cardiac electrical properties part 1

[FOUNDATION] What are the two major functional categories of cardiac cells?

Contractile myocytes and specialized autorhythmic (pacemaker) cells

[FOUNDATION] Approximately what percentage of cardiac cells are contractile myocytes?

Approximately 99% of cardiac cells

[FOUNDATION] Approximately what percentage of cardiac cells are specialized pacemaker cells?

Approximately 1% of cardiac cells

What is the primary function of contractile cardiac myocytes?

Generate force through contraction to pump blood through the cardiovascular system

What is the primary function of autorhythmic cardiac cells?

Initiate action potentials spontaneously without requiring neural stimulation

What electrical property is shared by all cardiac cells?

They can conduct and propagate action potentials

What is automaticity?

The ability of specialized cardiac cells to spontaneously depolarize to threshold and initiate an action potential without external stimulation

Does the heart require nervous system input to initiate each heartbeat?

No. Pacemaker cells can spontaneously generate action potentials, although autonomic input modifies their activity

What three electrical properties establish the normal cardiac cycle?

Automaticity, the cardiac conduction system, and the functional syncytium

What is the functional syncytium of the heart?

Cardiac cells are electrically coupled through gap junctions, allowing depolarization to propagate between cells so contractile myocardium functions as a coordinated unit

What structures allow electrical communication between adjacent cardiac myocytes?

Gap junctions

Why are gap junctions important for cardiac conduction?

They provide low-resistance pathways for electrical current to spread from one cell to another

What is the all-or-none law of the heart?

Normally, electrical excitation spreads throughout the coupled myocardium so cardiac contractile cells respond as a coordinated unit rather than through individual motor-unit recruitment

How does cardiac muscle differ from skeletal muscle regarding motor-unit recruitment?

Cardiac muscle cannot increase force by recruiting additional motor units, unlike skeletal muscle

How can cardiac muscle alter force production without motor-unit recruitment?

By changes in preload and contractility, including changes in calcium availability

What is preload in the context introduced by the lecture?

The prestretch of cardiac muscle associated with the volume of blood filling the heart

What is contractility in the context introduced by the lecture?

The ability of cardiac muscle to develop force, influenced by factors including autonomic regulation of intracellular calcium

[HIGH-YIELD CONCEPT] What must occur before a mechanical cardiac event?

An electrical event must occur first

What mechanical event follows cardiac depolarization?

Contraction, or systole

What mechanical event follows cardiac repolarization?

Relaxation, or diastole

What determines the direction of blood flow through the heart?

Blood flows from regions of higher pressure toward regions of lower pressure

What does systole generally refer to unless otherwise specified?

Ventricular contraction

What does diastole generally refer to unless otherwise specified?

Ventricular relaxation

What is the normal sequence of electrical conduction through the heart?

SA node → atrial myocardium → AV node → AV bundle (bundle of His) → right and left bundle branches → Purkinje fibers → ventricular myocardium

[DRAW/MAP] Map normal cardiac electrical conduction.

SA node initiates AP → AP spreads through atria → AV node delays conduction → AV bundle crosses into ventricles → bundle branches conduct toward apex → Purkinje fibers rapidly distribute excitation → ventricular myocardium depolarizes

Which cardiac structure normally initiates the heartbeat?

Sinoatrial (SA) node

Why is the SA node normally the dominant pacemaker?

It has the fastest intrinsic rate of spontaneous Phase 4 depolarization to threshold

What is the normal electrical connection between the atria and ventricles emphasized in the lecture?

The AV node and associated normal AV conduction pathway

Why must conduction through the AV node be delayed?

To allow adequate time for ventricular filling before ventricular depolarization and contraction

Which cardiac structure has the slowest conduction velocity?

The AV node

Which cardiac conduction cells have the fastest conduction velocity?

Purkinje fibers

Why do AV nodal cells conduct electrical impulses slowly?

Their relatively small diameter produces high internal resistance to current flow

Why do Purkinje fibers conduct impulses rapidly?

Their relatively large diameter reduces internal resistance to current flow

What is conduction velocity?

The distance an action potential propagates per unit time

How does conduction velocity differ from automaticity?

