Sympathetic nervous system

The sympathetic nervous system (SNS), mediated by the sympathetic chain (trunk) and ganglia, is a major division of the autonomic nervous system. It is composed of general visceral afferent and efferent axons that allow for involuntary control of bodily functions via the hypothalamus. 

The overarching function of the sympathetic system is to control the 'fight, fright or flight' (sympathoadrenal) response.

The major preganglionic neurotransmitter in the sympathetic system is acetylcholine (ACh). The major postganglionic neurotransmitter is norepinephrine, except in sweat glands where acetylcholine is released ¹.

Gross anatomy

Location

The sympathetic chain is located anterolateral to the vertebral bodies within the perivertebral space. It is comprised of paired, longitudinally arranged, paravertebral sympathetic ganglia linked together by myelinated axons forming the sympathetic trunk. It extends from the upper neck to the coccyx.

Branches from the prevertebral sympathetic ganglia may communicate with paravertebral sympathetic ganglia and/or form a plexus prior to innervating the target organ.

Sympathetic chain and ganglia:

• within the perivertebral space, lateral to the vertebral bodies and anterior to the intervertebral foramen at their respective spinal level

• extends from the upper neck to the coccyx

• 23 paired sympathetic ganglia and 1 unpaired coccygeal sympathetic ganglion (ganglion impar)

Origin

The sympathetic chain and ganglia are comprised of discrete sympathetic ganglia (containing neuronal cell bodies) which communicate with each ganglion via the sympathetic trunk (containing myelinated axons).

• cervical: three discrete interconnected ganglia:

  • superior cervical ganglion - elongated cylindrical shape formed by embryologically fused C1 to C4 sympathetic ganglia

  • middle cervical ganglion

  • inferior cervical ganglion which in the majority of people partially fuses with the T1 sympathetic ganglion forming the stellate ganglion- located within the cervicothoracic junction

• thorax: T2 to T12 thoracic sympathetic ganglia adjacent to their respective vertebral bodies, located within the posterior mediastinum

• abdomen: L1 to L5 lumbar sympathetic ganglia. Located anteriorly to the thoracolumbar fascia within the perivertebral space and posteromedial to the psoas major muscle

• pelvis: S1 to S5 sacral sympathetic ganglia. Located on the anterior surface of the sacrum medial to the S1-S4 sacral foramina

• unpaired coccygeal ganglion - also known as ganglion impar

Course

Efferent pathway

Intra-axial component:

• sympathetic nuclei are located within the hypothalamus

• white matter tracts course to spinal cord segments between T1 and L2

• axons synapse at cell bodies within the intermediolateral nucleus of the grey matter lateral horns between T1 and L2 spinal segments

Thoracolumbar outflow:

• preganglionic myelinated fibers enter the sympathetic ganglia at that spinal level by coursing through the ventral roots, spinal nerve and then white ramus communicans

• above T1 and below L2, there are only grey ramus communicans between the spinal nerve and the sympathetic ganglion

Within the paravertebral sympathetic ganglion, the preganglionic axons may then behave in one of several ways:

1. preganglionic axons may synapse at their respective ganglia, and then re-enter the spinal nerve as postganglionic axons (via grey ramus communicans). This route supplies dermatomes and myotomes

2. preganglionic axons may travel superiorly or inferiorly along the sympathetic chain to synapse with one (or many) of a more rostral or caudal ganglia

3. preganglionic axons can leave the sympathetic ganglion directly via sympathetic branches (e.g. to celiac or superior mesenteric ganglia)

It should be noted that more than one preganglionic axon can synapse with single post-ganglionic neuron.

Afferent pathway

• cell bodies are located within the dorsal root ganglion

• axons travel from the target organ to the dorsal root ganglion via the same plexus and nerves that carry efferent fibers

• sympathetic axons can synapse at the intermediolateral nucleus to form a sympathetic reflex arc, or ascend to the hypothalamus within the spinal cord

Branches

Branches of the paravertebral ganglia, prevertebral ganglia and plexus (from superior to inferior):

Head

• superior cervical ganglia send branches to the head via sympathetic branches that accompany the internal and external carotid arteries

  • sympathetics course with the ophthalmic artery to innervate the dilator pupillae and levator palpebrae superioris muscles

  • forms the deep petrosal nerve, which combines with the greater petrosal nerve to form the nerve of the pterygoid canal

