Cardiac Muscles, Conducting System, and Electrocardiogram

Flashcards covering the physiology of cardiac muscles, the electrical conduction system of the heart, and the principles/components of the electrocardiogram (ECG).

Myogenic heart

A heart where the cardiac impulse is independent of the nervous system and relies on periodic self-generated impulses to trigger contraction.

Autorhythmicity

The ability of certain heart cells to generate periodic self-generated impulses to trigger contraction.

Contractile cells

Cardiomyocytes making up $99\%$ of the heart that act as "pumpers" by generating forceful and coordinated contractions to pump blood.

Autorhythmic cells

Specialized cardiomyocytes that act as the heart's "electric generator" or pacemakers, initiating and conducting electrical impulses to establish heart rhythm.

Cardiac syncytium

The property of cardiac muscle acting as one unit, where signals spread easily between cells via intercalated discs to allow the heart to contract together.

Intercalated disc

The junction between cardiac cells providing mechanical attachment and electrical connection, consisting of desmosomes and gap junctions.

Desmosome

A structure within the intercalated disc that provides mechanical strength and support to keep cardiac cells together during contraction.

Gap junction

A component of the intercalated disc that allows the rapid movement of ions or current between cells, enabling synchronized contraction.

Funny ($I_f$) channel

Voltage-gated channels that open with hyperpolarization at the end of repolarization, causing slow $Na^{+}$ entry known as diastolic depolarization.

T-type calcium channel

Transient voltage-gated calcium channels that open briefly during early depolarization to help the membrane reach threshold.

L-type calcium channel

Long-lasting voltage-gated calcium channels that open at threshold (approximately $-40\,mV$) to allow a large influx of $Ca^{2+}$, producing rapid depolarization.

Phase 4 (Diastolic Depolarization)

The slow spontaneous depolarization of pacemaker cells from approximately $-60\,mV$ to the threshold of $-40\,mV$.

AV nodal delay

An approximately $0.1\,s$ ($100\,msec$) delay in action potential conduction that allows complete atrial depolarization and contraction before ventricular activation.

Excitation-contraction coupling

The process where electrical depolarization leads to $Ca^{2+}$ entry, triggering cardiac-induced-calcium-release (CICR) from the sarcoplasmic reticulum to initiate contraction.

Refractory period

A prolonged period (about $250\,msec$ in contractile cells) during which sodium channels are inactivated, preventing tetanus and ensuring rhythmic filling and emptying.

Electrocardiogram (ECG)

A recording of the cumulative spread of electrical current throughout depolarization and repolarization of the heart that passes to the skin surface.

P wave

The ECG wave representing atrial depolarization, with a normal duration of approximately $0.08 - 0.11\,s$.

QRS complex

The ECG waveform representing ventricular depolarization, typically lasting between $0.06 - 0.10\,s$.

T wave

The ECG wave representing ventricular repolarization, occurring after the ST segment.

PR interval

The time from the start of atrial depolarization to the start of ventricular depolarization ($0.12 - 0.20\,s$), reflecting AV nodal conduction time.

ST segment

An isoelectric period on the ECG representing the time when the ventricles are fully depolarized, correlating with the plateau phase (Phase 2).

QT interval

The total time for ventricular depolarization and repolarization, normally lasting approximately $0.35 - 0.44\,s$.

RR interval

The time between two consecutive R waves, used to calculate heart rate ($\text{HR} = 60 / \text{RR in seconds}$).

Limb leads

The six ECG leads consisting of bipolar leads (I, II, III) and unipolar augmented leads ($aVR, aVL, aVF$).

Precordial leads

Also known as chest leads ($V_1, V_2, V_3, V_4, V_5, V_6$), they provide different anatomical perspectives like septal, anterior, and lateral views of the heart.