Difficulty: Intermediate | Prerequisites: Basic cell membrane physiology (resting membrane potential, ion channels), Chapter 12 notes on cardiac muscle tissue.
This material covers how the heart generates its own electrical signals, how those signals spread through the conduction system, and how the resulting mechanical events (contraction and relaxation) produce the cardiac cycle. It sits at the core of cardiovascular physiology: you cannot reason about cardiac output, blood pressure, or clinical interventions (drugs, pacemakers) without understanding what drives each heartbeat electrically and mechanically. You should already be comfortable with the idea that ions moving across membranes create voltage changes, and that cardiac muscle is distinct from skeletal muscle.
The SA node fires spontaneously, setting heart rate. The signal passes through the AV node (which delays it so the atria finish filling the ventricles), then spreads rapidly through the ventricles. The ECG records this electrical activity from the body surface: P wave = atrial depolarisation, QRS = ventricular depolarisation, T wave = ventricular repolarisation. Mechanically, the ventricles cycle through filling, isovolumetric contraction, ejection, and isovolumetric relaxation, with valve openings and closings producing the heart sounds S1 and S2.
SA node (sinoatrial node)
The heart's primary pacemaker, located in the right atrium. It spontaneously depolarises to initiate each heartbeat. Think of it as the spark plug that fires on its own, setting the pace for the entire heart.
Funny (If) channels
Ion channels in pacemaker cells that open at hyperpolarised (negative) membrane potentials and allow a slow inward current of Na+ and K+. They are the main reason the SA node depolarises spontaneously. In simple terms, these channels slowly push the membrane potential toward threshold without any outside stimulus.
AV node (atrioventricular node)
A small cluster of specialised cells between the atria and ventricles that delays the electrical signal by roughly 0.1 seconds before passing it to the ventricles. Think of it as a deliberate bottleneck: it holds the signal just long enough for the atria to finish contracting and topping off ventricular volume.
AV nodal delay
The brief pause in conduction at the AV node. Its primary function is to allow ventricular filling before ventricular contraction.
Plateau phase
The prolonged phase 2 of the ventricular contractile cell action potential, maintained by a sustained influx of Ca2+ through L-type calcium channels while K+ efflux is temporarily reduced. This keeps the cell depolarised far longer than a skeletal muscle fibre and prevents tetanus. In simple terms, calcium keeps streaming in, holding the voltage up so the muscle stays contracted long enough to eject blood.
Depolarisation
A shift in membrane potential toward a more positive value, triggered by inward ion flow (Na+ or Ca2+). In the heart, depolarisation is the electrical event that precedes and triggers contraction.
Repolarisation
The return of membrane potential to its resting (negative) value, mainly through K+ efflux. In the ventricles, repolarisation allows the muscle to relax so the chambers can refill.
P wave
The ECG deflection representing atrial depolarisation, the spread of the electrical impulse from the SA node across both atria.
QRS complex
The ECG deflection representing ventricular depolarisation. Because the ventricular muscle mass is large, the QRS signal is the tallest waveform on a normal ECG.
T wave
The ECG deflection representing ventricular repolarisation, when the ventricles are recovering electrically and beginning to relax.
PR interval
The time from the start of the P wave to the start of the QRS complex. It includes atrial depolarisation plus the AV nodal delay, and reflects how long the signal takes to travel from atria to ventricles.
Isovolumetric contraction
The brief phase after the AV valves close but before the semilunar valves open. Ventricular pressure rises sharply while volume stays constant because all four valves are closed.
Isovolumetric relaxation
The brief phase after the semilunar valves close but before the AV valves open. Ventricular pressure drops while volume stays constant, again because all four valves are closed.
Ventricular ejection
The phase when ventricular pressure exceeds arterial pressure, the semilunar valves are open, and blood is pumped out of the ventricles into the aorta and pulmonary trunk.
Ventricular filling
The phase when atrial pressure exceeds ventricular pressure, the AV valves are open, and blood flows passively (and then actively, during atrial systole) into the ventricles.
