Automatism and excitation conduction
A crucial physiological property of the heart is automatism. The organ is capable of generating impulses and contracting without external neural stimuli. This process is driven by slow diastolic depolarization (SDD) of membranes occurring in pacemaker cells of the sinoatrial node.
Unlike skeletal muscle, the law of isolated conduction of excitation does not apply to the heart. Cardiomyocytes are integrated into a single functional syncytium through specialized gap junctions known as nexus junctions. Thanks to them, excitation is transmitted instantaneously from cell to cell.
It is precisely the presence of nexus junctions that causes the myocardium to obey the "all-or-none" law:
- With a subthreshold stimulus, the muscle does not respond ("none").
- With a threshold or suprathreshold stimulus, all cells without exception are excited, and the heart responds with the maximum possible contraction ("all").
Note: Skeletal muscle does not obey this law because each of its fibers has its own threshold, and the overall force depends on the number of recruited fibers. Furthermore, a new myocardial contraction is possible only after the complete completion of the previous one, which is associated with a prolonged refractory period.
Regulation of contraction force
In every cardiac cycle, there are phases of isometric contraction and isometric relaxation that are unique to the myocardium. The force and speed of the contractile act are flexibly regulated by several intracardiac mechanisms.
The first mechanism is the chronoinotropic relationship (Bowditch staircase phenomenon). As the heart rate increases, the force, speed of contraction, and speed of subsequent myocardial relaxation naturally increase, even if the initial fiber length remains unchanged.
The second mechanism is based on the Starling's law, according to which the force of contraction depends on the degree of stretch of the muscle fibers.
- Weak contraction occurs with inadequate blood filling or a small load (e.g., during an early extrasystole), since the actin and myosin filaments are in a suboptimal position.
- Post-extrasystolic potentiation demonstrates the opposite effect. If a compensatory pause occurs after a ventricular extrasystole, the heart fills excessively with blood. The myocardium is strongly stretched, leading to an anomalously powerful subsequent contraction.
Hemodynamic self-regulation
The baseline rhythm integrates the heart's function, automatically adapting it to the body's current needs. The main task is to maintain an ideal balance of blood flow where blood volumes in the systemic and pulmonary circulations are equal.
Coordination of the power of the right and left halves of the heart occurs exclusively through hemodynamic self-regulation, completely without the involvement of the nervous system.
This fact was brilliantly proven in a physiological experiment using two isolated hearts. During the experiment, the left and right halves of an animal's heart were replaced with two separate donor hearts (one working for the systemic circulation, the other for the pulmonary). Initially, these organs had different automatism, different initial stroke volumes, and asynchronous systoles. However, over time, self-regulation mechanisms caused the cardiac output (CO) passing through both organs to equalize and become completely identical.
Metabolism and oxygen supply
The myocardial energy balance is maintained through the oxidation of three main substrates: glucose, free fatty acids (FFAs), and lactic acid.
- At rest, the heart uses glucose, FFAs, and lactic acid in approximately equal proportions, with the role of lactic acid being minimal.
- During physical exertion, a key feature becomes the active utilization of lactic acid supplied by skeletal muscles. The energy consumption structure changes cardinally:
- 2/3 of energy is provided by the oxidation of lactic acid (the main source);
- 1/5 of energy — oxidation of FFAs;
- 1/7 of energy — oxidation of glucose.
Myocardial blood supply has strict phasic characteristics: blood flows through the coronary vessels only during diastole. During systole, the powerfully contracting myocardium compresses the arteries, blocking the delivery of oxygen from hemoglobin. This problem is solved thanks to the intracellular protein myoglobin. During diastole, it binds and stores oxygen, and during systole, it releases it, ensuring the continuity of oxidative reactions in cardiomyocytes.
Briefly: answers to topical questions
What ionic currents provide slow diastolic depolarization (SDD) in sinoatrial node cells?
Slow diastolic depolarization in sinoatrial node cells is provided by inward sodium and calcium currents against the background of a decreasing outward potassium current.
- Sodium current (Na⁺) — a constant slow inward current through specific If-channels ("funny" currents) initiating the process.
- Calcium current (Ca²⁺) — a slow inward current that increases at the end of SDD and accelerates spontaneous depolarization.
- Potassium current (K⁺) — the membrane permeability for these ions gradually decreases during diastole, leading to the depolarization of the membrane potential.
What phases does the action potential of a typical working cardiomyocyte consist of?
The action potential of a typical working cardiomyocyte consists of five sequential phases.
- Phase zero — depolarization.
- Phase one — initial rapid repolarization.
- Phase two — slow repolarization (plateau).
- Phase three — final rapid repolarization.
- Phase four — resting potential.
To remember myocardial energy supply during exertion, use the rule of decreasing fractions: "Milk, Fat, Sugar" (Lactic acid — 2/3, Free fatty acids — 1/5, Glucose — 1/7).
Class and exam questions
Why does the heart muscle contract according to the "all-or-none" law?
Due to the presence of nexus junctions (gap junctions), cardiomyocytes form a functional syncytium. Threshold excitation is instantaneously transmitted to all cells, causing their simultaneous contraction with maximum force.
How does the myocardium receive oxygen during systole if the coronary vessels are compressed?
The oxidation process during systole is supported by oxygen released from the intracellular protein myoglobin. The storage of oxygen by myoglobin occurs exclusively during diastole.
What is the essence of the Bowditch staircase phenomenon?
This is the chronoinotropic relationship: when heart rate increases, not only the force but also the speed of contraction, as well as the rate of relaxation of the myocardium, increase.
Does the nervous system participate in equalizing cardiac output between the circulations?
No, the coordination of the power of the right and left ventricles to maintain equal blood flow in the systemic and pulmonary circulations is ensured solely by mechanisms of hemodynamic self-regulation.
What else to study on this topic
- Mechanisms of formation of slow diastolic depolarization (SDD)
- Physiological role of the prolonged refractory period
- Significance of the phases of isometric contraction and relaxation in the cycle
- Detailed analysis of hemodynamic parameters in the two-isolated-hearts experiment
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