Conduction velocity describes how quickly an action potential spreads, whereas automaticity describes how quickly a pacemaker cell spontaneously reaches threshold

Why is rapid Purkinje fiber conduction physiologically important?

It allows rapid distribution of ventricular excitation and coordinated ventricular contraction

Why is AV nodal conduction intentionally slower than Purkinje conduction?

AV nodal delay allows ventricular filling, whereas rapid Purkinje conduction supports efficient ventricular activation

What is the approximate normal resting heart rate range discussed in the lecture?

60–100 beats per minute

What is bradycardia?

A heart rate below 60 beats per minute

What is tachycardia?

A heart rate above 100 beats per minute

What may happen if the SA node fails to generate action potentials?

A slower secondary or tertiary pacemaker may initiate cardiac impulses

Which structure normally serves as the secondary pacemaker when the SA node fails?

The AV nodal region

What is an ectopic pacemaker?

An abnormal site outside the normal dominant pacemaker that initiates electrical activity

How can an ectopic pacemaker cause tachycardia?

It may reach threshold and initiate impulses faster than the SA node, allowing it to dominate cardiac rhythm

Why are cells with slower conduction velocities more susceptible to conduction block?

Their slower propagation makes electrical transmission more vulnerable to failure

What is the normal initial direction of interventricular septal depolarization?

From left to right

In what general direction does ventricular depolarization progress through the anteroseptal region?

From base toward apex

In what direction does ventricular myocardial depolarization generally spread across the wall?

From endocardium toward epicardium

Which region of the ventricles is among the last to depolarize?

The basal ventricular region

How does the sequence of ventricular repolarization compare with depolarization?

Repolarization occurs in the opposite general order

Why does ventricular repolarization occur in a different order than depolarization?

Differences in action potential duration and potassium-channel expression among ventricular cell populations

Why does ventricular repolarization generally take longer than depolarization?

Repolarization depends on slower, variable ionic processes across different myocardial cells

[DRAW/MAP] Map the ventricular depolarization sequence emphasized in the lecture.

Interventricular septum depolarizes left → right → anteroseptal excitation progresses base → apex → ventricular wall depolarizes endocardium → epicardium → basal regions complete depolarization

How does abnormal ventricular depolarization affect the QRS complex?

It can widen the QRS complex because ventricular activation takes longer

Why does an ectopic ventricular impulse often produce a widened QRS?

Excitation spreads relatively slowly from myocyte to myocyte rather than rapidly through the normal Purkinje conduction system

What ECG finding suggests delayed ventricular depolarization?

A widened QRS complex

What is a premature ventricular contraction (PVC)?

An early ventricular beat initiated from an ectopic ventricular focus

Why can PVCs produce widened QRS complexes?

Depolarization begins outside the normal ventricular conduction sequence and spreads more slowly through ventricular myocardium

Why can ventricular tachycardia produce widened QRS complexes?

Ventricular excitation may propagate abnormally rather than following the rapid normal conduction pathway

What is Wolff-Parkinson-White (WPW) syndrome?

A pre-excitation syndrome involving an abnormal accessory electrical pathway between the atria and ventricles

What is the key anatomical conduction abnormality in WPW?

An accessory pathway of cardiac myocytes allows impulses to bypass the normal AV nodal delay

What happens to atrioventricular conduction time in WPW?

It becomes shorter because the accessory pathway bypasses the AV node

What is the characteristic PR interval abnormality in WPW?

A shortened PR interval, less than 0.12 seconds

What is the characteristic QRS duration abnormality described in WPW?

A widened QRS complex, greater than 0.12 seconds

What is a delta wave in WPW?

A slurred, slowly rising initial portion of the QRS complex associated with ventricular pre-excitation

Why does WPW produce a shortened PR interval?

The accessory pathway conducts atrial impulses to ventricular tissue without the normal AV nodal delay

Why does WPW produce a widened QRS complex?

Pre-excited ventricular tissue is activated through relatively slow cell-to-cell myocardial conduction instead of exclusively through the normal rapid Purkinje system

What type of action potential occurs in the accessory myocyte pathway of WPW?

A fast-response action potential

Which ion is responsible for Phase 0 depolarization in the WPW accessory myocyte pathway?

