The autonomic nervous system (ANS) regulates involuntary physiological processes — heart rate, blood pressure, digestion, respiration, body temperature, and glandular secretion. It operates largely below conscious awareness, maintaining the internal environment within the narrow ranges required for survival.

Gray's Anatomy illustration of the sympathetic nervous system — The sympathetic trunk and its connections to the spinal cord and viscera. Preganglionic fibers arise from the thoracolumbar region.

Divisions of the Autonomic Nervous System

The ANS has three anatomically and functionally distinct divisions:

Sympathetic division (thoracolumbar outflow). Prepares the body for action — the fight-or-flight response. It is catabolic, mobilizing energy stores and increasing cardiac output, blood flow to skeletal muscle, and alertness.

Parasympathetic division (craniosacral outflow). Promotes rest, repair, and energy conservation — the rest-and-digest response. It is anabolic, slowing the heart, promoting digestion, and eliminating waste.

Enteric division. The intrinsic nervous system of the gastrointestinal tract. It can function independently but is modulated by the sympathetic and parasympathetic systems.

The Two-Neuron Chain

Unlike the somatic motor system (which uses a single neuron from CNS to effector), the ANS always uses a chain of two neurons:

Preganglionic neuron. Its cell body lies in the CNS (brainstem or spinal cord). Its axon is lightly myelinated and projects to an autonomic ganglion outside the CNS.

Postganglionic neuron. Its cell body lies in the autonomic ganglion. Its unmyelinated axon extends to the target organ (smooth muscle, cardiac muscle, or gland).

Ganglion. A collection of postganglionic neuron cell bodies where the synapse between preganglionic and postganglionic neurons occurs.

Sympathetic Division

Organization

Sympathetic preganglionic neurons are located in the intermediolateral cell column (lateral horn) of spinal cord segments T1 through L2. Their axons exit through the ventral roots and reach the sympathetic chain via white rami communicantes.

The sympathetic chain (paravertebral ganglia) consists of two chains of interconnected ganglia running alongside the vertebral column, from the base of the skull to the coccyx. There are typically 3 cervical, 11 thoracic, 4 lumbar, and 4 sacral ganglia on each side.

Pathways

A preganglionic sympathetic fiber has three possible destinations after entering the sympathetic chain:

Synapse in the chain ganglion at the same level. The postganglionic fiber returns to the spinal nerve via a gray ramus communicans and travels to peripheral targets (blood vessels, sweat glands, arrector pili muscles).
Ascend or descend within the chain to synapse at a different level. For example, T1 preganglionic fibers ascend to synapse in the superior cervical ganglion, providing sympathetic innervation to the head.
Pass through the chain without synapsing and travel via a splanchnic nerve to a prevertebral ganglion. The splanchnic nerves (greater, lesser, least, lumbar, sacral) carry preganglionic fibers to the celiac, superior mesenteric, inferior mesenteric, and hypogastric ganglia.

Prevertebral (collateral) ganglia. These lie in the abdominal cavity near the origins of the major arteries:

Celiac ganglion. Innervates the stomach, liver, pancreas, spleen, and proximal duodenum.
Superior mesenteric ganglion. Innervates the small intestine and proximal colon.
Inferior mesenteric ganglion. Innervates the distal colon, rectum, and pelvic organs.
Aorticorenal ganglia. Innervate the kidneys and adrenal glands.

Key Features

Preganglionic axons are short (CNS to nearby chain or ganglia).
Postganglionic axons are long (ganglia to distant targets).
The ratio of preganglionic to postganglionic neurons is high (1:20 to 1:100 or more), allowing diffuse, widespread effects — the entire sympathetic system can be activated as a unit.
The adrenal medulla is unique: it receives direct preganglionic innervation (no postganglionic fiber) and releases epinephrine and norepinephrine directly into the bloodstream.

Gray's Anatomy illustration of the autonomic nervous system — Distribution of the autonomic nervous system. Sympathetic outflow (red) arises from the thoracolumbar region; parasympathetic outflow (blue) arises from the brainstem and sacral region.

