Cardiac Excitation and Conduction System

Vocabulary flashcards based on the lecture covering cardiac excitation, conduction pathways, membrane potentials, autonomic control, and related physiological phenomena.

Sinoatrial (SA) Node

A specialized elliptical strip of auto-excitatory muscle fibers measuring $3\,mm$ wide, $15\,mm$ long, and $1\,mm$ thick, located in the posterolateral upper wall of the right atrium that acts as the heart's natural pacemaker at $70$ to $80\text{ beats per minute}$.

Atrioventricular (AV) Node

A element of the cardiac conduction system that delays the electrical impulse by $0.09\,s$ to allow atrial emptying before ventricular contraction, and functions as an ectopic pacemaker at $40$ to $60\text{ beats per minute}$ if the SA node fails.

Purkinje Fibers

Large specialized conduction fibers with high conduction velocities ($1.5$ to $4\,m/s$) that rapidly distribute action potentials through the ventricles and can act as an intrinsic escape pacemaker at $15$ to $40\text{ beats per minute}$.

Stokes-Adams Syndrome

A condition caused by total AV block where ventricular escape takes $5$ to $20\,s$ to initiate, resulting in brief cerebral hypoperfusion and syncope (fainting).

Internodal Pathways

Specialized atrial conduction tracts that transmit impulses from the SA node to the AV node at $1\,m/s$, taking $0.03\,s$ to reach the AV node.

Carotid Sinus

A dilated area at the bifurcation of the carotid artery containing pressure receptors that, when stimulated by pressure or massage, trigger a vagal response that lowers heart rate and blood pressure.

Calcium Channel Blockers

Pharmacological agents that partially block calcium channels, slowing sinus action potential initiation and decreasing heart rate (e.g., reducing rate from $110\text{ bpm}$ to $90\text{ bpm}$).

Funny Currents ($I_f$)

Inward sodium currents through specialized channels in SA node cells that cause a slow self-depolarization drift up to the threshold of $-40\,mV$.

Ventricular Action Potential Plateau

A prolonged depolarization phase lasting $0.2$ to $0.3\,s$ in ventricular muscle cells caused primarily by slow inward calcium current.

Parasympathetic Stimulation of the Heart

Nervous regulation via acetylcholine that opens potassium channels, hyperpolarizing the resting potential from $-55\text{ to }-60\,mV$ down to $-65\text{ to }-75\,mV$, thereby slowing heart rate and conduction speed.

Sympathetic Stimulation of the Heart

Nervous regulation via norepinephrine acting on beta-1 adrenergic receptors to increase sodium and calcium permeability, raising membrane potential faster to reach threshold and increasing heart rate and contraction force.

Total Impulse Delay to Ventricles

The cumulative time delay of $0.16\,s$ experienced by an impulse from its origin in the SA node ($0.03\,s$), through the AV node ($0.09\,s$), and through the penetrating AV bundle ($0.04\,s$).

Total Cardiac Conduction Time

The overall time duration of $0.22\,s$ required for an action potential generated at the SA node to spread completely through the entire heart muscle to the last epicardial fibers.

Valsalva Maneuver

A vagal stimulation maneuver performed by forced exhalation against a closed glottis to increase intra-abdominal pressure, activating parasympathetic tone to decrease heart rate and blood pressure.