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Autonomic Medicine

Autonomic Ganglia and Neurotransmitters

Autonomic Ganglia and Neurotransmitters

Autonomic ganglia are relay structures where preganglionic nerve fibers originating from the central nervous system form synapses with postganglionic neurons. They are divided into sympathetic and parasympathetic ganglia, transmitting autonomic signals to target organs via neurotransmitters such as acetylcholine and norepinephrine.

2026-03-28

At a Glance

Autonomic ganglia are relay stations through which autonomic signals from the brain and spinal cord pass before reaching each organ. All preganglionic fibers of autonomic ganglia use acetylcholine as their neurotransmitter, transmitting signals via nicotinic receptors. At the postganglionic level, sympathetic nerves release norepinephrine while parasympathetic nerves release acetylcholine. The sympathetic postganglionic fibers innervating sweat glands and the adrenal medulla are notable exceptions to this principle. Understanding these neurotransmitters and receptors forms the essential foundation for diagnosing autonomic disorders and guiding pharmacological treatment.

Definition and Overview

Autonomic ganglia are neural relay structures where autonomic fibers (preganglionic fibers) originating from the central nervous system form synapses with peripheral neurons (postganglionic neurons). The motor pathway of the autonomic nervous system must pass through two neurons to reach the target organ, and the autonomic ganglion is the site where this relay occurs [1].

Autonomic ganglia are not mere signal relay stations. Interneurons within the ganglia integrate and modulate signals. A single preganglionic fiber may synapse with multiple postganglionic neurons, or several preganglionic fibers may converge on a single postganglionic neuron, enabling signal amplification and fine-tuned regulation [2]. Understanding the types of neurotransmitters and receptors used in autonomic ganglia is a key element in comprehending the pathophysiology of autonomic disorders and the principles of pharmacological treatment.

Structure of Autonomic Ganglia

Sympathetic Ganglia

The preganglionic neurons of the sympathetic nervous system are located in the intermediolateral column of the spinal cord from T1 to L2. Their axons exit through the ventral roots and form synapses with postganglionic neurons in sympathetic ganglia [1].

Sympathetic ganglia are classified into two types based on their location.

  • Paravertebral ganglia: Approximately 22 pairs of ganglia arranged in a chain along both sides of the vertebral column. The superior, middle, and inferior cervical ganglia, along with the thoracic, lumbar, and sacral ganglia, constitute the sympathetic trunk. The inferior cervical ganglion merges with the first thoracic ganglion to form the stellate ganglion, which serves as a major relay point for sympathetic nerves projecting to the head, neck, and upper extremities [5].
  • Prevertebral ganglia: Located anterior to the abdominal aorta, these include the celiac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion, which relay sympathetic signals to the abdominal and pelvic organs [2].

A structural characteristic of the sympathetic nervous system is that preganglionic fibers are short while postganglionic fibers are long, because the ganglia are located close to the central nervous system. A single preganglionic neuron synapses with an average of 20 to 30 postganglionic neurons, enabling a small number of central signals to produce widespread peripheral responses [1].

Parasympathetic Ganglia

The preganglionic neurons of the parasympathetic nervous system are located in the brainstem (midbrain, pons, medulla) and the sacral spinal cord (S2-S4). Parasympathetic fibers originating from the brainstem travel to target organs via cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus) [2].

Unlike sympathetic ganglia, parasympathetic ganglia are located near or within the walls of the target organs (intramural ganglia). Consequently, preganglionic fibers are long while postganglionic fibers are very short. A single preganglionic neuron synapses with only a few postganglionic neurons, allowing more localized and precise regulation compared to the sympathetic system [1]. The vagus nerve accounts for approximately 75% of parasympathetic output and innervates a wide range of visceral organs including the heart, lungs, and gastrointestinal tract [5].

