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General Characteristics of the Autonomic Nervous System

Systema nervosum autonomicum

For medical students2 min readUpdated 2026-10-10

The autonomic nervous system (ANS) is a specialized division of the nervous system that regulates all visceral ("vegetative") body functions. It controls metabolism, tissue trophism, and growth, providing long-lasting and stable effects that are normally outside conscious control.

Target structuresSmooth muscles, glands, internal organs, blood vessels, and lymphatic vessels.

Three divisionsSympathetic, parasympathetic, and enteric (metasympathetic) nervous systems.

Highest centerThe hypothalamus serves as the primary subcortical center of autonomic regulation.

AutonomyANS activity is subject to voluntary control only to a very limited degree.

Main Functions and Features

The autonomic nervous system is responsible for maintaining homeostasis and regulating organ function. Unlike the somatic system, the ANS operates autonomously: signals from peripheral receptors are transmitted to central processing areas, but this information typically does not reach the level of consciousness.

Key features of the ANS include:

Broad target spectrum: The system controls smooth muscle, glandular tissue, visceral organs, and the vascular bed.
Additional influence: Beyond internal organs, the ANS partially modulates skeletal muscle activity and external genitalia.
Character of action: Autonomic effects are typically long-lasting and sustained.
Integration: During whole-body responses (such as stress adaptation), autonomic functions are closely integrated with somatic responses to form a unified adaptive reaction.

Morphofunctional Divisions and Innervation

Based on structural and functional characteristics, the ANS is traditionally divided into three major divisions:

Sympathetic — includes both central and peripheral structures.
Parasympathetic — also consists of central and peripheral parts.
Enteric (Metasympathetic) — located primarily within the walls of hollow visceral organs.

Effector fibers from these divisions innervate virtually all tissues. Most organs receive dual innervation (both sympathetic and parasympathetic). However, triple innervation also exists, involving the enteric division. A classic example is the muscular apparatus of the gastrointestinal tract, regulated by three mechanisms: adrenergic, cholinergic, and serotonergic.

Afferent Pathway and Neurotransmitters

The afferent limb of the ANS collects and transmits information from peripheral receptors to the central nervous system.

Fast-acting neurotransmitters operate at the terminals of these sensory pathways:

Excitatory amino acids: Primarily glutamate.
Peptides: Substance P, angiotensin II, arginine vasopressin, cholecystokinin, and bombesin.

Organization of Centers and the Monoaminergic System

A fundamental difference between the ANS and the somatic nervous system is that the ANS lacks a segmental organization. Its neurons are localized in specific regions of the central nervous system, forming autonomic centers.

Interaction among these centers follows a strict hierarchical principle. Levels of regulation (from lowest to highest):

Spinal level.
Bulbar level (medulla oblongata structures).
Mesencephalic level (midbrain).
Hypothalamus (the supreme subcortical center controlling all visceral functions).
Limbic system.
Cerebellum.
Cerebral cortex.

The monoaminergic system plays a special regulatory role. This network of brainstem neurons (serotonergic, dopaminergic, and noradrenergic) projects to the neocortex, thalamus, hypothalamus, and limbic structures. Through connections with the limbic system, it mediates emotional states. Certain sources highlight distinct specificities: serotonin is implicated in negative emotions, whereas norepinephrine mediates positive emotional states.

Mnemonic

Internal organs often have dual innervation: the sympathetic and parasympathetic systems act like the gas pedal and the brakes.

Frequently asked questions

Where are the central neurons of the sympathetic division of the ANS located?

Central (first, preganglionic) neurons of the sympathetic nervous system are located in the spinal cord.

Their cell bodies lie in the lateral horns (cornu laterale) of the thoracic and upper lumbar segments, from T1 to L2/L3.

Where are the centers of the parasympathetic nervous system located?

The centers of the parasympathetic nervous system are located in the brainstem and the sacral spinal cord.

The brainstem contains the following parasympathetic cranial nerve nuclei:

Edinger-Westphal nucleus (Accessory oculomotor nucleus) (CN III).
Superior salivatory nucleus (CN VII).
Inferior salivatory nucleus (CN IX).
Dorsal motor nucleus of vagus nerve (CN X).

In the spinal cord, sacral centers are localized in segments S2–S4.

What are the anatomical and functional differences between the autonomic and somatic nervous systems?

The main differences between the autonomic and somatic nervous systems involve control, innervation targets, center organization, and efferent pathway structure.

Feature	Somatic Nervous System	Autonomic Nervous System
Control	Through lower CNS connections to the cerebral cortex, an individual is conscious of stimuli and performs voluntary movements.	Functions predominantly autonomously; activity is regulated by consciousness only to a minor degree.
Innervation / Target Objects	Efferent CNS neurons innervate skeletal muscle; the somatic pathway leads directly to the muscle.	Regulates internal organs, smooth muscle, glands, blood, and lymphatic vessels; partially influences skeletal muscle and external genitalia.
Organization of Centers	Organized according to a segmental principle.	Lacks segmental organization; neurons reside in specialized CNS regions, forming autonomic centers.
Efferent Pathway	Efferent cell bodies lie in the anterior horns; the pathway to the muscle is direct without peripheral ganglia.	Two-neuron efferent pathway: a central neuron sends a preganglionic fiber to an autonomic ganglion, and a ganglionic neuron sends a postganglionic fiber to the effector organ.
Effector Neuron Localization	Effector cell bodies for skeletal muscle innervation are located within the CNS.	Effector neuron cell bodies are always located outside the CNS—within autonomic ganglia.

Which brain region serves as the highest center of the ANS?

The hypothalamus is the highest subcortical center that directly directs all autonomic functions of the body.

Which neurotransmitters function in the afferent pathway of the ANS?

Information from peripheral receptors is transmitted by fast-acting neurotransmitters such as glutamate and various peptides (substance P, cholecystokinin, bombesin, and angiotensin II).

What is the function of the monoaminergic system?

It connects the brainstem with the cortex and limbic structures, participating in the formation of emotional states via the release of dopamine, norepinephrine, and serotonin.

Go deeper

Mechanisms of forming autonomic components in integrated whole-body reactions
Specifics of sympathetic and parasympathetic innervation (dual innervation)
Role of the enteric division in triple innervation of GI tract musculature
Connection between the monoaminergic system and emotion generation