M01.02.022 Sympathetic Innervation

The sympathetic nervous system (SNS) is a division of the autonomic nervous system that functions involuntarily. Working alongside the parasympathetic system, it helps maintain body homeostasis. It is best known for mediating the “fight or flight” response.

This article focuses on the anatomy of sympathetic innervation to the head and neck, including its structure, course, and clinical relevance.

Anatomical Structure and Course

Origin

• Spinal cord region: Thoracic (T1-T6).
• Sympathetic chain: Extends from the base of the skull to the coccyx. Contains ganglia where preganglionic fibers synapse with postganglionic neurons.

Ganglia of Interest

There are three key ganglia in the cervical region:

1. Superior cervical ganglion
2. Middle cervical ganglion
3. Inferior cervical ganglionIn some individuals, the middle cervical ganglion may be absent, and the inferior ganglion may fuse with the first thoracic ganglion to form the cervicothoracic ganglion.

Pathways

• Postganglionic fibers “hitch-hike” along arteries to reach their target organs.
• Associated arteries include the common, external, and internal carotid arteries, inferior thyroid artery, subclavian artery, and vertebral artery.

Ganglia in Detail

1. Superior Cervical Ganglion

• Location:
  • Posterior to the carotid artery.
  • Anterior to C1-C4 vertebrae.
• Postganglionic branches:
  • Internal carotid nerve: Forms a plexus, innervates eye structures and blood vessels.
  • External carotid nerve: Targets smooth muscle of arteries.
  • Nerve to pharyngeal plexus: Combines with vagus and glossopharyngeal nerves.
  • Superior cardiac branch: Contributes to the cardiac plexus.
  • Gray rami communicantes: Distributes fibers to C1-C4.

2. Middle Cervical Ganglion

• Location: Anterior to the inferior thyroid artery and C6 vertebra (may be absent).
• Postganglionic branches:
  • Gray rami communicantes: Distributes fibers to C5 and C6.
  • Thyroid branches: Innervate the larynx, trachea, pharynx, and upper esophagus.
  • Middle cardiac branch: Contributes to the cardiac plexus.

3. Inferior Cervical Ganglion

• Location: Anterior to the C7 vertebra (fused with T1 in the cervicothoracic ganglion).
• Postganglionic branches:
  • Gray rami communicantes: Distributes fibers to C7, C8, and T1.
  • Branches to subclavian and vertebral arteries: Innervate arterial smooth muscle.
  • Inferior cardiac nerve: Contributes to the cardiac plexus.

Clinical Correlation: Horner’s Syndrome

Horner’s syndrome results from damage to sympathetic fibers traveling to the head and neck. It manifests as a triad of symptoms:

1. Partial ptosis: Drooping of the upper eyelid due to paralysis of the superior tarsal muscle.
2. Miosis: Pupil constriction from dysfunction of the dilator pupillae muscle.
3. Anhidrosis: Loss of sweating on the affected side of the face due to denervation of sweat glands.

Common Causes

• Spinal cord lesions
• Traumatic injury
• Pancoast tumor: Cancer affecting the lung apex.

Summary Table

Points to Remember

• Sympathetic fibers originate from T1-T6 and ascend via the sympathetic chain.
• Key cervical ganglia: Superior, Middle, and Inferior.
• Clinical relevance includes Horner’s syndrome, often caused by nerve injury or compression.

Bibliography

• TeachMeSeries Ltd, “Sympathetic Nervous System: Anatomy of the Head and Neck.”
• Wikimedia Commons [CC-BY-2.5], “Horner’s Syndrome Illustrations.”