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Physiology of the Sympathetic Nervous System

Q: Which higher autonomic centers regulate the activity of the sympathetic nervous system?

The key higher autonomic center is the hypothalamus. Activity within its associated mechanisms can also be modulated by influences from the limbic system and cerebellum. Hypothalamus — the primary autonomic center coordinating the body's response; corticotropin-releasing hormone participates in central regulation of autonomic functions, including activation of the sympathetic ANS.Limbic system — activates neurosecretory cells of the hypothalamus, influencing the regulation of corticotropin-releasing hormone synthesis.Cerebellum — participates in the regulation of visceral functions via connections with autonomic hypothalamic centers; its neurons evaluate the state of internal organs and help adapt organ activity to current metabolic demands.

Pars sympathica systematis nervosi autonomici

For medical students2 min readUpdated 2026-10-10

The sympathetic division of the autonomic nervous system is responsible for mobilizing the body's resources for active interaction with the environment, executing the "fight-or-flight" response. The system exerts a powerful generalized effect by activating catabolic processes, stimulating heart function, and suppressing digestive functions.

LocalizationLateral horns of the spinal cord from the 1st thoracic (Th1) to the 2nd–3rd lumbar (L2–L3) segments

ConductionPreganglionic (slow B-fibers) and postganglionic (C-fibers)

NeurotransmittersIn ganglia — acetylcholine; at effectors — norepinephrine (90%)

DivergenceOne axon branches to innervate 10–30 ganglionic neurons

Structural and Functional Organization

The sympathetic nervous system has a distinct hierarchical structure. The bodies of preganglionic neurons are located in the gray matter of the lateral horns of the spinal cord from the Th1 to L2–L3 segments.

Their axons are myelinated B-fibers with a slow conduction velocity. They travel to peripheral autonomic ganglia, which are divided into two types:

Paravertebral (chain) ganglia: located in chains along the sides of the vertebral column, forming the right and left sympathetic trunks. They are interconnected by longitudinal and transverse (commissural) branches, as well as with spinal nerves to supply skeletal muscle, blood vessels, and glands.
Prevertebral ganglia: located further from the spine, though still at a significant distance from the target organs (e.g., heart or stomach). Fibers pass through the sympathetic trunk to reach them in transit.

A crucial feature of the sympathetic division is divergence (multiplication). A single preganglionic axon forms synapses with 10–30 ganglionic neurons, ensuring a broad, generalized spread of excitation. The ganglia themselves are protected from circulating blood-borne substances by a specialized blood-ganglion barrier.

Neurotransmitters and Receptor Apparatus

Signal transmission in the sympathetic nervous system occurs in two stages and utilizes different neurotransmitters.

In ganglia (preganglionic synapses): acetylcholine is released, binding to nicotinic cholinergic receptors (N-receptors) and producing a fast excitatory postsynaptic potential (EPSP). Slow inhibitory postsynaptic potentials (IPSPs), mediated by dopamine released from small interneurons, are also recorded in ganglia.
At target organs (postganglionic synapses): axons of ganglionic neurons (unmyelinated C-fibers) release predominantly norepinephrine (90%), along with epinephrine (7%) and dopamine (3%). Exceptions include sweat glands and certain blood vessels, which are innervated by cholinergic sympathetic fibers (releasing acetylcholine).

The postsynaptic membrane of target organs contains α1- and β1-adrenergic receptors, which trigger intracellular cascades (via cAMP and inositol trisphosphate). About 80% of the released norepinephrine undergoes reuptake into vesicles, while the remainder is degraded by MAO and COMT enzymes.

Physiological Effects

According to W. Cannon's theory, the sympathetic system mobilizes the body. Its effects are more prolonged compared to the parasympathetic system and have a pronounced catabolic character.

Effects on Body Systems:

Cardiovascular: increased heart rate and myocardial contractility, elevated blood pressure.
Respiratory: bronchodilation, increased ventilation volume, and improved oxygenation.
Metabolism: hyperglycemia (increased blood glucose levels), enhanced overall metabolism.
Musculoskeletal: increased muscle tone and impulse conduction velocity.
Gastrointestinal: inhibitory effect (reduced tone, decreased absorption and enzymatic digestion).
Special senses: pupil dilation (mydriasis).

The system plays an active role in mediating emotions, stress responses, pain reactions, and aggression.

Mnemonic

To easily recall sympathetic effects, imagine a reaction to a sudden threat: pupils dilate (to see better), bronchi open up (to breathe), the heart beats faster (to pump blood), and digestion shuts down (it's not the time for eating).

Frequently asked questions

What types of adrenergic receptors are localized on the postsynaptic membrane of effector organs?

The postsynaptic membrane of effector target organs predominantly contains $\alpha_1$- and $\beta_1$-adrenergic receptors, which are activated by norepinephrine to execute physiological responses.

$\alpha_1$-Adrenergic receptors — located in vascular smooth muscle, the radial muscle of the iris, sphincters of the GI and urogenital tracts, and the splenic capsule; their activation triggers inositol trisphosphate production.
$\beta_1$-Adrenergic receptors — upon binding with the neurotransmitter, they activate adenylate cyclase, an enzyme converting ATP to cAMP.

Which higher autonomic centers regulate the activity of the sympathetic nervous system?

The key higher autonomic center is the hypothalamus. Activity within its associated mechanisms can also be modulated by influences from the limbic system and cerebellum.

Hypothalamus — the primary autonomic center coordinating the body's response; corticotropin-releasing hormone participates in central regulation of autonomic functions, including activation of the sympathetic ANS.
Limbic system — activates neurosecretory cells of the hypothalamus, influencing the regulation of corticotropin-releasing hormone synthesis.
Cerebellum — participates in the regulation of visceral functions via connections with autonomic hypothalamic centers; its neurons evaluate the state of internal organs and help adapt organ activity to current metabolic demands.

What mechanism achieves the generalized influence of the sympathetic system?

Through divergence: a single preganglionic axon branches extensively to contact 10–30 neurons in the autonomic ganglion simultaneously, affecting multiple organs at once.

How is norepinephrine release regulated at the synapse?

Via presynaptic receptors on the fiber itself: α2-receptors inhibit further norepinephrine release (negative feedback), while β2-receptors enhance it (positive feedback).

Which neurotransmitters are released in sympathetic ganglia?

The primary neurotransmitter is acetylcholine acting on N-cholinergic receptors. Dopamine is also involved, secreted by interneurons to generate slow inhibitory potentials.

Go deeper

Mechanisms of fast and slow postsynaptic potentials (EPSP and IPSP).
Role of second messengers: cAMP (upon β1 activation) and inositol trisphosphate (upon α1 activation).
Cotransmitters of the sympathetic nervous system: ATP, somatostatin, and neuropeptide Y.
Presynaptic heteroregulation of neurotransmitter release involving M2 muscarinic receptors.
Enzymatic degradation of catecholamines involving MAO and COMT.