Parasympathetic Division

Organization

Parasympathetic preganglionic neurons are located in two regions:

Cranial outflow. Nuclei of cranial nerves III, VII, IX, and X in the brainstem.
Sacral outflow. The intermediolateral cell column of spinal cord segments S2–S4 (the sacral parasympathetic nucleus).

Cranial Parasympathetic Pathways

CN III (oculomotor). Preganglionic fibers travel to the ciliary ganglion (in the orbit). Postganglionic fibers innervate the sphincter pupillae (pupillary constriction) and ciliary muscle (accommodation for near vision).

CN VII (facial). Preganglionic fibers travel to two ganglia:

Pterygopalatine ganglion. Postganglionic fibers innervate the lacrimal gland and nasal mucosa.
Submandibular ganglion. Postganglionic fibers innervate the submandibular and sublingual salivary glands.

CN IX (glossopharyngeal). Preganglionic fibers travel to the otic ganglion. Postganglionic fibers innervate the parotid gland.

CN X (vagus). The vagus nerve carries approximately 75% of all parasympathetic outflow. Its preganglionic fibers travel to terminal (intramural) ganglia in the walls of thoracic and abdominal organs — the heart, lungs, esophagus, stomach, liver, pancreas, small intestine, and proximal colon. Postganglionic fibers are extremely short, as the ganglia are within the target organs.

Sacral Parasympathetic Outflow

Preganglionic fibers from S2–S4 form the pelvic splanchnic nerves (nervi erigentes). These fibers synapse in terminal ganglia in the walls of the distal colon, rectum, bladder, and reproductive organs. Sacral parasympathetic activity controls urination, defecation, erection, and ejaculation.

Key Features

Preganglionic axons are long (CNS to ganglia near or within the target organ).
Postganglionic axons are very short (ganglia to target cells).
The ratio of preganglionic to postganglionic neurons is low (1:1 to 1:5), allowing discrete, localized effects.
All parasympathetic ganglia are terminal ganglia (located near or within the target organ).

ⓘ Information

After a meal, the parasympathetic system dominates. The vagus nerve increases gastric motility and secretion, stimulates pancreatic enzyme release, and promotes gallbladder contraction. Heart rate slows, bronchioles constrict slightly, and blood flow is redirected to the digestive tract. This state of calm, restorative activity is essential for efficient digestion and energy storage.

Enteric Nervous System

The enteric nervous system (ENS) is sometimes called the “second brain.” It consists of approximately 500 million neurons arranged in two plexuses within the wall of the GI tract:

Myenteric plexus (Auerbach’s plexus). Between the longitudinal and circular muscle layers. Controls motility — peristalsis and segmentation.

Submucosal plexus (Meissner’s plexus). Within the submucosa. Controls secretion, absorption, and blood flow.

The ENS can generate reflex activity independently of the CNS (which is why a transplanted intestine can still function), but it receives modulatory input from the sympathetic (inhibitory) and parasympathetic (excitatory) systems.

Neurotransmitters and Receptors

Sympathetic Neurotransmission

Preganglionic neurons. Release acetylcholine (ACh), which activates nicotinic receptors on postganglionic neurons.

Postganglionic neurons. Most release norepinephrine (noradrenaline) onto target organs, acting on adrenergic receptors:

α₁ receptors. Vasoconstriction, pupillary dilation (mydriasis), contraction of sphincters.
α₂ receptors. Presynaptic inhibition of norepinephrine release (negative feedback).
β₁ receptors. Increased heart rate and contractility, increased renin release.
β₂ receptors. Bronchodilation, vasodilation (in skeletal muscle), relaxation of uterine smooth muscle, glycogenolysis.
β₃ receptors. Lipolysis, thermogenesis.

Exception. Sympathetic postganglionic fibers to sweat glands and some blood vessels release ACh (cholinergic sympathetic fibers).

Adrenal medulla. Releases epinephrine (80%) and norepinephrine (20%) directly into the bloodstream, acting as a hormone.

Parasympathetic Neurotransmission

Preganglionic neurons. Release ACh, activating nicotinic receptors on postganglionic neurons.

