a Ca2+ signal. 3) Ca2+ binds to troponin for contraction, unbinds for relaxation. 4) Ca2+ is pumped back into the SR. 5) Ca2+ is exchanged with Na+ via NCX antiporter. 6) Na+ gradient is maintained by Na+K+ATPase.

What is phase 0 of the action potential in myocardial contractile cells?

Depolarization due to Na+ inflow via gap junctions.

What happens during phase 1 of the action potential in myocardial contractile cells?

Initial repolarization occurs as Na+ channels close.

What characterizes phase 2 of the action potential in myocardial contractile cells?

Plateau phase; long action potential due to Ca2+ inflow, sustaining refractory period and preventing tetanus.

What happens during phase 3 of the action potential in contractile cells?

Rapid repolarization occurs due to K+ outflow.

What is phase 4 of action potentials in myocardial contractile cells?

Resting membrane potential (RMP) of -90 mV.

What is depolarization initially due to in myocardial autorhythmic cells?

Opening of If channels leading to Na+ inflow.

What occurs as voltage-gated Ca2+ channels open in myocardial autorhythmic cells?

Ca2+ inflow occurs, closing If channels, at threshold a second type of voltage-gated Ca2+ channels opens, causing steep depolarization.

What happens at the peak of an action potential in myocardial autorhythmic cells?

Ca2+ channels close, slow K+ channels open, decreasing Ca2+ inflow and increasing K+ outflow.

What does a long refractory period in cardiac muscle prevent?

Tetanus.

Describe the conduction of electrical signals through the heart.

Depolarization travels from SA node to internodal pathways to AV node, then to AV bundle, bundle branches, and finally to Purkinje fibers.

What does the P wave of an ECG represent?

Atrial depolarization.

What is represented by the P-Q or P-R segment?

Conduction through AV node and AV bundle, during which the atria contract.

What does the R wave on an ECG indicate?

Atrial repolarization.

What does the S wave represent?

Ventricular contraction.

What does the T wave on an ECG indicate?

Ventricular repolarization.

What is diastole?

The phase when the heart relaxes.

What is systole?

The phase when the heart contracts.

What does EDV stand for and what does it indicate?

End diastolic volume; the amount of blood in the heart after relaxation.

What does ESV stand for and what does it indicate?

End systolic volume; the amount of blood in the heart after contraction.

What is SV?

Stroke volume; the amount of blood ejected from the ventricle with one contraction of the heart.

What is the formula for stroke volume?

SV = EDV - ESV.

When do you hear the 1st heart sound?

After atrial systole, when AV valves close, producing the 'lub' sound.

When do you hear the 2nd heart sound?

After ventricular ejection, when semilunar valves shut, producing the 'dub' sound.

Describe the heart at rest during atrial and ventricular diastole.

Atria fill with blood from veins, AV valves open, and ventricles fill.

What marks the completion of ventricular filling?

Atrial systole, which sets end diastolic volume (EDV).

What happens during isovolumetric contraction?

No blood enters or exits; both AV and semilunar valves are closed and pressure in the left ventricle increases due to systole.

What occurs during ventricular ejection?

Semilunar valves open and blood is ejected into arteries, causing a decrease in volume in the left ventricle resulting in end systolic volume (ESV).

What occurs during isovolumetric relaxation?

Pressure in the left ventricle decreases but the volume remains unchanged; semilunar valves close and the second heart sound is heard, with no blood entering or exiting.

When do the AV valves open again?

When atrial pressure exceeds ventricular pressure.

What is the average stroke volume (SV)?

Approximately 70 mL.

What is cardiac output (CO)?

The volume of blood pumped by one ventricle in a given period, averaging 5 L/min.

What is the formula for cardiac output?

CO = Heart Rate x Stroke Volume.

How does the parasympathetic control affect the heart rate?

It decreases heart rate by increasing K+ permeability and decreasing Ca2+ permeability in autorhythmic cells.

What is tonic control normally dominated by?

Parasympathetic activity.

How does sympathetic control of the heart affect heart rate?

It increases heart rate by increasing Na+ and Ca2+ permeability through B1 adrenergic receptors on autorhythmic cells.

What does parasympathetic stimulation do to the membrane potential of the autorhythmic cell?

It hyperpolarizes the membrane potential and slows depolarization, thereby slowing the heart rate.

What effect does sympathetic stimulation and epinephrine have on the autorhythmic cell?

They depolarize the autorhythmic cell and speed up the pacemaker potential, increasing the heart rate.

What are length-tension relationships?

They are determined by the volume of blood at the beginning of contraction.

What is the degree of stretch in length-tension relationships termed?

Preload.

What is the Frank-Starling law of the heart?

Stroke volume is proportional to end diastolic volume (EDV); the heart pumps all the blood that is returned to it, although stroke volume is less than EDV.

What is EDV determined by?

Venous return.

What affects venous return?

Skeletal muscle pump, respiratory pump, and sympathetic innervation of veins.