Neurotransmitters

Preganglionic Fibers: Acetylcholine and Nicotinic Receptors

In both sympathetic and parasympathetic divisions, the neurotransmitter of preganglionic fibers is acetylcholine (ACh). Acetylcholine released from preganglionic nerve terminals binds to nicotinic acetylcholine receptors (nAChRs) on the postganglionic neuronal membrane, generating excitatory postsynaptic potentials [2].

The nicotinic receptors in autonomic ganglia differ in subtype from those at the neuromuscular junction. Ganglionic nAChRs are predominantly of the alpha-3/beta-4 subtype, distinct from the alpha-1/beta-1/delta/epsilon subtype found at the neuromuscular junction. This subtype difference determines differences in drug sensitivity [3].

Postganglionic Fibers: Sympathetic

The primary neurotransmitter of sympathetic postganglionic fibers is norepinephrine (NE). Norepinephrine binds to adrenergic receptors on target organs to exert its effects. Adrenergic receptors are broadly classified into alpha and beta types, each with subtypes (alpha-1, alpha-2, beta-1, beta-2, beta-3) [6].

The distribution and functions of the major adrenergic receptors are as follows.

  • Alpha-1 receptors: Located on vascular smooth muscle; activation causes vasoconstriction and blood pressure elevation.
  • Alpha-2 receptors: Located on preganglionic nerve terminals; they perform a negative feedback function by inhibiting norepinephrine release.
  • Beta-1 receptors: Located in the heart; activation increases heart rate (positive chronotropic effect) and myocardial contractility (positive inotropic effect).
  • Beta-2 receptors: Located on bronchial and vascular smooth muscle; activation causes bronchodilation and vasodilation [6].

Research has confirmed the phenomenon of cotransmission, in which sympathetic postganglionic nerves simultaneously release ATP, neuropeptide Y, and other substances in addition to norepinephrine [4]. These cotransmitters modulate or complement the effects of the primary neurotransmitter.

Postganglionic Fibers: Parasympathetic

The neurotransmitter of parasympathetic postganglionic fibers is acetylcholine. Acetylcholine released from postganglionic nerve terminals binds to muscarinic acetylcholine receptors (mAChRs) on target organs. Five subtypes of muscarinic receptors (M1 through M5) exist, with varying distributions across different organs [2].

  • M2 receptors: Located in the heart; activation decreases heart rate and delays atrioventricular conduction.
  • M3 receptors: Located on bronchial smooth muscle, gastrointestinal smooth muscle, and exocrine glands; activation causes bronchoconstriction, enhanced intestinal motility, and secretion of saliva and gastric acid.

Parasympathetic postganglionic nerves also release cotransmitters such as nitric oxide (NO) and vasoactive intestinal peptide (VIP) in addition to acetylcholine [4].

Exceptions: Sweat Glands and Adrenal Medulla

There are two notable exceptions to the general principles of autonomic neurotransmission.

First, the sympathetic postganglionic fibers innervating eccrine sweat glands use acetylcholine rather than norepinephrine as their neurotransmitter. The receptors on sweat glands are muscarinic receptors. This is known to result from neurotransmitter switching to a cholinergic phenotype during development [1].

Second, the adrenal medulla is embryologically equivalent to a sympathetic ganglion. Chromaffin cells of the adrenal medulla are modified postganglionic neurons that, upon acetylcholine stimulation from preganglionic sympathetic nerves, directly secrete epinephrine (approximately 80%) and norepinephrine (approximately 20%) into the bloodstream [5]. This endocrine mode of signal transmission distinguishes it from typical postganglionic neurons.

Receptors and Pharmacology

Most autonomic drugs act on neurotransmitter receptors. Since drug effects vary depending on the type and location of receptors, receptor pharmacology is central to autonomic therapeutics [6].

The major autonomic receptors and their associated drugs are as follows.

