Central regulation of autonomic functions and autonomic tone

Hierarchy of central regulation

The autonomic nervous system is organized hierarchically. Lower levels possess a certain degree of autonomy and are responsible for the local regulation of internal organs. Higher levels provide complex integration of autonomic responses with somatic and emotional processes.

Main structures controlling autonomic functions:

• Hypothalamus. It receives information via two pathways: neural (through spinal afferent tracts from peripheral receptors) and humoral (directly via the blood). The hypothalamic nuclei have a clear functional specialization: stimulation of the anterior nuclear group triggers parasympathetic responses, while stimulation of the posterior group triggers sympathetic responses. In addition, it houses centers of biological motivations (such as fear or hunger), 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 shapes emotional states, directly driving the accompanying autonomic shifts.
• Cerebellum. Its main task in this context is the autonomic support of behavior. The cerebellum is capable of exerting both activating and inhibitory influences on the work of internal organs.
• Thalamus. Its posteroventral nucleus is the site of collection and processing of visceral signals. The lateral part of the nucleus serves as the primary collector of impulses originating from abdominal organs (via the celiac nerve), while the representation of the vagus nerve is located in the medial part.
• Cerebral cortex. The highest level of integration. The frontal cortex acts as the primary center for autonomic innervation, regulating autonomic tone during learning and the execution of complex movements. 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

Centers of the autonomic nervous system are constantly in a state of tonic activity. This autonomic tone is maintained by three key factors:

1. Spontaneous electrical activity of the nerve cells themselves.
2. Continuous afferent impulsation arriving both from the somatic system and from the receptors of the ANS itself.
3. Direct action of humoral biologically active substances circulating in the blood (vitamins, hormones, oligopeptides).

A clear demonstration of the existence of constant sympathetic tone is Horner's syndrome. In experimental physiology, it is modeled by transecting the postganglionic fibers of the superior and middle cervical sympathetic ganglia. Characteristic symptoms immediately appear on the side of the lesion: pupillary 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 end of the transected nerve, these symptoms disappear completely, confirming their tonic nature.

Methods for assessing autonomic tone

Autonomic tone is a relatively stable characteristic of a person's current autonomic parameters. In practice, calculated indices based on cardiovascular system parameters are most commonly used to assess it.

Kerdo's vegetative index (KVI) Allows determining the balance between the sympathetic and parasympathetic systems 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).
• More than +10%: Sympathicotonia (sympathetic division prevails).
• Less than -10%: Vagotonia (parasympathetic division prevails).

Cardiointervalometry according to R.M. Baevsky (1992) The method is based on electrocardiogram analysis and calculation of the Stress Index (SI), which deeply reflects the state of 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 occurrence rate of the mode (in %). This indicator directly reflects the tone of the sympathetic division.
• $\Delta X$ (Variation range) — the difference between the longest and shortest cardiointerval. Characterizes the activity of the parasympathetic division.

Evaluation of the Stress Index (in arbitrary units):

• Less than 50: Vagotonia.
• From 50 to 200: Normotonia.
• From 200 to 500: Sympathicotonia.
• Over 500: Hypersympathicotonia (indicates critical overstrain of the body's regulatory systems).

Class and exam questions

What else to study on this topic

• Mechanisms of humoral influence on the hypothalamus: the role of specific oligopeptides and hormones in tone formation.
• Detailed topical anatomy of autonomic cortical centers: somatosensory areas SI and SII.
• The connection of biological motivations (hunger, fear) with the activity of hypothalamic nuclei.
• Pathophysiology of hypersympathicotonia and exhaustion of regulatory systems according to Baevsky.

Other topics in the «Autonomic Nervous System» section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.