Ch. 17.3-17.4 Heart Electrophysiology & Cardiac Cycle

What do most cells in the myocardium consist of?

Most cells (~99%) are Myocytes.

What is the primary function of myocytes?

They generate action potentials and produce force for heart muscle contraction.

What percentage of myocardial cells are pacemaker cells?

Approximately 1%.

What is the role of pacemaker cells in the heart?

They generate spontaneous, rhythmic action potentials that signal myocyte contraction.

What are intercalated discs and their components?

Intercalated discs join cardiac muscle cells and consist of desmosomes (hold cells together) and gap junctions (allow ions to pass rapidly).

What is the significance of the AV node in cardiac conduction?

The AV node allows the atria to fully depolarize and contract before the ventricles, ensuring efficient filling.

What initiates the slow initial depolarization phase in pacemaker cells?

Voltage-gated HCN channels open in response to membrane hyperpolarization.

What occurs during the rapid depolarization phase of pacemaker cells?

Voltage-gated Ca2+ channels open, allowing Ca2+ to flow into the cell.

What happens during the repolarization phase of pacemaker cells?

Ca2+ channels close and voltage-gated K+ channels open, allowing K+ to flow out.

What is the plateau phase in myocyte action potentials?

Ca2+ channels open while K+ channels remain open, balancing the influx of Ca2+ with the efflux of K+.

What is excitation-contraction coupling in cardiac myocytes?

It is the mechanism by which action potentials trigger contraction in cardiac muscle cells.

What does an electrocardiogram (ECG) measure?

It reflects the electrical activity of all cardiac muscle cells over time.

What does the P wave in an ECG represent?

The depolarization of the left and right atria.

What does the QRS complex in an ECG indicate?

The depolarization of the left and right ventricles.

What does the T wave in an ECG reflect?

The repolarization of the left and right ventricles.

What is the cardiac cycle?

It describes the mechanical events of the heart, consisting of one contraction (systole) and one relaxation (diastole) period for each chamber.

What occurs during the ventricular filling phase of the cardiac cycle?

Blood flows from the atria into the ventricles, with AV valves open and semilunar valves closed.

What defines the isovolumetric contraction phase?

All valves are closed, and the pressure in the ventricles is not yet high enough to open the semilunar valves.

What happens during the ventricular ejection phase?

Blood is ejected from the ventricles as the atrial and pulmonary valves open.

What characterizes the isovolumetric relaxation phase?

All valves are closed, and the volume in the ventricles remains constant as they begin to relax.

What is end-diastolic volume?

The total amount of blood in the ventricles at the end of ventricular diastole.

What is end-systolic volume?

The amount of blood remaining in the ventricle at the end of ventricular ejection.

What is the role of gap junctions in cardiac myocytes?

They allow electrical signals to spread quickly between myocytes.

How does the cardiac conduction system affect heart function?

It focuses the electrical signal along a path and allows for delays, ensuring synchronized contraction.

What is the importance of the orientation of myocytes in the myocardium?

It facilitates the coordinated contraction and relaxation necessary for effective blood flow.

What is the function of mitochondria in cardiac myocytes?

They provide the high energy demands required for continuous contraction.

What is the significance of the plateau phase in cardiac action potentials?

It prolongs the action potential duration, allowing time for the heart to fill with blood before the next contraction.