  • Nicotinic receptors (autonomic ganglia): Ganglionic blocking agents (e.g., trimethaphan) block these receptors, inhibiting both sympathetic and parasympathetic transmission. They were previously used for severe hypertensive emergencies but are now used only in limited settings due to nonselective side effects [5].
  • Muscarinic receptors (parasympathetic postganglionic effectors): Atropine blocks muscarinic receptors, increasing heart rate, suppressing salivary and gastric acid secretion, and dilating the pupils. It is used as a first-line drug in bradycardia emergencies [2].
  • Alpha-adrenergic receptors: Alpha-1 agonists (phenylephrine) cause vasoconstriction and are used for nasal decongestion and hypotension treatment. Alpha-1 blockers (prazosin) treat hypertension through vasodilation [6].
  • Beta-adrenergic receptors: Beta-1 blockers (atenolol, metoprolol) reduce heart rate and myocardial contractility for the treatment of hypertension, angina, and arrhythmias. Beta-2 agonists (salbutamol) dilate the bronchi for asthma treatment [6].

Receptor subtype selectivity is important in drug selection. Nonselective beta-blockers (propranolol) block both beta-1 and beta-2 receptors and may cause bronchoconstriction; therefore, beta-1 selective blockers are used in patients with asthma [5].

Clinical Significance

Targets of Autonomic Drugs

Many commonly used medications act on autonomic ganglia and neurotransmitter receptors. Beta-blockers used for hypertension are among the most widely prescribed drug classes worldwide, selectively blocking cardiac beta-1 receptors to reduce heart rate by approximately 10-15 beats per minute [6]. Salbutamol, a bronchodilator, acts selectively on beta-2 receptors and has been shown to improve forced expiratory volume in one second (FEV1) by 15-30% in asthma patients [5].

Autoimmune Autonomic Ganglionopathy

Autoimmune autonomic ganglionopathy (AAG) is a condition in which autoantibodies form against the alpha-3 subtype of nicotinic receptors in autonomic ganglia, blocking preganglionic-to-postganglionic signal transmission [3]. In a 2000 study by Vernino et al., ganglionic AChR antibodies were detected in approximately 50% of patients with autonomic failure, and a significant correlation was found between antibody titers and the severity of autonomic dysfunction [3].

Major symptoms of AAG include orthostatic hypotension, gastroparesis, pupillary abnormalities (anisocoria), hypohidrosis, dry mouth and eyes, and bladder dysfunction. Diagnostic workup includes serum ganglionic AChR antibody testing and autonomic function tests (heart rate variability analysis, tilt table test, sudomotor function test) [3].

Treatment involves immunotherapy including intravenous immunoglobulin, plasma exchange, and immunosuppressants, with meaningful symptom improvement reported in some patients [3].

Lifestyle Management

The following lifestyle practices help maintain healthy autonomic ganglia and neurotransmitter function.

  • Regular aerobic exercise: Walking, swimming, cycling, or similar activities for 30 minutes or more, 3-5 times per week, can help restore autonomic balance. Exercise has been reported to enhance parasympathetic activity and improve heart rate variability [1].
  • Adequate sleep: 7-8 hours of regular sleep is essential for autonomic nervous system recovery. During sleep, parasympathetic dominance lowers heart rate and promotes tissue repair.
  • Stress management: Chronic stress continuously increases norepinephrine release from sympathetic postganglionic nerves, causing autonomic imbalance. Relaxation techniques such as diaphragmatic breathing and meditation can help suppress sympathetic overactivity.
  • Balanced diet: Excessive caffeine stimulates the sympathetic nervous system, while alcohol blunts autonomic reflexes. Adequate fluid and electrolyte intake is important for blood pressure regulation.
  • Caution with medications: Drugs that act on autonomic receptors must be taken only as prescribed by a specialist. Abrupt discontinuation or dose changes may cause rebound effects.

If autonomic symptoms persist, objective evaluation through heart rate variability testing and specialist consultation is recommended.

