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Regulation of Cardiac Activity

Regulatio activitatis cardiacae

For medical students2 min readUpdated 2026-10-10

Regulation of cardiac activity is a complex of neurohumoral mechanisms ensuring the adequate function of the organ. It includes intracardiac processes, the influence of parasympathetic nerves, and the action of biologically active substances circulating in the coronary vessels.

Intracardiac controlHighly dependent on chamber filling and venous pressure
Vagus nerveDecreases the excitability of the sinuatrial and atrioventricular nodes
Humoral regulationDelivered via the blood of the coronary vessels
Ionic mechanismDecreased membrane permeability for calcium ions

Intracardiac Neural Regulation

A complex system regulating cardiac contractions begins directly at the organ level. Intracardiac neural regulation depends directly on hemodynamic parameters, primarily the degree of cardiac chamber filling, which is determined by venous pressure. Depending on how much the heart chambers are stretched by incoming venous blood, the rate and force of contractions are finely tuned. An important feature of this local mechanism is the ability to switch between different neurotransmitter systems, allowing the myocardium to rapidly adapt to changing conditions of blood filling.

Effects of the Vagus Nerves (*nervus vagus*)

The parasympathetic nervous system exerts a powerful inhibitory effect on the heart muscle. This influence is mediated via the vagus nerves (n. vagus), whose fibers predominantly innervate the atria.

Mechanism of vagal action:

As a result of these cellular and ionic changes, the heart exhibits several negative effects: negative chronotropic (reduced heart rate), dromotropic (slower conduction), as well as inotropic, bathmotropic, and lusitropic effects.

The Heart «Escape» Phenomenon

An interesting physiological feature is the so-called heart escape phenomenon from vagal inhibition. This phenomenon means that during continuous, prolonged rhythmic stimulation of the vagus nerves, their initial inhibitory effect on the myocardium gradually weakens and eventually disappears entirely. As a result, the heart resumes its contractions despite ongoing stimulation of n. vagus.

Why does escape occur? Physiologists identify four main causes for this protective mechanism:

  1. Reflex excitation of the sympathetic nervous system, which begins to counteract parasympathetic inhibition.
  2. Physical depletion of neurotransmitter (acetylcholine) stores in the presynaptic terminals of the vagus nerves.
  3. Decreased sensitivity of myocardial cholinergic receptors to the released neurotransmitter (desensitization).
  4. Active production of specific anticholinergic substances by the cardiomyocytes themselves, blocking the action of acetylcholine.

Humoral Regulation

In addition to neural mechanisms, humoral regulation plays a critical role in heart function. It is carried out not via nerve impulses, but through chemical substances delivered to myocardial cells. These substances are transported exclusively through the blood of the coronary vessels supplying the heart muscle itself. The regulators include hormones, various ions (including the aforementioned calcium), and other biologically active substances (BAS). Coronary blood flow serves as the medium through which humoral factors reach cardiomyocytes and alter their operational parameters.

Mnemonic

To easily remember the four causes of the heart escape phenomenon, use the phrase «Sympathetic Exhaustion Feels Withdrawal»: Sympathetics (reflex excitation), Exhaustion (of acetylcholine stores), Sensitivity (decrease in receptors), Withdrawal/Production (of anticholinergic substances).

Frequently asked questions

Which specific hormones participate in the humoral regulation of cardiac activity?

Sources indicate that humoral regulation of heart function is mediated via coronary blood flow by hormones, ions, and BAS.

Specifically named:

  • Adrenaline — increases heart rate and contraction force.
  • Thyroxine — stimulates heart function, metabolic processes, and increases myocardial sensitivity to adrenaline.
  • Glucagon — exerts a positive inotropic effect.

Additionally, neurotransmitters acetylcholine and noradrenaline exert opposing effects on the heart.

What is the complete ionic mechanism of vagal action on pacemaker cells?

The ionic mechanism of vagal (n. vagus) action on the heart involves decreasing the permeability of cardiomyocyte cell membranes to calcium ions ($Ca^{2+}$).

Electrophysiologically, this leads to a reduction in the rate of slow diastolic depolarization and decreased excitability of the sinuatrial (SA) and atrioventricular (AV) node cells, resulting in negative cardiac effects (chronotropic, dromotropic, etc.).

What is the mechanism of intracardiac heterometric regulation (Frank-Starling law)?

Heterometric regulation (the Frank-Starling law) regulates the force of cardiac contraction in response to changes in cardiomyocyte length.

Core principle: the more blood flows into the heart during diastole, the more the heart muscle fibers (myocardium) are stretched, and the stronger they contract during the next systole. This leads to an increase in stroke volume. With low filling, actin and myosin filaments are in a suboptimal overlap position, leading to weaker contractions.

What influence does the sympathetic nervous system exert on the heart?

Sympathetic nerves innervate the entire heart and, when excited, enhance cardiac activity.

Mechanisms of sympathetic nervous system action include:

  • acceleration of slow diastolic depolarization (SDD) in sinus node cells;
  • increased excitability of typical and atypical cardiomyocytes;
  • increased entry of $Ca^{2+}$ ions into cardiomyocytes.

Effects:

  • positive chronotropic;
  • positive bathmotropic;
  • positive dromotropic;
  • increased duration of action potentials (AP) in ventricular cardiomyocytes.
How does the vagus nerve affect the ion permeability of cardiomyocytes?

The vagus nerve decreases the permeability of cardiomyocyte cell membranes to calcium ions ($Ca^{2+}$). This, in turn, leads to a reduction in the rate of slow diastolic depolarization.

What is the core of the heart escape phenomenon?

During prolonged and rhythmic stimulation of the vagus nerves, their inhibitory effect on the heart weakens and disappears. The heart begins to contract again despite ongoing stimulation.

Which nodes of the cardiac conduction system are inhibited by the vagus nerve?

Vagal influence decreases the excitability of cells in the sinuatrial (SA) and atrioventricular (AV) nodes, while primarily innervating the atria.

How do biologically active substances reach cardiomyocytes during humoral regulation?

Humoral regulation of the heart by hormones, ions, and BAS is carried out directly via the blood of the coronary vessels.

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