Cardiac conduction system

What are the two types of cardiac cells?

Conduction cells and contraction (contractile) cells.

What percentage of the myocardium consists of conduction cells?

Approximately 1%.

What is the primary role of conduction cells?

Coordinate and direct cardiac contraction by generating and conducting electrical impulses.

What percentage of the myocardium consists of contraction cells?

Approximately 99%.

What is the primary role of contraction cells?

Generate force to propel blood through the circulation.

What does it mean for a cardiac cell to be polarised?

It has an electrical voltage across its membrane.

What is resting membrane potential?

The electrical voltage across the membrane when the cell is at rest.

Which ions are found in higher concentration inside cardiac cells at rest?

Potassium (K+).

Which ions are found in higher concentration outside cardiac cells at rest?

Sodium (Na+) and Calcium (Ca2+).

What maintains ionic concentration gradients across cardiac cell membranes?

Ion pumps that move Na+ and Ca2+ out and K+ into the cell.

Define action potential.

A brief reversal of membrane polarity caused by voltage-gated ion channels.

What is depolarisation?

A rise in membrane voltage making the inside of the cell less negative.

What is repolarisation?

A return to a more negative membrane voltage.

What is automaticity?

The ability of pacemaker cells to generate spontaneous action potentials without external stimulation.

Which cardiac cells exhibit automaticity?

Pacemaker cells within the conduction system.

What threshold voltage must pacemaker cells reach to trigger an action potential?

Approximately -40 mV.

What membrane voltage do pacemaker cells start from?

Approximately -60 mV.

Why do pacemaker cells not have a true resting membrane potential?

Their membrane voltage slowly rises spontaneously toward threshold.

What causes spontaneous depolarisation in pacemaker cells?

Funny currents (If channels).

What are funny channels?

Special sodium channels that open when membrane voltage falls below about -40 mV.

What ion enters through funny channels?

Sodium (Na+).

What is pacemaker potential?

The spontaneous depolarisation caused by funny currents.

What occurs during Phase 4 of a pacemaker action potential?

Slow spontaneous depolarisation toward threshold.

What occurs during Phase 0 of a pacemaker action potential?

Calcium influx causing rapid depolarisation.

What occurs during Phase 3 of a pacemaker action potential?

Potassium efflux causing repolarisation.

Which ion is primarily responsible for depolarisation in pacemaker cells?

Calcium (Ca2+).

Which ion is primarily responsible for repolarisation in pacemaker cells?

Potassium (K+).

What happens when pacemaker cells reach -40 mV?

Calcium channels open and depolarisation accelerates.

What membrane potential is reached at peak pacemaker depolarisation?

Approximately +10 mV.

What happens to calcium channels at peak depolarisation?

They close.

What happens to potassium channels at peak depolarisation?

They open.

What is the resting membrane potential of contractile myocytes?

Approximately -90 mV.

How are contractile cells stimulated to depolarise?

By neighbouring cells through gap junctions.

What are gap junctions?

Channels connecting adjacent cells allowing ion movement.

What is electrical coupling?

The spread of electrical activity between neighbouring cells through gap junctions.

What threshold triggers a contractile cell action potential?

Approximately -70 mV.

What opens first during depolarisation of contractile cells?

Fast sodium channels.

Which ion is responsible for the rapid upstroke in contractile cells?

Sodium (Na+).

At what voltage do slow calcium channels open in contractile cells?

Approximately -40 mV.

What occurs during Phase 0 of the contractile cell action potential?

Rapid depolarisation due to sodium influx.

What occurs during Phase 1 of the contractile cell action potential?

Early partial repolarisation.

What occurs during Phase 2 of the contractile cell action potential?

Plateau phase.

What occurs during Phase 3 of the contractile cell action potential?

Repolarisation back to resting potential.

What occurs during Phase 4 of the contractile cell action potential?

Resting membrane potential (-90 mV).

Why is the plateau phase important?

It prolongs contraction and prevents tetany.

Which ions balance each other during the plateau phase?

Calcium entering and potassium leaving.

How long does the plateau phase last?

