Cardiac Muscle and Electrical Activity (Section 19.2)

Vocabulary flashcards covering the structure of cardiac muscle, its conducting system, ion movements, ECG components, and related clinical concepts from section 19.2.

autorhythmicity

The heart’s ability to spontaneously generate electrical impulses that trigger contraction and spread from cell to cell; SA node is the primary pacemaker and rate is modulated by nerves and hormones.

myocardial contractile cells

The bulk (~99%) of atrial and ventricular cells responsible for contraction and pumping blood.

myocardial conducting cells

Specialized cardiac cells (~1% of cells) that initiate and propagate action potentials; form the conduction system (e.g., SA node, AV node, Purkinje fibers).

intercalated discs

Junctions between cardiac cells containing desmosomes and gap junctions that enable synchronized contraction.

desmosomes

Adhesive protein structures in intercalated discs that mechanically bind adjacent cardiac cells.

gap junctions

Channels in intercalated discs that allow ions to pass between cells, enabling electrical coupling.

T tubules (transverse tubules)

Membrane invaginations that transmit impulses into the cell interior; in cardiac muscle they are at Z discs and are fewer than in skeletal muscle.

sarcoplasmic reticulum (SR)

Stores relatively little Ca2+ in cardiac cells; most Ca2+ for contraction comes from outside the cell.

sarcomere

The contractile unit of muscle fibers; organized along the length of cardiomyocytes, similar to skeletal muscle.

cardiomyocytes

Cardiac muscle cells that form the heart muscle and include both contractile and conducting types.

Bachmann’s bundle

Interatrial pathway that conducts the impulse directly from the right atrium to the left atrium.

Sinoatrial (SA) node

The heart’s natural pacemaker located in the upper right atrium; highest inherent depolarization rate initiating the sinus rhythm.

Atrioventricular (AV) node

Second cluster of conducting cells in the inferior right atrium; introduces a critical delay (~100 ms) to allow atrial contraction before ventricular stimulation.

Atrioventricular bundle (Bundle of His)

Conduction pathway arising from the AV node that travels through the interventricular septum to the bundle branches.

bundle branches (left and right)

Conduction paths from the AV bundle that travel to the ventricles; left bundle is larger and supplies the left ventricle.

Purkinje fibers

Fast-conducting fibers that spread the impulse through the ventricles, causing contraction from apex to base.

moderator band

Structure in the right ventricle linking part of the Purkinje network to the right papillary muscle to synchronize timing.

internodal pathways

Three bundles (anterior, middle, posterior) connecting the SA node to the AV node; ~50 ms transit time.

prepotential (pacemaker potential)

The gradual depolarization in cardiac conducting cells due to slow Na+ influx; there is no stable resting potential.

Ca2+ entry in SA node firing

Ca2+ channels open after prepotential, causing rapid depolarization to ~+15 mV, then K+ efflux repolarizes.

Membrane potential of SA node

Lacks a true resting potential; gradual depolarization toward threshold triggers spontaneous firing.

action potential in cardiac conductive cells

Slow Na+ influx to threshold, Ca2+ influx to ~+15 mV, Ca2+ channels close, K+ efflux repolarizes.

action potential in cardiac contractile cells

Rapid Na+ influx to ~+30 mV, plateau due to Ca2+ influx with limited K+ efflux, followed by repolarization.

absolute refractory period (cardiac)

About 200 ms when no new action potential can be initiated to prevent tetany.

relative refractory period (cardiac)

Approximately 50 ms after the absolute period, during which a strong stimulus can trigger a response.

calcium’s role in contraction

Ca2+ contributes to the plateau phase and enables cross-bridge cycling via troponin-tropomyosin.

troponin-tropomyosin complex

Regulatory proteins on actin that regulate myosin cross-bridge formation and contraction.

electrocardiogram (ECG/EKG)

Recording of the heart’s electrical activity using leads; 3, 5, or 12 leads provide electrical information about the cardiac cycle.

P wave

Atrial depolarization; the atria begin contracting about 25 ms after the start of the P wave.

QRS complex

Ventricular depolarization; larger due to the greater mass of ventricular muscle.

T wave

Ventricular repolarization; atrial repolarization occurs during the QRS and is usually masked.

PR interval

Time from the beginning of the P wave to the beginning of the QRS complex; reflects AV node conduction delay.

PR segment

Time from the end of the P wave to the start of the QRS complex.

QT interval

From the start of the QRS complex to the end of the T wave; represents ventricular depolarization and repolarization.

ST segment

Segment between the end of the S wave and the start of the T wave; used in assessing myocardial injury.

12-lead ECG

ECG using 10 skin electrodes to yield 12 different leads for comprehensive electrical view of the heart.

Holter monitor

Portable device that continuously records ECG activity, typically for 24 hours during normal activity.

ectopic focus/pacemaker

Area outside the SA node that initiates impulses, potentially causing premature contractions.

defibrillation

External electric shock used to terminate fibrillation and restore a normal sinus rhythm.

artificial pacemaker

Implanted device that delivers electrical impulses to stimulate heart contractions when needed; programmable and may include a defibrillator.

heart block (blocks in conduction pathway)

Interruption of the normal conduction pathway, including SA nodal, AV nodal, infra-Hisian, and bundle branch blocks.

first-degree block

Delayed conduction between the SA and AV nodes, seen as a prolonged PR interval.

second-degree block

Some P waves are not followed by QRS complexes, indicating intermittent conduction failure.

third-degree (complete) block

No correlation between atrial (P) and ventricular (QRS) activity; the AV node or lower takes over pacing at 40–60 bpm.

bradycardia

Abnormally slow heart rate, typically below 50 beats per minute in adults.

cardiac metabolism

Normally aerobic metabolism; heart uses fatty acids and glucose with oxygen supplied by blood; myoglobin stores oxygen.