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Morphology of the Sympathetic Nervous System

Q: What types of communicating rami connect to and arise from the sympathetic trunk?

Two types of communicating rami (rami communicantes) are associated with the sympathetic trunk: White communicating rami (rami communicantes albi) — contain myelinated preganglionic fibers, emerge from spinal nerves T1–L2, and run toward the sympathetic trunk ganglia.Gray communicating rami (rami communicantes grisei) — contain unmyelinated postganglionic fibers, arise from all ganglia of the sympathetic trunk, and return to spinal nerves.

Pars sympathica

For medical students2 min readUpdated 2026-10-10

The sympathetic nervous system is a division of the autonomic nervous system characterized morphologically by a specific arrangement of centers and ganglia. Its anatomical hallmark is the thoracolumbar outflow and a unique ratio in length between preganglionic and postganglionic nerve fibers.

Central componentCell bodies of the first neurons lie in the lateral horns of the spinal cord from T1 to L2/L3

Preganglionic fibersShort fibers extending to the sympathetic trunk or prevertebral ganglia

Postganglionic fibersLong fibers extending from the ganglion directly to the target organ

Adrenal glandsReceive direct innervation without prior synaptic relay in ganglia

Central and Peripheral Structures

Anatomically, the sympathetic division (Pars sympathica) is strictly divided into a central apparatus and a peripheral part.

The cell bodies of the first (preganglionic) neurons are localized exclusively in the lateral horns of the spinal cord. This innervation zone spans segments from the first thoracic (T1) down to the second or third lumbar (L2/L3). Due to this nuclear localization, the sympathetic nervous system features a thoracolumbar outflow.

The peripheral part of the system is represented by two main groups of ganglia:

Sympathetic trunk (Truncus sympathicus). This forms a long chain of paravertebral ganglia positioned symmetrically on either side of the vertebral column throughout its entire length.
Prevertebral ganglia. These ganglia lie further away from the spine, situated primarily within the abdominal cavity. The most prominent include the celiac plexus ganglia and the mesenteric ganglia.

Fiber Morphology and Exceptions

A crucial morphological criterion of the sympathetic division is the difference in length between nerve conductors:

Preganglionic fibers are short. Their task is to cover only a brief distance from the spinal cord to the nearest sympathetic trunk ganglion or prevertebral ganglion.
Postganglionic fibers, by contrast, are long. They must travel a considerable distance from the peripheral ganglion directly to the target organ to transmit the signal.

There is one critically important exception to this system: the innervation of the adrenal medulla. The preganglionic sympathetic fiber projects directly to the adrenal gland, without synapsing or relaying on a second neuron in either the sympathetic trunk or the prevertebral ganglia. In this case, the adrenal medulla itself functionally replaces the postganglionic neuron, releasing hormones into the bloodstream in response to the impulse.

Topography of Segmental Innervation

Sympathetic influences are distributed to target organs depending on the level of fiber origin:

Head and neck: Innervation is provided by fibers from the upper thoracic segments that synapse in the superior cervical ganglion. From there, signals travel to the eye (causing pupillary dilation) as well as to the lacrimal and salivary glands (inhibiting secretion and altering saliva composition).
Thoracic cavity: Sympathetic fibers reach the heart, where their activation increases heart rate and contractility. In the lungs, sympathetic stimulation causes bronchodilation.
Abdominal cavity: Fibers pass through to synapse in the celiac and mesenteric ganglia. They innervate the kidneys as well as organs of the gastrointestinal tract—the stomach, liver, pancreas, and intestines. The result is inhibition of motility and a decrease in digestive secretions.
Pelvic region: Impulses originate from the lumbar segments, controlling the reproductive organs and urinary bladder. In the urinary bladder, sympathetic input relaxes the detrusor muscle and simultaneously contracts the sphincter, preventing urination.

Mnemonic

To easily remember fiber lengths, picture the anatomical layout: the sympathetic trunk lies directly adjacent to the spine. Therefore, the preganglionic fiber from the spinal cord only needs to travel a «short» path to the ganglion, whereas the postganglionic fiber faces a «long» road all the way to the target organ.

Frequently asked questions

From which specific spinal cord segments does the sympathetic innervation of the head and neck originate?

Generally, sympathetic innervation for the head and neck originates from the upper thoracic segments via the superior cervical ganglion.

Specific segments are identified for the sympathetic pathway to the eye: the lateral horns of C8 and T1–T2 form the ciliospinal center, from which preganglionic fibers ascend to the superior cervical sympathetic ganglion.

Which neurotransmitters are released in sympathetic ganglia and at target organs?

Acetylcholine is released within sympathetic ganglia. At target organs, the primary neurotransmitter is norepinephrine (accounting for 90% of released catecholamines, along with epinephrine at 7%, dopamine at 3%, and co-transmitters).

Exceptions include:

Sweat glands and skeletal muscle blood vessels — innervated by fibers releasing acetylcholine.
Gastrointestinal tract — contains serotonergic fibers.

What types of communicating rami connect to and arise from the sympathetic trunk?

Two types of communicating rami (rami communicantes) are associated with the sympathetic trunk:

White communicating rami (rami communicantes albi) — contain myelinated preganglionic fibers, emerge from spinal nerves T1–L2, and run toward the sympathetic trunk ganglia.
Gray communicating rami (rami communicantes grisei) — contain unmyelinated postganglionic fibers, arise from all ganglia of the sympathetic trunk, and return to spinal nerves.

Where are the cell bodies of the first sympathetic neurons localized?

They are located in the lateral horns of the spinal cord strictly at segments T1 through L2/L3.

How is the adrenal medulla innervated?

It is reached directly by preganglionic fibers without a synaptic relay in ganglia. The medulla itself functionally acts as the postganglionic neuron.

What type of outflow characterizes the sympathetic division?

The sympathetic system exhibits a thoracolumbar outflow.

Go deeper

Structure and topography of the sympathetic trunk (Truncus sympathicus)
Prevertebral abdominal ganglia and the celiac plexus
Sympathetic regulation of the visual apparatus and salivary glands
Influence of the sympathetic division on the smooth musculature of the urinary bladder
Functional connection of the adrenal medulla with the nervous system