Intracardiac nervous regulation
The complex system regulating heart contractions begins at the level of the organ itself. Intracardiac nervous regulation directly depends on hemodynamic parameters, primarily on the degree of filling of the heart chambers with blood, which is determined by venous pressure. Depending on how strongly the heart chambers are stretched by the incoming venous blood, the rhythm and force of contractions are finely tuned. An important feature of this local mechanism is the ability to switch between different mediator systems, allowing the myocardium to rapidly adapt to changing blood-filling conditions.
Influence of the vagus nerves (nervus vagus)
The parasympathetic nervous system exerts a powerful inhibitory effect on the heart muscle. This influence is mediated through the vagus nerves (n. vagus), the fibers of which innervate predominantly the atria.
Mechanism of action of the vagus nerve:
- Decrease in excitability: N. vagus inhibits the activity of the main pacemakers — cells of the sinoatrial (SA) and atrioventricular (AV) nodes.
- Ionic mechanism: The inhibitory effect is based on changes in ionic permeability. Under parasympathetic stimulation, the permeability of cardiomyocyte cell membranes to calcium ions ($Ca^{2+}$) decreases.
- Electrophysiological effect: The reduction in calcium influx leads to a decrease in the rate of slow diastolic depolarization — a process critical for generating spontaneous action potentials in pacemaker cells.
As a result of these cellular and ionic changes, a number of negative effects occur in the heart: negative chronotropic (rate reduction), dromotropic (conduction slowing), as well as inotropic, bathmotropic, and lusitropic effects.
Cardiac «escape» phenomenon
An interesting physiological feature is the so-called cardiac «escape» phenomenon from the influence of the vagus nerves. The essence of this phenomenon is that with continuous, rhythmic stimulation of the vagus nerves, their initial inhibitory effect on the myocardium gradually weakens and eventually disappears completely. As a result, the heart resumes its contractions despite the ongoing stimulation of n. vagus.
Why does «escape» occur? Physiologists identify four main causes of this protective mechanism:
- Reflex excitation of the sympathetic nervous system, which begins to counteract parasympathetic inhibition.
- Physical depletion of mediator (acetylcholine) stores in the presynaptic terminals of the vagus nerves.
- Decreased sensitivity of myocardial cholinergic receptors to the released mediator (desensitization).
- Active production of specific anticholinergic substances by the cardiomyocytes themselves, blocking the action of acetylcholine.
Humoral regulation
In addition to neural mechanisms, humoral regulation of heart function plays a crucial role. It is carried out not through nerve impulses, but via chemical substances delivered to the myocardial cells. These substances are delivered exclusively through the blood of the coronary vessels supplying the heart muscle itself. Hormones, various ions (including the aforementioned calcium), and other biologically active substances (BAS) act as regulators. It is the blood of the coronary vessels that serves as the medium through which humoral factors reach cardiomyocytes and alter their operating parameters.
Briefly: answers to topical questions
Which specific hormones are involved in the humoral regulation of cardiac activity?
Sources indicate that humoral regulation of heart function is carried out through the blood of coronary vessels by hormones, ions, and BAS.
Specifically mentioned are:
- 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.
Also, the mediators acetylcholine and noradrenaline are mentioned: they exert opposite effects on the heart.
What is the complete ionic mechanism of action of the vagus nerve on pacemaker cells?
The ionic mechanism of action of the vagus nerves (n. vagus) on the heart consists of decreasing the permeability of cardiomyocyte cell membranes to calcium ions (Ca²⁺).
Electrophysiologically, this leads to a decrease in the rate of slow diastolic depolarization and a reduction in the excitability of sinoatrial (SA) and atrioventricular (AV) node cells, resulting in negative cardiac effects (chronotropic, dromotropic, and others).
What is the mechanism of intracardiac heterometric regulation (Frank-Starling law)?
The mechanism of heterometric regulation (Frank-Starling law) consists of regulating the force of heart contraction by changing the length of cardiomyocytes.
The essence of the law: the more blood flows to the heart during diastole, the more the heart muscle fibers (myocardium) are stretched, and the stronger it contracts during the next systole. This leads to an increase in cardiac output. With low blood filling, actin and myosin filaments are in a suboptimal position, leading to weak contraction.
What is the effect of the sympathetic nervous system on heart function?
Sympathetic nerves innervate the entire heart and, when excited, enhance cardiac activity.
Mechanisms of action of the sympathetic nervous system include:
- acceleration of slow diastolic depolarization (SDD) in sinus node cells;
- increase in the excitability of typical and atypical cardiomyocytes;
- increase in $Ca^{2+}$ ion influx into cardiomyocytes.
Effects:
- positive chronotropic;
- positive bathmotropic;
- positive dromotropic;
- increase in the duration of ventricular cardiomyocyte action potentials (APs).
To easily remember the four causes of the «escape» phenomenon, use the mnemonic: Sympathetic (reflex excitation), Depletion (of acetylcholine stores), Sensitivity (decrease in receptors), Production (of anticholinergic substances).
Class and exam questions
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 decrease in the rate of slow diastolic depolarization.
What is the essence of the cardiac escape phenomenon?
With 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?
The influence of the vagus nerve decreases the excitability of sinoatrial (SA) and atrioventricular (AV) node cells while innervating predominantly 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 through the blood of the coronary vessels.
What else to study on this topic
- Negative chronotropic and dromotropic effects: mechanisms
- Mechanism of slow diastolic depolarization in nodes
- Switching of intracardiac neurotransmitter systems
- Relationship between venous pressure and heart chamber filling
- Anticholinergic substances produced by cardiomyocytes
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