Frequently Asked Questions

Autonomic ganglia are relay stations through which autonomic signals from the brain and spinal cord pass before reaching organs such as the heart, blood vessels, and gastrointestinal tract. Both sympathetic and parasympathetic nerves must pass through these relay stations to reach their target organs. If signal transmission at the autonomic ganglia is impaired, various autonomic dysfunction symptoms can occur.

Sympathetic ganglia are located alongside or in front of the vertebral column, relatively close to the central nervous system, whereas parasympathetic ganglia are situated near or within the walls of target organs, very close to the organs themselves. Due to this difference in location, sympathetic nerves have short preganglionic fibers and long postganglionic fibers, while parasympathetic nerves have long preganglionic fibers and short postganglionic fibers.

Acetylcholine is released from the preganglionic fibers of both the sympathetic and parasympathetic divisions. The difference lies at the postganglionic level: sympathetic postganglionic nerves release norepinephrine, while parasympathetic postganglionic nerves release acetylcholine. However, the sympathetic postganglionic fibers innervating sweat glands are an exception, using acetylcholine instead.

When autonomic ganglia function is compromised, symptoms may include dizziness and syncope from orthostatic hypotension, pupillary dysfunction, dry mouth, indigestion, sweating abnormalities, and bladder dysfunction. In autoimmune autonomic ganglionopathy in particular, autoantibodies against nicotinic receptors in the ganglia cause widespread autonomic failure. If you experience multiple such symptoms simultaneously, please consult a specialist.

Most autonomic drugs act on the receptors to which neurotransmitters bind. For example, beta-blockers used in hypertension treatment block beta-adrenergic receptors on sympathetic postganglionic effectors, lowering heart rate and blood pressure. Atropine blocks muscarinic receptors, increasing heart rate and reducing secretions. Since drug effects vary depending on the receptor type, medications must always be taken as prescribed by a specialist.

Autoimmune autonomic ganglionopathy (AAG) is a condition in which autoantibodies form against nicotinic receptors in autonomic ganglia, blocking autonomic signal transmission. It presents with widespread autonomic failure symptoms including orthostatic hypotension, gastrointestinal motility disorders, pupillary abnormalities, and hypohidrosis. Antibodies can be identified through blood testing, and since immunotherapy may improve symptoms, early diagnosis is important. If you experience suspicious symptoms, please visit a medical center specializing in autonomic disorders.

The adrenal medulla is anatomically a structure that shares the same embryological origin as sympathetic ganglia. Unlike typical sympathetic ganglia, it receives acetylcholine stimulation from preganglionic nerves and directly secretes epinephrine (adrenaline) and norepinephrine into the bloodstream. What makes it unique is that rather than sending signals to specific organs through nerves, it acts on the entire body simultaneously in the form of hormones.

References

  1. [1] Wehrwein EA, Orer HS, Barman SM (2016). "Overview of the anatomy, physiology, and pharmacology of the autonomic nervous system." Comprehensive Physiology, 6: 1239-1278. DOI PubMed
  2. [2] McCorry LK (2007). "Physiology of the autonomic nervous system." American Journal of Pharmaceutical Education, 71: 78. DOI PubMed
  3. [3] Vernino S, Low PA, Fealey RD, Stewart JD, Farrugia G, Lennon VA (2000). "Autoantibodies to ganglionic acetylcholine receptors in autoimmune autonomic neuropathies." New England Journal of Medicine, 343: 847-855. DOI PubMed
  4. [4] Burnstock G (2013). "Cotransmission in the autonomic nervous system." Handbook of Clinical Neurology, 117: 23-35. DOI PubMed
  5. [5] Robertson D, Biaggioni I, Burnstock G, Low PA, Paton JFR (2012). "Primer on the autonomic nervous system." Academic Press (Elsevier): 1-572. DOI
  6. [6] Bylund DB (2007). "Alpha- and beta-adrenergic receptors: Ahlquist's landmark hypothesis of a single mediator with two receptors." American Journal of Physiology - Endocrinology and Metabolism, 293: E1479-E1481. DOI PubMed

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