Describe the conduction system (SA, AV node)
WHAT is the conduction system?
HOW the impulse travels (step by step)
1. SA node fires.
- Located in the wall of the right atrium, near the opening of the superior vena cava.
- It depolarises spontaneously at the fastest rate (~60–100 times/min), so it sets the pace → called the pacemaker.
- Why fastest sets the pace? Whichever region reaches threshold first fires the rest before they can fire themselves. The SA node's leaky Na⁺ ("funny") channels make it reach threshold first.
2. Impulse spreads across the atria.
- Travels cell-to-cell through gap junctions (intercalated discs) → both atria contract (atrial systole), pushing blood into the ventricles.
3. Impulse reaches the AV node.
- Located at the base of the right atrium, in the interatrial septum, near the AV valves.
- Here the signal is delayed ~0.1 second.
- Why the delay? AV nodal cells conduct slowly. This gives the atria time to finish emptying into the ventricles before the ventricles contract. Without the delay, chambers would squeeze at the same time and pumping would fail.
4. Bundle of His → bundle branches.
- The AV node is the only electrical bridge between atria and ventricles (the rest is insulated by non-conducting fibrous tissue). The signal passes down the Bundle of His through the interventricular septum, splitting into left and right bundle branches.
5. Purkinje fibres.
- These spread rapidly through the ventricle walls, from the apex upward.
- Why apex-first? So contraction squeezes blood upward toward the arteries (aorta & pulmonary artery) — like squeezing a toothpaste tube from the bottom. This is ventricular systole.
The pacemaker hierarchy (WHY backups exist)
Worked examples
Common mistakes (Steel-man + fix)
Active recall
Recall Quick self-test (hide answers)
- Where is the SA node located? → Wall of the right atrium, near the superior vena cava.
- What is the function of the AV node? → Delays the impulse ~0.1 s.
- Only electrical bridge between atria and ventricles? → AV node → Bundle of His.
- Which fibres fire first in ventricles? → Purkinje fibres at the apex.
- Why is the SA node the pacemaker? → It depolarises fastest, so it fires the rest first.
Recall Feynman: explain to a 12-year-old
Your heart is like a marching band. The SA node is the drummer who sets the beat — nobody tells him to, he just drums on his own, and everyone follows his fastest beat. The signal spreads so the top two rooms (atria) squeeze first, pushing blood into the bottom rooms. Then the signal hits a traffic light called the AV node that makes it wait a tiny moment — this gives the top rooms time to finish pouring blood down. After the wait, the signal zooms down special wires (Bundle of His → Purkinje fibres) to the bottom of the heart, so the big bottom rooms (ventricles) squeeze from the bottom up, shooting blood out to the body. If the drummer ever quits, a slower backup drummer takes over so the music never fully stops.
Flashcards
What generates the heartbeat without any brain signal (myogenic)?
Where is the SA node located?
Why is the SA node called the pacemaker?
What is the key function of the AV node?
Why is a delay at the AV node necessary?
What is the only electrical connection between atria and ventricles?
List the conduction pathway in order.
Why do Purkinje fibres cause contraction to start at the apex?
What happens if the SA node fails?
Intrinsic rates: SA node vs AV node vs ventricles?
Are conduction cells nerves or muscle?
How do impulses spread across atrial muscle?
Connections
- Cardiac Cycle — the delay explains the timing of atrial vs ventricular systole.
- ECG / Electrocardiogram — P wave (atrial), QRS (ventricular) map directly onto this pathway.
- Autonomic Nervous System — sympathetic/parasympathetic modulation of SA node rate.
- Structure of the Heart — location of nodes within right atrium/septum.
- Cardiac Muscle & Intercalated Discs — gap junctions enable rapid spread.
- Blood Pressure & Cardiac Output — rate × stroke volume depends on this rhythm.
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, dil ek pump hai jise bina brain ke command ke apne aap dhadakna padta hai. Iske liye dil ke andar ek chhoti si "electrical wiring" hoti hai — ye normal nerves nahi, balki special cardiac muscle cells hain jo khud spontaneously fire karti hain (isko myogenic ya autorhythmic kehte hain). Sabse pehle SA node fire karta hai, jo right atrium mein superior vena cava ke paas hota hai. Ye sabse fast fire karta hai, isliye ise pacemaker kehte hain — jo fastest fire kare wahi sab ko lead karta hai.
SA node ka signal dono atria mein gap junctions ke through spread hota hai, atria squeeze hote hain aur blood ventricles mein jaata hai. Fir signal AV node pe pahunchta hai (right atrium ke base pe, septum ke paas). Yahan ek chhota sa delay (~0.1 second) hota hai. Ye delay bahut important hai — isse atria pura khaali ho jaate hain ventricles mein, tabhi ventricles squeeze karte hain. Agar delay na ho to sab ek saath dab jaayenge aur pumping bekaar ho jaayegi.
Delay ke baad signal Bundle of His se neeche jaata hai, phir left aur right bundle branches mein baant jaata hai, aur Purkinje fibres se poore ventricles mein spread hota hai — khaas baat: apex (neeche) se upar ki taraf, taaki blood upar ki arteries (aorta, pulmonary artery) mein push ho, bilkul toothpaste ko neeche se dabane jaisa.
Yaad rakho: SA = Starts it, AV = A Wait (delay). Aur agar SA node fail ho jaaye to AV node backup pacemaker ban jaata hai, thoda slow rate pe — isliye dil turant nahi rukta. Exam mein order strong hona chahiye: SA -> atria -> AV -> Bundle of His -> bundle branches -> Purkinje -> ventricles.