Postganglionic neurons. Release ACh, activating muscarinic receptors on target organs:

M₁ receptors. Gastric acid secretion, salivary secretion.
M₂ receptors. Slowing of heart rate (bradycardia), reduction of atrial contractility.
M₃ receptors. Smooth muscle contraction (bronchoconstriction, GI motility, bladder contraction), pupillary constriction (miosis), lacrimation, salivation.
M₄ and M₅ receptors. Less well characterized; M₄ may modulate dopamine release in the brain.

Receptor	Location	Effect
Nicotinic	All autonomic ganglia, neuromuscular junction	Excitatory (EPSP)
Muscarinic M₂	Heart (SA node, AV node, atria)	Bradycardia, decreased contractility
Muscarinic M₃	Smooth muscle, glands	Contraction, secretion
α₁	Blood vessels (skin, viscera), pupil dilator	Vasoconstriction, mydriasis
β₁	Heart, kidney (juxtaglomerular cells)	Increased HR, contractility, renin
β₂	Lungs, skeletal muscle vessels, liver	Bronchodilation, vasodilation, glycogenolysis

Autonomic Innervation of Major Organs

Heart

Sympathetic (β₁). Increases heart rate (chronotropy), conduction velocity (dromotropy), and contractility (inotropy).
Parasympathetic (M₂ via vagus nerve). Decreases heart rate and atrial contractility. The ventricles have limited parasympathetic innervation.

Lungs

Sympathetic (β₂). Relaxes bronchial smooth muscle (bronchodilation).
Parasympathetic (M₃). Constricts bronchial smooth muscle (bronchoconstriction) and increases mucus secretion.

Gastrointestinal Tract

Sympathetic (α₁, β₂). Decreases motility, constricts sphincters, decreases secretion.
Parasympathetic (M₃). Increases motility, relaxes sphincters, increases secretion.

Bladder

Sympathetic (β₂, α₁). Relaxes detrusor muscle (β₂), contracts internal urethral sphincter (α₁) — promotes urine storage.
Parasympathetic (M₃). Contracts detrusor muscle, relaxes internal sphincter — promotes urination.

Eye

Sympathetic (α₁). Contracts the pupillary dilator muscle (mydriasis).
Parasympathetic (M₃). Contracts the sphincter pupillae (miosis) and ciliary muscle (accommodation).

Blood Vessels

Sympathetic (α₁). Vasoconstriction in skin, viscera, and mucous membranes.
Sympathetic (β₂). Vasodilation in skeletal muscle.
Parasympathetic. Minimal direct innervation of most blood vessels; notable exceptions include the salivary glands and genitalia (vasodilation).

Sweat Glands

Sympathetic (cholinergic — ACh). Eccrine sweat gland secretion (thermoregulatory sweating).
Parasympathetic. Apocrine sweat glands on limited body areas; minor role.

⚠ Clinical Correlation

Horner syndrome. Disruption of the sympathetic chain to the head (from a Pancoast tumor, carotid dissection, or iatrogenic injury) causes ipsilateral ptosis, miosis, anhydrosis, and enophthalmos. Orthostatic hypotension. Failure of sympathetic vasoconstriction upon standing produces a drop in blood pressure with dizziness or syncope. Common in diabetic autonomic neuropathy and Parkinson’s disease. Autonomic dysreflexia. In spinal cord injury above T6, a noxious stimulus below the lesion triggers massive sympathetic discharge, producing severe hypertension, bradycardia, and headache. This is a medical emergency.

Summary

The autonomic nervous system regulates all involuntary bodily functions through three divisions: sympathetic (fight or flight), parasympathetic (rest and digest), and enteric (gut brain). The two-neuron chain — preganglionic and postganglionic — allows precise or diffuse control depending on the division. Sympathetic effects are widespread and mediated primarily by norepinephrine on adrenergic receptors; parasympathetic effects are localized and mediated by acetylcholine on muscarinic receptors. Understanding autonomic anatomy and pharmacology is essential for comprehending how the body maintains homeostasis and responds to stress, and for managing a wide range of clinical conditions from hypertension to autonomic neuropathy.