Approximately 200 ms.

What is calcium-induced calcium release?

Calcium entry triggering larger calcium release from the sarcoplasmic reticulum.

What structure stores calcium inside contractile cells?

Sarcoplasmic reticulum.

When does contraction begin during the action potential?

About halfway through the plateau phase.

Why is cardiac muscle contraction longer than skeletal muscle contraction?

Because of the prolonged plateau phase.

What is the refractory period of cardiac muscle?

Approximately 250 ms.

What is the refractory period of skeletal muscle?

Approximately 1 ms.

Why is a long refractory period important in cardiac muscle?

It prevents sustained contraction and ensures filling between beats.

How is calcium removed after contraction?

Actively transported out of the cell and back into the sarcoplasmic reticulum.

What restores ionic balance after repolarisation?

The sodium-potassium pump.

Where is the SA node located?

In the posterior wall of the right atrium.

What is the normal pacemaker of the heart?

The sinoatrial (SA) node.

What initiates every normal heartbeat?

An action potential generated in the SA node.

What is the first chamber to depolarise during a normal heartbeat?

The right atrium.

What are the three internodal pathways?

Anterior, middle, and posterior internodal tracts.

What structure carries impulses from the right atrium to the left atrium?

Bachmann's bundle.

Where is the AV node located?

In the lower posterior interatrial septum.

What is the only normal electrical connection between atria and ventricles?

The AV node and Bundle of His.

Why is the AV node called the gatekeeper?

It delays conduction before impulses reach the ventricles.

How long is the AV nodal delay?

Approximately 100 ms.

Why is the AV nodal delay important?

It allows atrial contraction to complete before ventricular contraction.

What additional protective function does the AV node provide?

It limits transmission of excessively rapid atrial rates.

What is the Bundle of His?

A bundle of specialised conducting fibres connecting the AV node to the ventricles.

What artery primarily supplies the Bundle of His?

Left anterior descending artery (LAD).

Into what structures does the Bundle of His divide?

Left and right bundle branches.

Which bundle branch is larger?

Left bundle branch.

Into what does the left bundle branch divide?

Anterior fascicle and posterior fascicle.

What does the anterior fascicle supply?

Anterior papillary muscle and Purkinje fibres.

What does the posterior fascicle supply?

Posterior papillary muscle and Purkinje fibres.

What structure carries the right bundle branch through the right ventricle?

Moderator band.

What are Purkinje fibres?

Large, rapidly conducting fibres that distribute impulses throughout the ventricles.

Why do Purkinje fibres conduct rapidly?

They are the widest conducting cells in the heart.

In what direction does ventricular depolarisation spread through myocardium?

Generally from endocardium outward.

List the conduction pathway in order.

SA node → Internodal pathways → AV node → Bundle of His → Bundle branches → Purkinje fibres → Ventricular myocardium.

What does the P wave represent?

Atrial depolarisation.

What does the QRS complex represent?

Ventricular depolarisation.

What does the T wave represent?

Ventricular repolarisation.

What does the U wave represent?

Papillary muscle repolarisation.

What does the PR interval represent?

Conduction from the SA node through the AV node.

What ECG findings define sinus rhythm?

One P wave followed by one QRS complex and one T wave.

What is complete (third-degree) AV block?

Failure of impulses to conduct from atria to ventricles.

What is AV dissociation?

Atria and ventricles beating independently.

What symptoms may occur with complete heart block?

Bradycardia, dizziness, and syncope.

What ECG feature is characteristic of atrial fibrillation?

Irregularly irregular QRS complexes with no distinct P waves.

Why is the ventricular rhythm irregular in atrial fibrillation?

The AV node receives chaotic atrial impulses and conducts them variably.

Which structure electrically insulates the atria from the ventricles?

Annulus fibrosus.

Which coronary artery supplies the AV node in most individuals?

Right coronary artery (about 90%).

Which coronary artery supplies the SA node in most individuals?

Right coronary artery (about 59%).

What heart rate can occur in untreated atrial fibrillation within the atria?

Approximately 300–400 bpm.