#The sympathetic nervous system

#Define the sympathetic nervous system

The sympathetic nervous system (SNS) forms part of the autonomic nervous system, controlling many unconscious functions.

It is generally considered a ‘physiological accelerator’.

#Outline the origin of the sympathetic nervous system

#Higher centres

The SNS receives input from several higher centres in the brainstem and cerebral cortex, such as the:

• limbic system, which includes the hypothalamus, anterior thalamic nuclei, the cingulate gyrus, and the hippocampus
• rostroventrolateral medulla (RVLM), commonly referred to as the vasomotor area
• raphe nuclei
• nucleus tractus solitarius

#Preganglionic neurons and the sympathetic chain

The cell bodies of preganglionic nerves are located in the grey matter of the lateral horn of the spinal cord from T1-L2.

From here, the nerve fibres can take 4 paths:

1. Synapse at a paravertebral ganglion at the same spinal level

   • each paravertebral ganglion communicates with its corresponding spinal nerve via the white and grey rami communicans

   • preganglionic nerves exit the spinal cord via the ventral root and synapse at the paravertebral ganglion via the white ramus communicans

   • postganglionic nerves exit the paravertebral ganglion via the grey ramus communicans and travel to their target organ via spinal nerves

   • the paravertebral ganglia collectively form the sympathetic chain

   • the sympathetic chain traverses the anterolateral margin of the thoracic vertebral bodies, posterior to the aorta (left) and IVC (right)

2. Travel to a different paravertebral ganglion

   • preganglionic nerves enter the sympathetic chain via the white ramus communicans but do not synapse right away
   • they ascend or descend within the sympathetic chain before synapsing at a paravertebral ganglion at a different spinal level
   • postganglionic fibres then exit the sympathetic chain via the grey ramus communicans as usual

3. Pass through the paravertebral ganglia without synapsing

   • these preganglionic nerves synapse at prevertebral ganglia such as the:
     • coeliac ganglion
     • aorticorenal ganglion
     • superior mesenteric ganglion
     • inferior mesenteric ganglion

4. Synapse directly on chromaffin cells in the adrenal medulla

   • stimulates catecholamine synthesis and secretion
   • the adrenal medulla behaves similarly to a postganglionic neuron, releasing catecholamines into the bloodstream instead of directly into a target organ

#Describe the course and fibre types

#Cervical branches

The cervical ganglia receive input from the T1-7 spinal levels. There are three:

Superior cervical ganglion (C3-4)

• gives rise to the:
  • carotid plexus, which surrounds internal carotid artery
    • which in turn join parasympathetic fibres to feed the ciliary, pterygopalatine and otic ganglia via cranial nerves 7, 9, 10 and 12
  • superior cardiac nerve → superficial and deep cardiac plexuses

Middle cervical ganglion (C6)

• gives rise to the:
  • thyroid nerve
  • middle cardiac nerve → deep cardiac plexus

Inferior cervical ganglion (C7)

• in 80% of people, is fused with the first thoracic ganglion to form the stellate ganglion
• gives rise to the:
  • vertebral artery plexus → brain
  • inferior cardiac nerve → deep cardiac plexus

#Thoracic branches

Branches from the T1-4 paravertebral ganglia feed into the cardiac, pulmonary and oesophageal plexuses.

Branches from all spinal levels give branches to the aortic plexus.

#Thoracic splanchnic nerves

Thoracic splanchnic nerves are preganglionic nerves which originate in the thoracic spine and pierce the diaphragm to synapse on abdominal prevertebral ganglia, to innervate abdominal viscera.

Branches from T5-9 form the greater splanchnic nerve

• joins the coeliac ganglion → coeliac plexus → coeliac axis and foregut (liver, biliary tree, pancrease, spleen, stomach, proximal duodenum)
• also gives branches to the adrenal medulla

Branches from T10-11 form the lesser splanchnic nerve

• joins the superior mesenteric and aorticorenal ganglia to innervate the midgut and kidneys

Branches from T12 form the least splanchnic nerve

• gives branches to the renal plexus

#Lumbosacral splanchnic nerves

Lumbar splanchnic nerves arise from lumbar sympathetic ganglia. They feed the inferior mesenteric ganglion, sending branches to the:

• hindgut
• superior hypogastric plexus → iliac vessels and pelvic viscera

Sacral splanchnic nerves arise from sacral sympathetic ganglia. They feed the inferior hypogastric plexus → pelvic viscera (bladder, rectum, uterus, prostate)

#Nerve fibre types

	Preganglionic	Postganglionic
Length	Short	Long
Fibre type	Myelinated B-fibres	Unmyelinated C-fibres
Synapse location	Autonomic ganglia, such as para- and prevertebral ganglia	On target organ
Neurotransmitter released	Acetylcholine (ACh), which is taken up by type-2 nicotinic receptors (N2 receptors) on post-ganglionic nerves	Predominantly releases noradrenaline, but also adrenaline and dopamine in smaller amounts. Releases ACh in sweat glands and erector pili in skin, important for thermoregulation.

#Outline the receptors and neurotransmitters

• N2 receptors
• Adrenergic receptors
• Dopamine receptors

#References

Ellis, Feldman, Harrop-Griffiths, Lawson. Anatomy for anaesthetists, 8th edition. Blackwell Publishing. 2004.