  • may course with branches of the external carotid artery to supply sublingual gland, submandibular gland and parotid gland

Neck and thorax

• superior, middle and inferior cardiac nerves (T1-4): arise from the superior cervical ganglion, middle cervical ganglion, inferior cervical ganglion respectively

  • form the superficial/deep cardiac plexus and pulmonary plexus which supplies the heart and bronchi respectively

• thoracic cardiac branches: arising from T1-4 sympathetic ganglia which innervate the cardiac plexus

  • contribute to esophageal plexus and thoracic aortic plexus which supplies the esophagus and thoracic aorta

Abdomen

• greater (T5-10) and lesser (T10-11) splanchnic nerves - arise from the respective thoracic sympathetic ganglia within the posterior mediastinum

  • pierce the crura of the diaphragm to enter the abdominal cavity, join the celiac and superior mesenteric ganglia - communicates with the inferior mesenteric ganglia via the intermesenteric plexus

  • celiac plexus - supplies the liver (via hepatic plexus), gallbladder, bile duct, spleen, pancreas, adrenal glands (via suprarenal plexus), and foregut

  • superior mesenteric plexus - supplies the midgut (via mesenteric branches) by following the superior mesenteric artery

  • aorticorenal plexus and renal plexus- supplies the kidneys via branches from lesser splanchnic (T10-11) - sympathetics course with the renal arteries to innervate the kidneys

  • fibers course with the gonadal arteries to innervate the ovaries/testes

• least splanchnic nerve (T12)

  • aorticorenal plexus and renal plexus- supplies the kidneys - sympathetics course with the renal arteries to innervate the kidneys

• lumbar splanchnic nerves (L1 and L2)

  • communicate with inferior mesenteric ganglia and course with the inferior mesenteric artery to supply the hindgut

• superior hypogastric plexus: formed by descending fibers from the inferior mesenteric ganglion

Pelvis

• right and left inferior hypogastric plexus - formed as the inferior continuation of superior hypogastric plexus - each courses anterolateral to the sacral promontory

• sacral splanchnic nerves arising from the S1 to S5 sacral sympathetic ganglia provide branches to the inferior hypogastric plexus

  • supplies bladder (via vesical plexus), rectum (via rectal plexus), uterus/prostate (via uterovaginal/prostatic plexus)

Relations

Neck

• anterior: scalenus anterior

• anterolateral: sternocleidomastoid, carotid sheath (IJV, ICA, vagus nerve)

• lateral: posterior triangle, fat, brachial plexus

• posterior: transverse process, vertebral artery

Root of the neck

• posterior: T1 spinal nerve

• lateral: suprapleural membrane, vertebral artery

• anterior: carotid sheath, stellate ganglion is located opposite to the neck of the 1st rib, phrenic nerve, scalenus anterior

Thorax

• posterior: intercostal nerves, intercostal vessels, 

  • neck of the 1st rib, head of 2nd-10th ribs and lateral to bodies of T11 and T12 vertebrae

• anterior: descending thoracic aorta, azygous veins, thoracic duct

• lateral: parietal pleura

Abdomen

• anterior: inferior vena cava, abdominal aorta

• posterior: lumbar spinal nerves, lumbar arteries

  • anterolateral to lumbar vertebrae

• lateral: psoas major, quadratus lumborum muscles

Pelvis

• lateral: sacral foramina, spinal nerves

• posterior: sacrum

Arterial supply

• neck: ascending pharyngeal and ascending cervical arteries, thyrocervical trunk and supreme intercostal arteries ⁴

• thorax: posterior intercostal arteries

• abdomen: lumbar arteries

• pelvis: lateral and median sacral arteries

Venous drainage

• neck: small posterior veins into internal jugular vein ⁴

• to be completed

Lymphatic drainage

• neck: deep cervical chain lymph nodes

• thorax: posterior mediastinal lymph nodes

• abdomen: para-aortic lymph nodes, cisterna chyli

• pelvis: internal iliac lymph nodes

Variant anatomy

• unfused T1 and inferior cervical ganglia (absent stellate ganglion)

• doubled superior cervical ganglion

• absent superior cardiac branch (usually on the right)

• accessory L3 white ramus communicans

Interventional procedures

• celiac plexus block

• percutaneous splanchnic nerve radiofrequency ablation

• renal sympathetic denervation

• stellate ganglion block

• superior hypogastric plexus block