Cardiovascular III: Cardiac Cycle and Electrical Activity

Vocabulary flashcards reviewing electrical activity, conduction systems, action potentials, arrhythmias, and cardiac cycle phases from Cardiovascular Lecture III.

Electrocardiogram (EKG)

A recording of the electrical activity of the heart, where electrical signals cause the heart muscle to contract and relax to create the heartbeat.

P wave

The wave on an EKG that represents atrial depolarization, starting at the SA node and leading to atrial contraction.

QRS complex

The wave complex on an EKG that represents ventricular depolarization, which is followed by ventricular contraction and masks atrial repolarization.

T wave

The wave on an EKG that represents ventricular repolarization, during which the ventricles electrically reset and recover prior to relaxation.

Pacemaker potential

The slow rise in membrane potential toward threshold in autorhythmic cells caused by the slow entry of $\text{Na}^+$ through open HCN channels following repolarization.

HCN channels

Hyperpolarization-activated cyclic nucleotide-gated ("funny") channels in pacemaker cells that open after repolarization, allowing $\text{Na}^+$ to slowly enter the cell.

Pacemaker cells

Autorhythmic cells that generate action potentials on their own without a stable resting membrane potential to set the heart rhythm.

Cardiomyocytes

Contractile cardiac cells that make up $99\%$ of cardiac muscle, possess a stable resting membrane potential around $-90\,\text{mV}$, and exhibit a long $\text{Ca}^{2+}$ plateau during action potentials.

Automaticity

The ability of the heart to generate its own action potentials without requiring the nervous system to start each heartbeat.

Gap junctions

Protein structures connecting cardiac cells that allow electrical current and ions to pass directly from cell to cell without neurotransmitters.

Refractory period (cardiac)

An extended time period overlapping most of cardiac muscle contraction during which a cell cannot easily produce another action potential, preventing tetanus.

Tetanus (cardiac)

A sustained, maximal muscle contraction with no relaxation between contractions, which is prevented in the heart by a long refractory period.

Sinoatrial (SA) node

The natural pacemaker of the heart located in the right atrium, composed of autorhythmic cells that initiate action potentials to depolarize both atria.

Atrioventricular (AV) node

A conduction structure located between the atria and ventricles that briefly delays electrical signals so the atria can finish contracting before the ventricles contract.

AV bundle (Bundle of His)

A conduction pathway that carries action potentials from the AV node into the interventricular septum, dividing into right and left bundle branches toward the apex.

Purkinje fibers

Conduction fibers that extend from the bundle branches throughout the ventricular walls, causing ventricular depolarization from the apex upward.

Bradycardia

An abnormal heart rhythm characterized by a heart rate slower than normal, typically defined in adults as $< 60\,\text{beats/min}$.

Tachycardia

An abnormal heart rhythm characterized by a heart rate faster than normal at rest, typically defined in adults as $> 100\,\text{beats/min}$.

Fibrillation

Very rapid, disorganized electrical activity causing ineffective and uncoordinated contractions, with ventricular fibrillation being immediately life-threatening.

AV block

An arrhythmia characterized by electrical signals from the atria being delayed or blocked from reaching the ventricles through the AV conduction pathway.

Isovolumetric Contraction

The phase of ventricular systole where ventricles begin contracting, raising pressure to close the AV valves ("LUB" / S1) while all valves remain closed and blood volume stays constant.

Ventricular Ejection

The phase of ventricular systole where ventricular pressure exceeds arterial pressure, opening the semilunar valves and pumping blood out into the aorta and pulmonary trunk.

Isovolumetric Relaxation

The phase of ventricular diastole where ventricles relax and pressure drops below arterial pressure, closing the semilunar valves ("DUB" / S2) while all valves stay closed and volume remains constant.

Dicrotic notch

A small bump or change on the aortic pressure graph caused by the closure of the aortic valve.

Ventricular Filling

The phase of ventricular diastole where decreasing ventricular pressure allows atrial pressure to exceed it, opening AV valves so blood flows passively into the ventricles.

Atrial Contraction

The final step of ventricular diastole where the atria contract to push the remaining volume of blood into the ventricles.

Systole

The period of the cardiac cycle when the ventricles contract, comprising Isovolumetric Contraction and Ventricular Ejection.

Diastole

The period of the cardiac cycle when the ventricles relax and fill with blood, comprising Isovolumetric Relaxation, Ventricular Filling, and Atrial Contraction.

Plateau phase

The phase during a cardiomyocyte action potential where $\text{Ca}^{2+}$ influx balances $\text{K}^+$ efflux, maintaining depolarization longer to allow a strong, sustained contraction.