Hierarchy of Central Regulation
The autonomic nervous system is organized hierarchically. Lower levels possess a degree of autonomy and are responsible for local regulation of internal organs. Higher levels provide complex integration of autonomic responses with somatic and emotional processes.
Key structures controlling autonomic functions include:
- Hypothalamus. Receives information via two pathways: neural (via spinal afferent tracts from peripheral receptors) and humoral (directly through the blood). Hypothalamic nuclei have distinct functional specializations: excitation of the anterior group triggers parasympathetic responses, and the posterior group triggers sympathetic responses. Additionally, centers of biological motivation (such as fear or hunger) are located here, the activation of which is always accompanied by a powerful autonomic response.
- Limbic System. Due to its close functional connection with internal organs, it is often called the "visceral brain." This structure forms emotional states while directly controlling accompanying autonomic shifts.
- Cerebellum. Its main task in this context is the autonomic support of behavior. The cerebellum can exert both activating and inhibitory influences on internal organ function.
- Thalamus. Visceral signals are gathered and processed in its ventroposterior nucleus. The lateral part of the nucleus serves as the main collector of impulses coming from abdominal organs (via the celiac nerve), while the medial part contains the representation of the vagus nerve.
- Cerebral Cortex. The highest level of integration. The frontal cortex acts as a major center for autonomic innervation, regulating autonomic background during learning and complex movement execution. The temporal cortex contains motor centers controlling the heart and abdominal cavity. The celiac nerve projects to the somatosensory areas (zones SI and SII), while the pelvic nerve projects to zone SII and the anterior part of the cruciate sulcus.
Nature of Autonomic Tone
Autonomic nervous system centers are constantly in a state of tonic activity. This autonomic tone is maintained by three key factors:
- Spontaneous electrical activity of the nerve cells themselves.
- Continuous afferent input from both the somatic system and ANS receptors.
- Direct effects of circulating humoral biologically active substances in the blood (vitamins, hormones, oligopeptides).
A clear demonstration of constant sympathetic tone is Horner syndrome. In experimental physiology, it is modeled by cutting the postganglionic fibers of the superior and middle cervical sympathetic ganglia. Characteristic symptoms appear immediately on the affected side: pupil constriction (miosis), sinking of the eyeball (enophthalmos), and relaxation of the nictitating membrane (third eyelid). If low-frequency electrical stimulation is applied to the peripheral stump of the cut nerve, these symptoms completely disappear, confirming their tonic nature.
Methods for Assessing Autonomic Tone
Autonomic tone is a relatively stable characteristic of a person's current autonomic state. In clinical and physiological practice, calculated indices based on cardiovascular parameters are most commonly used to assess it.
Kerdö Vegetative Index (KVI) Allows determination of the balance between sympathetic and parasympathetic divisions using heart rate (HR) and diastolic blood pressure ($BP_d$). Calculation formula: $KVI = (1 - (BP_d / HR)) \times 100\%$.
- From -10% to +10%: Eutonia (autonomic balance).
- Greater than +10%: Sympathicotonia (sympathetic division predominates).
- Less than -10%: Vagotonia (parasympathetic division predominates).
Cardiointervalometry according to R.M. Baevsky (1992) This method is based on electrocardiogram analysis and calculation of the Stress Index (SI), which deeply reflects autonomic homeostasis. Calculation formula: $SI = AM_0 / (2 M_0 \times \Delta X)$.
In this formula:
- $M_0$ (Mode) — the most frequent R–R interval value in seconds.
- $AM_0$ (Mode Amplitude) — the frequency of occurrence of the mode (in %). This parameter directly reflects sympathetic tone.
- $\Delta X$ (Variation Range) — the difference between the longest and shortest cardiointerval. Characterizes parasympathetic activity.
Interpretation of the Stress Index (in conventional units):
- Less than 50: Vagotonia.
- 50 to 200: Normotonia.
- 200 to 500: Sympathicotonia.
- Greater than 500: Hypersympathicotonia (indicates critical overstrain of the body's regulatory systems).