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NCERT Class 11 lists the heart's wiring in one paragraph: SAN, AVN, AV bundle, bundle branches, Purkinje fibres. It reads like a list to memorise. In fact it was pieced together by several people over about seventy years, and the man who joined the middle pieces was a young Japanese doctor working in a foreign language, far from home. This post covers Sunao Tawara's discovery, why the AV node matters, and the timing questions examiners like to ask.
Who was Sunao Tawara?
A tribute in the Journal of Arrhythmia, published by the Japanese Heart Rhythm Society for his 150th birthday, records that Tawara was born in Oita, Japan, on 5 July 1873. He studied German from before high school through Tokyo University. In 1903 he travelled to Germany and joined the pathology department of Ludwig Aschoff at the University of Marburg, where he stayed until 1906. A 2020 article by De Almeida, Sánchez-Quintana and Anderson gives his dates as 1873 to 1952 and calls him the father of modern cardiac electrophysiology.
After returning to Japan in 1906 he became a professor at Kyushu University, the Journal of Arrhythmia tribute adds.
What was already known before Tawara?
A 2022 review by Cavero and Holzgrefe in Advances in Physiology Education sets out the timeline:
In 1839 Jan Evangelista Purkinje came across special fibres in the sheep ventricle, now called Purkinje fibres.
In 1893 Wilhelm His Jr described a muscular tract crossing from the atria to the ventricles, now the bundle of His.
But nobody knew how these pieces linked up, or where the signal entered the bundle.
What did Tawara find?
Tawara examined hearts from several mammals under a single light microscope. In a preliminary report in 1905 and a full monograph in 1906, Das Reizleitungssystem des Säugetierherzens (The conduction system of the mammalian heart), he described a compact knot of special muscle at the junction of atria and ventricles: the atrioventricular node. The tribute in the Journal of Arrhythmia explains that he traced this node into the bundle of His, followed the bundle as it split into right and left branches, and showed those branches spreading into the Purkinje fibres. He summarised the whole tree as a single conduction system.
According to Kozo Suma's 2001 article in Pacing and Clinical Electrophysiology, Tawara also reasoned from the anatomy that conduction would be fast everywhere in the system except the AV node, so that all parts of the ventricles would contract almost at the same time. Suma adds that these findings gave Willem Einthoven a theoretical basis for interpreting the electrocardiogram. Our post on Einthoven and the ECG tells that side of the story.
The node is often called the Aschoff-Tawara node, after both student and supervisor, but the description of the bundle branches carries Tawara's name alone.
Who found the pacemaker, then?
Not Tawara. As De Almeida (2023) describes, two groups were working at the same time. Tawara and Aschoff, with the physiologist Heinrich Ewald Hering, formed one; James Mackenzie, Arthur Keith and Martin Flack formed the other. Keith and Flack located the sinoatrial node in 1907. The complete pathway was assembled from both groups' work.
Why does the AV node matter if the SAN is the pacemaker?
The NCERT Class 11 chapter Body Fluids and Circulation explains that all nodal tissue is autoexcitable, but the SAN produces the most action potentials per minute, 70 to 75, so it sets the rhythm. The SAN signal makes both atria contract, which increases the flow of blood into the ventricles by about 30 per cent. The AVN and AV bundle then carry the signal to the ventricles.
Cavero and Holzgrefe add the key point: impulses are briefly delayed at the AV node, enabling the atria to contract before the high-speed His-Purkinje system triggers the ventricles. Without that pause, atria and ventricles would squeeze at once and the atria could not top up the ventricles.
Worked example: timing the cardiac cycle
NCERT says the heart beats about 72 times a minute and deduces a cardiac cycle of 0.8 seconds.
At 72 beats per minute: 60 ÷ 72 = 0.833 seconds per cycle, which NCERT rounds to 0.8.
At 75 beats per minute, the top of the SAN's range: 60 ÷ 75 = 0.80 seconds exactly.
At 100 beats per minute (a sample rate, as during exercise): 60 ÷ 100 = 0.60 seconds. Every phase of the cycle has to fit into less time.
Now an order question examiners like. Put these in sequence: Purkinje fibres, AVN, SAN, bundle of His, atrial contraction, bundle branches, ventricular contraction. Answer: SAN, atrial contraction, AVN, bundle of His, bundle branches, Purkinje fibres, ventricular contraction.
Which mistakes cost marks?
Calling the AVN the pacemaker. The SAN is the pacemaker because it fires fastest.
Placing the AVN in the left atrium. NCERT places it in the lower left corner of the right atrium, close to the atrio-ventricular septum.
Forgetting the delay. If asked why the AVN is significant, mention that it holds the signal back briefly so the atria finish contracting first.
If you can recite the pathway but cannot see why the delay matters, Fermi's AI tutor decides whether to explain, ask or show, and can draw the figure as it goes, so you can work through the sequence step by step. For the rest of the chapter, see our guide to body fluids and circulation.
What can a student take from Tawara?
Join the dots others left. Purkinje and His had found pieces; Tawara connected them.
Work through discomfort. He studied in a second language, in a culture not his own, and nearly ran out of time.
Reason from structure. He predicted fast and slow conduction just from what the tissue looked like.
Next step: draw the conduction pathway from memory, mark where the delay happens, and write one line on why it is needed.