S1 (first heart sound)
The sound produced by closure of the AV valves (mitral and tricuspid) at the onset of ventricular systole.
S2 (second heart sound)
The sound produced by closure of the semilunar valves (aortic and pulmonary) at the onset of ventricular diastole.
Mitral valve (bicuspid valve)
The AV valve between the left atrium and left ventricle. It prevents backflow from the left ventricle into the left atrium during systole.
Tricuspid valve
The AV valve between the right atrium and right ventricle.
Semilunar valves
The aortic and pulmonary valves, positioned at the exits of the left and right ventricles respectively.
Diastole
The period of ventricular relaxation and filling.
Systole
The period of ventricular contraction and ejection.
The SA node fires spontaneously because of funny (If) channels, which open at negative membrane potentials and allow a slow inward current that gradually brings the cell to threshold.
Once threshold is reached, L-type Ca2+ channels open and produce the upstroke of the pacemaker action potential (note: pacemaker cells do not rely on fast voltage-gated Na+ channels the way contractile cells do).
Repolarisation occurs via K+ efflux, and the cycle repeats.
The SA node's intrinsic rate is roughly 100 bpm, modulated downward at rest by tonic parasympathetic (vagal) input.
The AV node is the only normal electrical connection between atria and ventricles.
The AV nodal delay (approx. 0.1 s) ensures the atria finish contracting before the ventricles begin, maximising ventricular filling.
After the AV node, the signal travels rapidly through the bundle of His, left and right bundle branches, and Purkinje fibres to depolarise the ventricular myocardium almost simultaneously.
Phase 0 (rapid depolarisation): fast Na+ influx through voltage-gated Na+ channels.
Phase 1 (initial repolarisation): brief K+ efflux.
Phase 2 (plateau): prolonged Ca2+ influx through L-type Ca2+ channels balanced against K+ efflux. This is the defining feature of the cardiac action potential and is responsible for the long refractory period.
Phase 3 (repolarisation): Ca2+ channels close, K+ efflux dominates, membrane potential returns to resting level.
Phase 4 (resting): stable resting potential (~-90 mV), maintained by K+ leak channels.
P wave: atrial depolarisation. Appears just before atrial contraction.
QRS complex: ventricular depolarisation. Appears just before ventricular contraction begins. Atrial repolarisation occurs during this time but is hidden by the larger QRS signal.
T wave: ventricular repolarisation. Appears as the ventricles relax.
PR interval: atrial depolarisation + AV nodal delay. A prolonged PR interval suggests a conduction delay at the AV node.
Ventricular filling (mid-to-late diastole): AV valves open, semilunar valves closed. Ventricular pressure is lower than atrial pressure, so blood flows in passively. Atrial systole (the "atrial kick") adds the final ~20% of ventricular volume at the end of this phase.
Isovolumetric contraction: begins when AV valves close (producing S1). All four valves are now closed. Ventricular pressure rises rapidly at constant volume. Ends when ventricular pressure exceeds arterial pressure and the semilunar valves open.
Ventricular ejection: semilunar valves open, blood is expelled into the aorta/pulmonary trunk. Ventricular pressure peaks, then begins to fall as ejection slows.
Isovolumetric relaxation: begins when semilunar valves close (producing S2). All four valves are closed again. Ventricular pressure drops rapidly at constant volume. Ends when ventricular pressure falls below atrial pressure and the AV valves open, restarting filling.
S1 = AV valve closure = onset of systole.
S2 = semilunar valve closure = onset of diastole.
When heart rate drops (bradycardia), the phase that lengthens the most is diastolic filling time. Systolic phases are relatively fixed; it is the diastolic interval that compresses or stretches as rate changes.
Wiggers diagram (not reproduced here, but essential to review): a composite graph aligning aortic pressure, ventricular pressure, atrial pressure, ventricular volume, the ECG, and heart sounds against time. Every exam question about "what happens immediately before/after X" can be answered by reading the Wiggers diagram from left to right.
