Complex Control Systems
Besides the central nervous system (CNS), the autonomic nervous system (ANS) is the most important neuronal control unit in the organism. Its primary function is to adapt the body's internal environment to external and internal stress factors (stimuli) and to maintain a stable equilibrium within the organism.
The peripheral ANS is integrated in a complex system. It is not only connected to the brain stem, but also to the hypothalamus and other parts within the CNS.
The Autonomic Nervous System (ANS) With Sympathetic and Parasympathetic Nervous System
The peripheral part of the ANS essentially consists of the sympathetic and parasympathetic nervous systems. The sympathetic nervous system arises from the thoracic medulla and from the upper three segments of the lumbar spine and is therefore also called the thoracolumbal system. The parasympathetic nervous system arises from the brain stem and from the sacral spine and is therefore also called the craniosacral system.
Normal Values for Heart Rate Variability (HRV)
The sinu-atrial node is located on the inside of the rear wall of the right atrium. The impulses it generates are transmitted via the muscle cells to the atrioventricular node (AV node) and from there to the ventricles. The sinu-atrial node is the fastest and thus the primary pacemaker of the heart, with an intrinsic natural frequency of 80 to 120 beats per minute.
Modulation of Heart Rate by the Sympathetic and Parasympathetic Nervous Systems
The heart muscle is innervated by both sympathetic and parasympathetic components. The sympathetic nervous system exerts a rate-raising (positive chronotropic) effect, while the parasympathetic system exerts a rate-lowering (negative chronotropic) effect. A heart rate lower than the intrinsic excitation frequency of the sinoatrial node (80-120) indicates that the parasympathetic system is dominant; if higher, the sympathetic is dominant.
The signals from the baroreceptors reach the nucleus tractus solitarii (NTS) in the brainstem. From there the signals make their way to the nucleus ambiguus and to the rostral and caudal ventrolateral medulla, each of which send excitatory and inhibitory pulses to the heart and blood vessels in order to control arterial blood pressure.
Baroreflex / Baroreceptors - Effects on the HRV
Baroreflex activity and respiratory sinus arrhythmia (RSA) are considered the key mechanisms that influence HRV. Baroreflex control serves to permanently maintain an (median) arterial blood pressure adequate for supplying blood to all organ systems. The receptors located in the aortic arch and in the carotid sinus are extremely sensitive and respond to minimal changes in pressure. During periods of physical exertion or exercise, the sensitivity threshold shifts in order to adapt to the increased oxygen requirement. A change to this threshold caused by permanent sympathetic stimulation plays a significant role in the development of hypertension.
RSA - Respiratory Sinus Arrhythmia
The dependence of the heart rate on respiration is known as respiratory sinus arrhythmia (RSA):
- There is an increase in the heart rate during inspiration
- There is a decrease in the heart rate during expiration
The RSA is above all communicated by the changing activity of the vagus nerve.
Influences on the respiration-dependent heart rate variability:
- Pulmonary, vascular and cardiac stretch receptors
- Respiratory centers in the brain stem
- Different baroreflex sensitivity in the respective phases of the respiratory cycle
Due to an inspiratory vagal inhibition, fluctuations in the heart rate result at the same frequency as respiration. The inspiratory inhibition is primarily caused by the influence of the medullar respiratory center on the medullar cardiovascular center. In addition, peripheral reflexes are responsible due to hemodynamic changes and thoracic stretch receptors.
Respiratory Sinus Arrhythmia - A Resonance Phenomenon
The resonance phenomenon describes the superpositioning of what in this case are biological oscillations. The physiological respiratory sinus arrhythmia describes a frequency range of approximately 0.3 Hz.
Rhythmic breathing at a frequency of 6 breaths per minute causes this frequency to shift to the range of around 0.1 Hz, the same range in which, for example, the base frequency of baroreflex integration is found. This induces a change to the HRV that can be depicted directly in the analysis.
Clearly seen in the rhythmogram in the left chart: HRV during "normal" breathing from left to center of rhythmogram, then HRV with controlled rhythmic respiration
Over the long term, this respiratory modulation works to strengthen baroreflex control and improve parasympathetic activity.