Stannius ligatures experiment

Initial baseline: normal rhythm

Normally, when ligatures are absent and the heart is intact, excitation originates in the main pacemaker — the venous sinus (analogous to the sinoatrial node, Remak's ganglion). The impulse successively spreads to the atria and then to the ventricle.

The kymogram records a normal sinus rhythm with the maximum heart rate (HR) for this myocardium. Two waves are clearly visible:

• Small wave — atrial systole (AS).
• Large wave — ventricular systole (VS).

First Stannius ligature (isolating)

The first ligature is applied in the sulcus between the venous sinus and the atria. It is tied tightly, making it an isolating one.

• Effect: The venous sinus is completely separated from the rest of the heart chambers.
• Result: The sinus continues to contract at its usual rhythm (though this is often not recorded on the myogram due to isolation). Meanwhile, the atria and ventricle stop in the diastole phase, and a straight line (asystole) is traced on the graph.
• Physiological significance: The experiment clearly demonstrates that the venous sinus is indeed the 1st order pacemaker. The cessation of underlying parts occurs because their own automaticity centers have low lability. They need time to "warm up" (pre-automatic pause) because they were previously suppressed by the high-frequency impulses of the sinus (overdrive suppression).

Second Stannius ligature (irritating)

The second ligature is applied along the atrioventricular sulcus, while the first ligature remains in place. Important difference: this thread is not tied all the way, exerting only slight pressure. By type, it is an irritating ligature.

• Effect: Mechanical stimulation of the 2nd order pacemaker — the atrioventricular node (Bidder's ganglion) and its isolation from inhibitory influences.
• Result: The ventricles resume spontaneous contractions. The atria and ventricles contract simultaneously, merging into a single wave on the cardiogram.
• Rhythm rate: Becomes slow — approximately 2 times slower than the initial sinus rhythm.
• Physiological significance: The atrioventricular node possesses its own automaticity, but the frequency of impulse generation in it is lower. Note: sometimes this node can "turn on" independently some time after the application of the first ligature — this is called the "escape" phenomenon.

Third Stannius ligature (isolation of the apex)

The third ligature is applied to the lower third of the ventricles, isolating the apex of the heart. The previous ligatures remain. This ligature is once again isolating.

• Effect: Separation of the working (typical) myocardium of the apex from the conduction system.
• Result: The isolated apex does not contract, and a straight line is recorded on the myogram. In the remaining parts of the myocardium, very rare twitches occasionally occur due to the automaticity of Purkinje fibers.
• Physiological significance: Typical cardiomyocytes of the apex contain no pacemaker cells (P-cells). Without impulses from above, the apex has no automaticity.

Law of automaticity gradient

General conclusion of the experiment — the degree of automaticity decreases from the base of the heart to its apex:

1. Sinus-venous region possesses maximum automaticity (1st order pacemaker).
2. Atrioventricular region possesses moderate automaticity (2nd order pacemaker).
3. Heart apex lacks automaticity.

Class and exam questions

What else to study on this topic

• Mechanism of pacemaker overdrive suppression
• Pre-automatic pause: causes of occurrence
• Comparison of the conduction system of amphibians (Remak's and Bidder's ganglia) and humans
• The phenomenon of heart "escape" from the control of the sinus node
• Purkinje fibers as 3rd order pacemakers and their rhythm

Other topics in the "Cardiac Physiology" section

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