Classification of sympathetic trunk ganglia
The sympathetic trunk is topographically divided into several regions. The ganglia themselves represent clusters of postganglionic neuron bodies. The number of these ganglia is subject to individual variability (especially in the cervical and lumbar parts).
Main groups of trunk ganglia:
- Cervical ganglia — ganglia cervicalia
- Thoracic ganglia — ganglia thoracica
- Lumbar ganglia — ganglia lumbalia
- Sacral ganglia — ganglia sacralia
- Ganglion impar (unpaired coccygeal ganglion) — ganglion impar (terminates the sympathetic chains of both sides inferiorly).
Communicating branches (Rami communicantes)
To exchange fibers with spinal nerves, the sympathetic trunk uses special communicating branches. They are divided into two types based on their structure and fiber direction.
White communicating branches (rr. communicantes albi) These are the "input" pathways into the sympathetic trunk.
- They arise only from a limited number of spinal nerves: at the level from Th1 to L2.
- They contain preganglionic myelinated fibers (myelin is precisely what gives them their white color).
- They are directed straight to the ganglia of the sympathetic trunk.
Gray communicating branches (rr. communicantes grisei) These are the "output" pathways.
- They arise from all trunk ganglia without exception.
- They contain postganglionic unmyelinated fibers.
- They return back to the spinal nerves. Further within their composition, they participate in the innervation of skin, blood vessels, and sweat glands.
Fiber course and visceral branches
Preganglionic fibers, having entered a sympathetic trunk ganglion, can behave in different ways. Three main scenarios for their course are possible:
- Synapse — switching to neurons of this specific ganglion.
- Transit through the trunk — ascending or descending via interganglionic branches to other sympathetic trunk ganglia.
- Transit to the periphery — passing through the ganglion without synapsing (in transit) to proceed to visceral ganglia and plexuses.
From the ganglia and the trunk itself, visceral branches depart to internal organs, as well as to vascular and organ plexuses (plexus). A special role is played by major splanchnic nerves that predominantly carry preganglionic fibers to prevertebral ganglia:
- Thoracic splanchnic nerves (nn. splanchnici thoracici)
- Lumbar splanchnic nerves (nn. splanchnici lumbales)
- Sacral splanchnic nerves (nn. splanchnici sacrales)
Features of the cervical region
The cervical region of the sympathetic trunk has a number of unique characteristics. It typically distinguishes three ganglia:
- Superior cervical ganglion (ganglion cervicale superius)
- Middle cervical ganglion (ganglion cervicale medium)
- Cervicothoracic, or stellate ganglion (ganglion cervicothoracicum / stellatum)
The main topographical feature of the cervical region is the absence of white communicating branches (since they terminate at the Th1 level). Branches arise from the cervical ganglia that run predominantly to the vessels of the head and neck, and also descend to the thoracic organs, taking an active part in the formation of carotid and cardiac autonomic plexuses.
Briefly: answers to topical questions
Which specific branches arise from the superior cervical ganglion of the sympathetic trunk?
The following branches arise from the superior cervical ganglion of the sympathetic trunk:
- Superior cervical cardiac nerve (n. cardiacus cervicalis superior) — descends along the sheath of the common carotid artery.
- Internal carotid nerve (n. caroticus internus) — heads into the cranial cavity.
Also, the superior cervical ganglion has connections with the vagus nerve, glossopharyngeal nerve, and the cervical plexus of spinal nerves.
Which nerves are included in the group of thoracic splanchnic nerves?
The group of thoracic splanchnic nerves (nn. splanchnici thoracici) includes the greater and lesser splanchnic nerves.
- Greater splanchnic nerve (n. splanchnicus major) — formed from the V–IX thoracic ganglia of the sympathetic trunk.
- Lesser splanchnic nerve (n. splanchnicus minor) — arises from the X–XI thoracic ganglia.
These nerves run along the lateral surface of the thoracic vertebral bodies through the gap of the diaphragmatic crura to the ganglia of the celiac plexus.
Where is the stellate (cervicothoracic) ganglion located topographically?
The stellate, or cervicothoracic, ganglion (ganglion cervicothoracicum s. stellatum) is topographically located in the deep region of the neck, specifically in the scalenovertebral triangle (trigonum scalenovertebrale).
The ganglion lies between muscles beneath the fifth (prevertebral) fascia of the neck. The boundaries of the space where it is located are:
- medially — the longus colli muscle (m. longus colli);
- laterally — the anterior scalene muscle (m. scalenus anterior);
- base — the dome of the pleura;
- apex — the transverse process of the sixth cervical vertebra.
Which prevertebral ganglia receive preganglionic fibers from the splanchnic nerves?
Preganglionic fibers from the splanchnic nerves are directed toward prevertebral ganglia and plexuses, in particular:
- to the ganglia of the celiac plexus — fibers of the greater and lesser splanchnic nerves;
- to the aortorenal ganglion — branches of the lesser splanchnic nerve.
Sources also indicate that prevertebral structures include the superior and inferior mesenteric ganglia, although their receipt of preganglionic fibers specifically from the splanchnic nerves is not explicitly stated in the provided materials.
The white rami are the "ENTRANCE" to the trunk (present only where the lateral horns of the spinal cord are located: Th1–L2). The gray rami are the "EXIT" from the trunk (present in all ganglia to return fibers to the spinal nerves).
Class and exam questions
From clusters of postganglionic neurons.
The sympathetic trunk ganglia contain the cell bodies of postganglionic neurons upon which preganglionic fibers may synapse.
From which segments do the white communicating rami arise?
White communicating rami (rr. communicantes albi) branch off from the spinal nerves from the Th1 to L2 levels.
What is the main difference between gray and white communicating rami?
Gray rami contain unmyelinated postganglionic fibers and branch from all trunk ganglia. White rami contain myelinated preganglionic fibers and are present only at the Th1–L2 levels.
What is the function of the gray communicating rami?
They return postganglionic fibers to the spinal nerves for further sympathetic innervation of the skin, blood vessels, and sweat glands.
What else to study on this topic
- Topography and structure of the cervicothoracic (stellate) ganglion
- Formation of the carotid plexuses by branches of the cervical region
- Participation of sympathetic trunk branches in the cardiac plexuses
- Course and distribution of the thoracic splanchnic nerves (nn. splanchnici thoracici)
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