Cardiac Electrophysiology and Function Flashcards

Vocabulary flashcards covering cardiac cell classifications, action potential phases, pacemaker dynamics, refractory periods, autonomic modulation, and electrocardiogram components.

Contractile cells

Working cells of the heart constituting the majority of atrial and ventricular tissues, responsible for generating contraction, force, and pressure.

Conduction cells

Specialized muscle cells in the SA node, atrial internodal tracts, AV node, bundle of His, and Purkinje system that rapidly distribute action potentials throughout the myocardium.

Normal sinus rhythm

The normal pattern of cardiac electrical activation where the action potential originates in the SA node, impulses occur regularly at $60\text{--}100\,\text{impulses/min}$, and activation follows the correct sequence and timing.

Threshold potential

The potential difference at which net inward current exceeds net outward current, initiating a self-sustaining depolarization that leads to the action potential upstroke.

Phase 0 of ventricular action potential

The rapid upstroke phase caused by a transient increase in $Na^+$ conductance ($g_{\text{Na}}$) and inward $Na^+$ current ($I_{\text{Na}}$), driving the membrane potential toward approx $+20\,\text{mV}$.

Upstroke velocity ($dV/dt$)

The rate of change of membrane potential with respect to time during Phase 0, expressed in $\text{V/s}$, which reaches its maximum when the resting membrane potential is most hyperpolarized.

Phase 1 of ventricular action potential

The brief initial repolarization phase following the upstroke, resulting from the closure of $Na^+$ channel inactivation gates and an outward $K^+$ current.

Phase 2 of ventricular action potential

The sustained plateau phase lasting $150\text{--}200\,\text{ms}$, during which inward $Ca^{2+}$ current through L-type $Ca^{2+}$ channels is balanced by an outward $K^+$ current.

Phase 3 of ventricular action potential

The rapid repolarization phase caused by a decrease in $Ca^{2+}$ conductance ($g_{\text{Ca}}$) and a substantial increase in $K^+$ conductance ($g_{\text{K}}$), driving the membrane potential back to resting level.

Phase 4 of ventricular action potential

The resting membrane potential period (electrical diastole) stable at approx $-85\,\text{mV}$, maintained primarily by high resting $K^+$ conductance ($g_{\text{K1}}$) generating outward $I_{\text{K1}}$, balanced by small inward $Na^+$ and $Ca^{2+}$ currents.

Automaticity

The intrinsic ability of certain cardiac cells, such as those in the sinoatrial (SA) node, to spontaneously generate action potentials without neural input.

$I_{\text{f}}$ current

The slow inward $Na^+$ current activated by repolarization during Phase 4 in SA node cells, responsible for spontaneous pacemaker depolarization.

Latent pacemakers

Non-SA node conduction tissues (AV node, bundle of His, Purkinje fibers) capable of spontaneous Phase 4 depolarization that are normally suppressed by the faster SA node.

Overdrive suppression

The physiological mechanism where the pacemaker with the fastest rate of Phase 4 depolarization (normally the SA node) suppresses the spontaneous firing of all latent pacemakers.

Ectopic pacemaker

A latent pacemaker that assumes control of cardiac rhythm when the SA node firing rate decreases, SA node function ceases, latent intrinsic rate exceeds the SA node, or conduction pathways are blocked.

AV node delay

The slow conduction velocity ($0.01\text{--}0.05\,\text{m/s}$) through the AV node taking approximately $100\,\text{ms}$, ensuring adequate time for the ventricles to fill with blood before contraction.

Excitability

The capacity of myocardial cells to generate an action potential in response to an inward depolarizing current.

Absolute Refractory Period (ARP)

The period during the action potential when no stimulus, regardless of magnitude, can elicit a second action potential because most $Na^+$ channels are inactivated.

Effective Refractory Period (ERP)

A time span slightly longer than the ARP during which a stimulus cannot elicit a conducted or propagated action potential due to insufficient inward current.

Relative Refractory Period (RRP)

The phase following the ARP during which an action potential can be generated, but requires a larger-than-normal depolarizing stimulus.

Supranormal Period (SNP)

The window between $-70\,\text{mV}$ and full repolarization at $-85\,\text{mV}$ where myocardial cells are more excitable than normal because $Na^+$ channels have recovered and the membrane is closer to threshold.

Chronotropic effect

An autonomic nervous system effect that alters heart rate, where sympathetic stimulation increases rate and parasympathetic stimulation decreases rate.

Dromotropic effect

An autonomic nervous system effect that modifies conduction velocity, most notably across the AV node.

P wave

The electrocardiogram waveform representing atrial depolarization.

PR interval

The ECG interval from the onset of atrial depolarization to the onset of ventricular depolarization (normally $160\,\text{ms}$), containing the PR segment which represents AV node conduction.

QRS complex

The collection of waves (Q, R, and S) on an ECG representing total ventricular depolarization.

T wave

The ECG waveform representing ventricular repolarization.

QT interval

The ECG interval extending from the start of the QRS complex to the end of the T wave, representing total ventricular depolarization and repolarization duration.

ST segment

The isoelectric portion of the ECG between the QRS complex and T wave corresponding to the plateau phase (Phase 2) of the ventricular action potential.

Cycle length

The time interval between consecutive R waves (R-R interval) on an ECG, inversely related to heart rate ($\text{Heart Rate} = \frac{1}{\text{Cycle length}}$).