Autonomic Nervous System

How is the nervous system organised?


What do you understand by the autonomic nervous system?

The autonomic nervous system is the involuntary, automatic branch of the nervous system that is largely concerned with control and maintenance of homeostasis via reflex pathways, but can be influenced by higher centres and somatic activity as well. It is comprised of two antagonistic components – sympathetic and parasympathetic – which differ in their anatomy, function and pharmacology.

Anatomy

Both sympathetic and parasympathetic pathways have two nerves between the central nervous system and the target organ. Both have a myelinated preganglionic nerve, then a ganglion, then a non-myelinated postganglionic nerve reaching the target organ.

Most organs are supplied by both, however some organs are only supplied by one system and not the other, such as the piloerector muscles (sympathetic only) and lacrimal gland (parasympathetic only)

Parasympathetic outflow is cranio-sacral, while sympathetic outflow is thoracolumbar via T1 to L2.

Parasympathetic ganglia are close to the target organ, with long preglanglionic and short post ganglionic fibres, while sympathetic ganglia are further from target organ, usually in the sympathetic chain.

Function

For most organs, both systems exert effects, but usually one system has a dominant effect. The parasympathetic system has a role in physiological processes that occur at rest, so there is usually an underlying background level of activity, or parasympathetic ‘tone’. Meanwhile the sympathetic system tends to only activate in times of stress and high activity.

Pharmacology

Both sympathetic and parasympathetic pathways employ acetylcholine as their preganglionic synapses via nicotinic receptors. Postganglionic receptors differ, however, with the sympathetic system using noradrenaline or adrenaline and the parasympathetic system using muscarinic acetylcholine receptors.

The one exception is sweat glands = sympathetic but acetylcholine at muscarinic receptors.

Also the adrenal medulla is unique in that the pre-synaptic neuron synapses directly onto the chromaffin cells of the adrenal gland, with acetylcholine as a neurotransmitter.


Tell me about the sympathetic chain

  • A chain from occiput to coccyx
  • 2cm lateral to vertebrae
  • Cervical
    • Superior, middle and inferior ganglia
  • Thoracic
    • 12 ganglia
    • Supplies splanchnic and intercostals
  • Lumbar
    • 4 ganglia
  • Sacral
    • 4 ganglia

How many nuclei does the vagus nerve have?

  • Four nuclei in the medulla
    • Dorsal motor nucleus of the Vagus
    • Nucleus Ambiguus
    • Solitary tract nucleus
    • Spinal trigeminal nucleus (mainly trigeminal but some vagal input)

Tell me about the stellate ganglion, and why it is relevant to anaesthetic practice

  • Sympathetic ganglion formed from fusion of first thoracic and inferior cervical ganglion
  • Occurs in 75-80% of people
  • Found at the level of C7, anterior to the neck of the first rib, and medial to vertebral artery, behind the carotid sheath
  • Local anaesthetic applied to the brachial plexus can track round to the stellate ganglion producing Horner’s syndrome
    • Ptosis
    • Miosis
    • Anhidrosis
  • A stellate ganglion block can be used for:
    • Vascular insufficiency of the upper limb
    • Complex regional pain syndrome
    • Severe refractory angina
    • Post herpetic neuralgia

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