Thoracic Sympathetic Chain and Splanchnic Nerves

Contents

AN23.5: Identify and mention the location and extent of thoracic sympathetic chain.
AN23.6: Describe the splanchnic nerves.

Introduction

Sympathetic nervous system: Predominantly active during stressful situations and prepares the body for the fight-or-flight response.
Parasympathetic nervous system: Promotes rest, digestion, energy conservation, and metabolic activity.
The sympathetic pathway includes:

Preganglionic fibers: Arise from neurons in the lateral horn of the spinal cord, mainly from T1 to L2 spinal segments. Their axons leave through the ventral roots, enter the spinal nerves, and pass into the sympathetic trunk through the white rami communicantes.
Sympathetic ganglia: Form paired chains of autonomic ganglia along the vertebral column. Preganglionic fibers may synapse at the level of entry or ascend or descend before relaying.
Postganglionic fibers: Leave the sympathetic ganglia and reach their target organs by several routes. Some return to spinal nerves through the gray rami communicantes, while others accompany blood vessels or contribute to visceral autonomic plexuses.
The right and left sympathetic trunks extend along either side of the vertebral column and contain cervical, thoracic, lumbar, and sacral ganglia.
Inferiorly, the two trunks converge anterior to the coccyx and unite in a single ganglion impar.

The thoracic sympathetic chain consists of a series of ganglia located on either side of the thoracic vertebral column.
Superiorly, it is continuous with the cervical sympathetic trunk.
Inferiorly, it continues as the lumbar sympathetic trunk.

Developmentally, there are 12 thoracic sympathetic ganglia on each side.
The first thoracic ganglion commonly fuses with the inferior cervical ganglion to form the stellate (cervicothoracic) ganglion.
The remaining thoracic ganglia are generally related to the levels of the corresponding thoracic vertebrae and intervertebral discs.

Each thoracic sympathetic ganglion communicates with a corresponding spinal nerve through:

White ramus communicans: Carries myelinated preganglionic sympathetic fibers from the spinal cord to the sympathetic trunk.
Gray ramus communicans: Contains unmyelinated postganglionic sympathetic fibers returning to the spinal nerve.

Branches of the thoracic sympathetic trunk may be considered as medial visceral branches and lateral branches.

Medial Branches

These branches contribute to the autonomic supply of the thoracic and abdominal viscera.
Branches from the upper thoracic ganglia contain mainly postganglionic sympathetic fibers.
They reach the heart, great vessels, lungs, and esophagus through the:
- Cardiac plexus
- Pulmonary plexus
- Aortic plexuses
- Esophageal plexus
Visceral branches from the lower thoracic ganglia contain predominantly preganglionic sympathetic fibers.
They combine to form the greater, lesser, and least thoracic splanchnic nerves.

Lateral Branches

Each thoracic ganglion gives rise to gray rami communicantes that join the corresponding spinal nerves.
These fibers are distributed to the body wall and limbs.
Their functions include:
- Vasomotor activity: Regulation of cutaneous blood vessels
- Sudomotor activity: Control of sweat glands
- Pilomotor activity: Contraction of arrector pili muscles

Cardiac referred pain: Visceral pain fibers from the heart travel mainly with sympathetic fibers to the T1–T5 spinal segments. Pain may therefore be perceived over the upper anterior chest and along the medial aspect of the left upper limb.
Thoracoabdominal sympathectomy: Surgical interruption of selected sympathetic pathways may be performed for specific clinical indications. Historically, procedures involving lower thoracic ganglia and splanchnic nerves have been used to reduce sympathetic activity.
Horner syndrome: Interruption of the cervical sympathetic pathway, including fibers passing through the stellate ganglion, may produce ipsilateral miosis, partial ptosis, and anhidrosis. Apparent enophthalmos may also be described clinically.
Raynaud phenomenon: Episodic vasospasm of small arteries and arterioles, particularly in the fingers and toes, causes pallor and reduced blood flow. Reperfusion may be associated with discomfort or burning. Sympathectomy has occasionally been considered in severe, refractory cases.

Formation: Usually arises from preganglionic sympathetic fibers derived from the T5–T9 thoracic ganglia.
Course: Descends along the thoracic vertebral bodies and enters the abdomen by passing through the crus of the diaphragm.
Termination: Most fibers end in the celiac ganglion. Some fibers may reach the aorticorenal ganglion or pass directly to the suprarenal medulla.

Formation: Usually formed by fibers from the T10 and T11 thoracic ganglia.
Course: Descends through the thorax and pierces the crus of the diaphragm.
Termination: Fibers primarily reach the aorticorenal and superior mesenteric regions, with contributions to the celiac plexus.

Formation: Usually formed by fibers from the T10 and T11 thoracic ganglia.
Course: Descends through the thorax and pierces the crus of the diaphragm.
Termination: Fibers primarily reach the aorticorenal and superior mesenteric regions, with contributions to the celiac plexus.

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