Conduction system of the heart
Uninterrupted and rhythmic cardiac activity is ensured by two key factors: the function of the conduction system and the contractility of the myocardium itself. First, an impulse is rhythmically generated, then it is conducted to all cells, and only after that does contraction begin (excitation always strictly precedes muscle contraction).
The structural basis for impulse generation consists of poorly differentiated atypical muscle fibers. Their main functional feature is that they can spontaneously depolarize and conduct this signal further, while being completely incapable of contraction.
The conduction system sequentially includes:
- Sinoatrial node (sinus, or sinuatrial node).
- Atrial conduction fibers.
- Atrioventricular node.
- Bundle of His.
- Purkinje fibers.
Topography and pathway of the cardiac impulse
Every cardiac contraction is the result of a strictly ordered propagation of an electrical signal through the structures of the conduction system.
- Start in the sinoatrial node (SAN): This node is located beneath the epicardium, in the area between the right atrial appendage and the entrance of the superior vena cava. This is where excitation initially arises, making the SAN the primary focus of automatism (pacemaker).
- Atrial involvement: From the sinus node, the signal spreads through the atrial conduction fibers. It excites typical (contractile) atrial cardiomyocytes and simultaneously travels to the atrioventricular node.
- Transition to the ventricles: From the AV node, excitation descends to the ventricles via the bundle of His. Upon reaching the interventricular septum, the bundle of His divides into two branches — the right and left bundle branches.
- Finish: Along the bundle branches, the signal passes to the terminal Purkinje fibers. From there, excitation is transmitted to typical ventricular cardiomyocytes, triggering their powerful contraction.
Automatism gradient and cell hierarchy
Different parts of the conduction system possess varying capacities for spontaneous impulse generation. This phenomenon is termed the automatism gradient. The direction of this gradient is always decreasing: from the base of the heart to its apex.
The hierarchy is structured as follows:
- Maximum automatism: sinus node cells.
- Intermediate automatism: atrioventricular node cells.
- Minimum automatism: remaining conducting cardiomyocytes.
Interestingly, typical cardiomyocytes do not normally depolarize on their own, yet they possess potential automatism. This is easily demonstrated in tissue culture experiments: isolated contractile myocardial cells are capable of beating spontaneously, each at its own rhythm. However, if these cells are physically joined, they immediately begin to contract synchronously.
A strict synchronization principle applies: the overall rhythm is always set by the cell with the highest excitation frequency. Its signal manages to spread to neighboring cells before they can generate their own impulse. It is through this mechanism that the sinus node suppresses the automatism of all downstream structures.
Electrophysiological mechanism of automatism
The secret of automatism lies in the specific properties of sinus node cells (so-called P-cells). Their main feature is the complete absence of a stable resting membrane potential.
Ion transport processes constantly occur within pacemaker cells, gradually reducing the resting potential. Slow diastolic depolarization (SDD) develops on the outer membrane — a spontaneous and continuous decrease in membrane potential.
When this drop reaches the critical level of depolarization (about –40 mV), there is a sharp decrease in the threshold of excitation and a surge in excitability. As a result, a full-fledged action potential is generated, which becomes that spontaneous impulse.
Pathology: AV nodal automatism
If an impulse fails to arise in the sinus node for any reason or is blocked on its way to the AV node, a backup scenario is triggered. The atrioventricular node takes over the pacemaker function.
Such a escape rhythm has a number of serious features and consequences:
- The frequency of spontaneous excitations of the AV node is exactly half that of the sinus node.
- Excitation begins to spread in two directions simultaneously: physiologically (downward to the ventricles) and retrogradely (backward to the atria).
- Due to retrograde conduction, the atria and ventricles contract simultaneously rather than sequentially, as is normally the case.
The hemodynamic outcome of this condition is a sharp drop in the efficiency of the heart's pumping function, which is characteristic of various pathological states.
Briefly: answers to topical questions
Which ion currents provide slow diastolic depolarization in P-cells?
Slow diastolic depolarization (SDD) in pacemaker cells is provided by inward sodium and calcium currents against the background of decreased potassium permeability of the membrane.
- Decreased permeability — during diastole, membrane permeability to potassium gradually decreases.
- Sodium current — a constant slow inward sodium current passes through specific If-channels (so-called "funny" currents).
- Calcium current — the influx of calcium into the cell is initiated by the preceding sodium current, and at the end of SDD, an additional increase in the inward sodium and calcium currents accelerates depolarization.
What is Stannius's automatism gradient and by what experiments is it proven?
The automatism gradient is the varying degree of automatism across the parts of the conduction system, where its expression decreases from the base of the heart to its apex. It is proven by the experiment of applying Stannius ligatures to an exposed frog heart with myogram recording.
- First ligature — isolates the sinoatrial node, leading to cardiac arrest.
- Second ligature — applied in the area of the atrioventricular node, causes its irritation and resumes ventricular contractions at a slower rhythm.
- Third ligature — isolates the apex of the heart, while its contractions are absent due to the lack of automatism.
Remembering the impulse pathway is simple: "SAN atria AVN bundle branches Purkinje" (Sinoatrial node → atria → Atrioventricular node → bundle of His → bundle branches → Purkinje fibers).
Class and exam questions
What is the function of the pacemaker?
The pacemaker primarily generates excitation and imposes its high rhythm on the entire heart, thereby suppressing the automatism of all downstream parts of the conduction system.
What is an objective measure of automatism?
The measure of automatism is the frequency of excitation generation at the site where spontaneous impulses originate.
Can ordinary cardiomyocytes contract spontaneously?
In a healthy intact heart — no. However, in experimental tissue culture, isolated typical cardiomyocytes demonstrate potential automatism and contract, each at its own rhythm.
What else to study on this topic
- Analysis of electrical activity graphs of atypical cardiomyocytes (P-cells)
- Ionic mechanisms of slow diastolic depolarization
- Specifics of ion transport reducing the resting potential to –40 mV
- Detailed hemodynamic changes during simultaneous contraction of the heart chambers
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