The ECG is one of the most widely used diagnostic tools in medicine. Clinicians read the PR interval to detect AV block, the QRS width to detect bundle-branch blocks, and the T wave morphology to spot ischaemia or electrolyte disturbances. Understanding which mechanical event maps to which ECG feature is the foundation for interpreting what is going wrong when a patient's rhythm looks abnormal.
Students often think the QRS complex represents ventricular repolarisation. It does not. The QRS is depolarisation; the T wave is repolarisation.
Students frequently confuse which valve closure produces which heart sound. S1 = AV valves closing (start of systole). S2 = semilunar valves closing (start of diastole). A useful mnemonic: the numbers go in order with the cycle.
The AV nodal delay is sometimes mistaken for a defect. It is a normal and essential feature: without it, the atria and ventricles would contract nearly simultaneously and filling would be incomplete.
Students sometimes assume isovolumetric contraction means no pressure change. Pressure rises dramatically; it is the volume that stays constant because all valves are shut.
⚠️ Know exactly what the P wave, QRS complex, and T wave represent. This is tested repeatedly.
⚠️ Be able to state what happens immediately before and after each valve event (AV close, semilunar open, semilunar close, AV open). Walk through the Wiggers diagram.
⚠️ Understand that the plateau phase of the ventricular action potential is due to prolonged Ca2+ influx, not Na+.
⚠️ The AV nodal delay exists to allow ventricular filling. Expect a question phrased as "the primary function of the AV nodal delay."
⚠️ Bradycardia lengthens diastolic filling time, not ejection time.
⚠️ The event immediately after the QRS complex is the onset of ventricular contraction (the electrical event triggers the mechanical event with a very short delay).
True or false: The T wave represents atrial repolarisation. ___
Fill in the blank: During isovolumetric contraction, both ___ valves and ___ valves are closed.
True or false: The SA node relies on fast voltage-gated Na+ channels to generate its action potential. ___
Fill in the blank: Closure of the semilunar valves produces the ___ heart sound.
True or false: The event immediately before ventricular ejection is isovolumetric contraction. ___
Answers: 1. False (ventricular repolarisation). 2. AV, semilunar. 3. False (funny/If channels and L-type Ca2+ channels). 4. S2. 5. True.
Q: The plateau phase of ventricular contractile cells primarily results from which ion movement?
A: Prolonged Ca2+ influx through L-type calcium channels.
Q: Which ECG component represents ventricular repolarisation?
A: The T wave.
Q: What is the primary function of the AV nodal delay?
A: To allow the ventricles to fill completely before they contract.
Q: Which event immediately follows ventricular depolarisation (the QRS complex)?
A: Ventricular contraction begins.
Q: What event marks the end of isovolumetric contraction?
A: The semilunar valves open (ventricular pressure has exceeded arterial pressure).
Q: During isovolumetric contraction, what is the status of the heart valves?
A: Both AV and semilunar valves are closed.
Q: Which event occurs immediately after the semilunar valves close?
A: Isovolumetric relaxation begins.
Q: The P wave on the ECG represents what electrical event?
A: Atrial depolarisation.
Q: In severe bradycardia, which phase of the cardiac cycle lengthens the most?
A: Diastolic filling time.
Q: During ventricular filling, how does ventricular pressure compare to atrial pressure?
A: Ventricular pressure is lower than atrial pressure (which is why blood flows in).
Q: Which valve prevents backflow from the left ventricle into the left atrium?
A: The mitral (bicuspid) valve.
Q: Closure of the AV valves produces which heart sound?
A: S1.
Q: What primarily drives SA node automaticity?
A: Funny (If) channels, which allow a slow inward current at negative membrane potentials, gradually depolarising the cell to threshold.
This material connects directly to cardiac output and stroke volume (Part 2 of these notes): the mechanical events here determine how much blood is ejected per beat. It also connects to blood pressure regulation (Part 3), because autonomic nerves that alter SA node firing rate and AV conduction speed are central to the baroreceptor reflex. Understanding the plateau phase and calcium handling will become important again if your course covers cardiac pharmacology (calcium channel blockers, beta-blockers).
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