Sodium influx through fast voltage-gated sodium channels

Why does the accessory pathway conduct faster than the AV node?

Accessory myocytes exhibit fast sodium-dependent Phase 0 depolarization, whereas AV nodal cells exhibit slower calcium-dependent Phase 0 depolarization

[CLINICAL APPLICATION] A 22-year-old has palpitations, dizziness, PR interval less than 0.12 seconds, and a delta wave. What condition is suggested?

Wolff-Parkinson-White syndrome

[CLINICAL APPLICATION] Why can ventricular activation be prolonged despite rapid atrioventricular transmission in WPW?

The impulse reaches ventricular tissue early but initially spreads through ordinary myocardium rather than exclusively through the rapid His-Purkinje system

[CLINICAL APPLICATION] What abnormal electrical mechanism can occur in WPW?

Re-entry, in which an impulse can circulate through an abnormal conduction circuit

What does re-entry mean in cardiac electrophysiology?

An electrical impulse repeatedly propagates through a circuit and can re-excite previously activated tissue

What structural feature of WPW creates the possibility of re-entry?

The presence of an accessory atrioventricular pathway in addition to the normal conduction pathway

What contractile proteins are found in cardiac muscle?

Thick and thin filaments organized into sarcomeres

How are sarcomeres defined in cardiac muscle?

The region extending from one Z line to the next Z line

Why is cardiac muscle described as striated?

Its actin and myosin filaments are organized into repeating sarcomeres

How many nuclei do cardiac myocytes typically contain according to the lecture?

Typically one nucleus

Why do cardiac myocytes contain numerous mitochondria?

They require a continuous supply of ATP to sustain repeated contractions

What two intracellular membrane systems participate in cardiac excitation–contraction coupling?

Transverse (T) tubules and the sarcoplasmic reticulum

What is the major difference between cardiac and skeletal muscle calcium sources?

Cardiac contraction depends on calcium from both extracellular fluid and the sarcoplasmic reticulum

What is calcium-induced calcium release?

Extracellular calcium enters cardiac myocytes and stimulates additional calcium release from the sarcoplasmic reticulum

Why is extracellular calcium influx essential for normal cardiac contraction?

It triggers additional sarcoplasmic reticulum calcium release and contributes calcium for cross-bridge activation

What ion directly supports cardiac cross-bridge activation?

Calcium

Which action potential phase provides an important source of extracellular calcium for contraction in ventricular myocytes?

Phase 2, the plateau phase

What channel type carries the major inward calcium current during the ventricular action potential plateau?

L-type voltage-gated calcium channels

What does L-type mean when describing calcium channels?

Long-lasting, reflecting relatively slow opening and closing kinetics

[DRAW/MAP] Map calcium-induced calcium release in cardiac myocytes.

Cardiac action potential → L-type calcium channels open → extracellular Ca2+ enters → sarcoplasmic reticulum releases additional Ca2+ → intracellular Ca2+ rises → cross-bridge cycling and contraction

What must happen to intracellular calcium for cardiac muscle to relax?

Cytosolic calcium concentration must decrease

What is the role of the sarcoplasmic reticulum during cardiac relaxation?

Reuptake cytosolic calcium for storage

What is the role of the sodium-calcium exchanger in cardiac relaxation?

Extrude calcium from the cell while sodium enters

What is the exchange ratio of the sodium-calcium exchanger described in the lecture?

Three sodium ions enter for each calcium ion extruded

Why is the sodium gradient important for calcium removal from cardiac myocytes?

It supplies the driving force for sodium-calcium exchange

What is the role of the sodium-potassium ATPase in maintaining cardiac ion gradients?

It uses ATP to pump sodium out and potassium into the cell

What is the sodium-potassium ATPase transport ratio?

Three sodium ions out for every two potassium ions in

Why is ATP required for long-term maintenance of cardiac electrical function?

ATP-dependent transport maintains the sodium and potassium gradients needed for electrical activity and secondary transport

What is the main intracellular cation in cardiac cells?

Potassium

What is the main extracellular cation involved in rapid cardiac depolarization?

Sodium

Why does sodium tend to enter a cardiac cell when sodium channels open?

Both its concentration gradient and the electrical gradient favor inward movement