Physiological significance of heart rate and its control
The activity of the heart is caused by automatic excitations formed in the sinoatrial node. Cardiac activity changes under different circumstances (when the position of the body changes, during physical work, during fatigue or emotions), because various internal and external influences also act on the creation of excitement. Changes in heart activity tend to be purposeful and thus adapt it to the needs of the organism.
The heart itself creates rhythmically repetitive impulses. Under normal circumstances, these impulses are generated by specialized cells of the cardiac conduction system. Impulse arises in the sinoatrial node, from where it spreads to the atrioventricular node, His bundle, Tawar's arms, and then to the Purkinje fibers, which transfer the impulse to the working myocardial ventricles. All cells of the cardiac conduction system are endowed with the ability to generate impulses, but the rhythm given by the sinoatrial node is normally the fastest and therefore affects hierarchically subordinate structures whose rhythm does not apply. Under certain circumstances, the lower sections of the transmission system may take on the role of source of impulse. However, their spontaneous diastolic depolarization is slower than in the sinoatrial node, and so the frequency of excitation is slower. The frequency indicated by the sinoatrial node, the main pacemaker, is about 70 pulses per minute, the frequency of the atrioventricular node is 40 - 60 pulses per minute and the His beam - the so-called tertiary pacemaker has a frequency of 30 - 40 pulses per minute.
Autonomous heart rate control
The heart is efferently innervated by parasympathetic nerve fibers (vagus nerve) and sympathetic nerve fibers (cardiaci nerves). By sympathetic stimulation, cardiac output can be increased by more than 100%. Conversely, vagal stimulation can reduce cardiac output to almost zero. Maximum stimulation of the vagus nerve stops the heart and causes so-called vagal death.
Influence of parasympathetic on heart rate control
The main goal of innervation of the right vagus nerve is the sinoatrial node, on which it has a negative chronotropic effect, i.e. it reduces the heart rate. The efferent fibers of the left vagus preferentially act on the atrioventricular node, where they slow down conduction and are thus negatively dromotropic. The vagal fibers are distributed mainly into the atria, but not into the ventricles, in which the force of contraction of the heart is created. Therefore, vagal stimulation reduces the heart rate rather than the strength of the heartbeat.
Influence of sympathetic on heart rate control
The sympathetic affects the whole heart. It has a positive chronotropic effect on the sinoatrial node (accelerates the heart rate). Outside the transmission system, it has a positive inotropic effect on the working myocardium (strengthens the contraction of the heart).
Both efferent systems are active at rest, but vagotonia generally predominates. According to Schmidt (1993), blockade of both systems increases the heart rate in a young adult from approximately 70 to 105 beats per minute. This is therefore a frequency free from the influence of sympathetic and parasympathetic. During physical activity, the tone of the sympathetic nervous system increases and at the same time the tone of the vagus decreases and vice versa.