The Autonomic Nervous System

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The cranial nerves | Main Anatomy Index

Last updated 30 March 2006

The Autonomic Nervous System

 

Sympathetic and Parasympathetic Systems

 

General Layout of the ANS

The simplified classical view of the design of the ANS is as follows:

The efferent pathway is made up of 2 neurons:

  1. a preganglionic neuron, (myelinated), located in the spinal cord or brainstem, synapsing with;
  2. postganglionic neuron(s) (unmyelinated) in an autonomic ganglion.
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The Sympathetic System

  1. Synapse with postganglionic fibres which leave the sympathetic trunk in the grey rums communicans, rejoins the same ventral ramus, then are distributed to the target organs (smooth muscles of blood vessels, sweat glands...)
  2. Travel up or down the trunk to synapse with postganglionic fibres in other ganglia (e.g. cervical, lumbar, sacral, which do not receive direct rami communcantes from the cervical, lower lumbar or sacral segments).
  3. Pass through the ganglion without relay to synapse in prevertebral ganglion (e.g. coeliac ganglion, known to lay people as "solar plexus").

 

Sympathetic Supply to the Head and Neck

The preganglionic fibres, which supply the head and neck, arise from the spinal segments T1-T2. They enter the sympathetic trunk and travel upward to synapse in one of the three ganglia in the neck:

  1. The superior cervical ganglion (located anterior to vertebrae C1, C2).
  2. The middle cervical ganglion (small, often absent, lying anterior to C6).
  3. The inferior cervical ganglion which is usually fused with the first thoracic ganglion to form the cervicothoracic (or stellate) ganglion (located anterior to C7 and the neck of the first rib).

Each of these ganglia gives rise to:

  1. Cardiac branches.
  2. Branches to blood vessels, sweat glands, hair follicles in the neck and head. The superior ganglion sends branches to spinal nerves C1-C4, the middle ganglion to spinal nerves C5-C6, and the cervicothoracic ganglion to spinal nerves C7, 8-T1.
  3. Vascular branches which pass to the vertebral, common, internal and external carotid arteries. Some of these branches "hitch-hike" along the arteries and their branches to reach their targets in the head and neck. The superior ganglion sends branches along the internal and external carotid arteries to reach structures in the orbit, face, nasal and oral cavities and pharynx. The middle ganglion sends branches along the inferior thyroid artery to reach the larynx, trachea and upper oesophagus. The inferior ganglion sends branches to the subclavian and vertebral arteries.
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The Parasympathetic System

The parasympathetic system outflow is craniosacral, emerging from:

  1. The oculomotor, facial, glossopharyngeal, and vagus nerves.
  2. The spinal cord segments S2, 3, 4. These fibres travel in the branches of sacral nerves S2-S4 (nervi erigentes) to the pelvic viscera.

 

Parasympathetic Supply to the Head and Neck

There are four ganglia in the head: ciliary, pteryopalataline, otic, and submandibular.

 

1. Oculomotor Nerve

 

2. Facial Nerve

 

3. Glossopharyngeal Nerve

 

4. Vagus Nerve

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Michael Tam (c) 1998