The sympathetic nervous system is a division of the autonomic nervous system. It is involuntary, and acts with the parasympathetic system to maintain body homeostasis.
The actions of the sympathetic nervous system are associated with the ‘fight or flight’ response.
In this article, we shall look at the anatomy of the sympathetic innervation to the head and neck – its structure, anatomical course, and its clinical correlations.
Anatomical Structure and Course
The sympathetic fibres to the head and neck begin in the spinal cord. They originate from the thoracic region (T1-6), and therefore need to ascend to reach the structures in the head and neck.
After leaving the spinal cord, the fibres enter the sympathetic chain. This structure spans from the base of the skull to the coccyx, and is formed by nerve fibres and ganglia (collections of nerve cell bodies). There are three ganglia within this chain that are of interest – the superior, middle and inferior cervical ganglia. The sympathetic fibres synapse with these ganglia, with post ganglionic branches continuing into the head and neck.
Each of the three ganglia are related to specific arteries in the head and neck. The post-ganglionic fibres hitch-hike along these arteries (and their branches) in order to reach their target organs.
We shall now look at the structure and function of the ganglia in more detail.
Note: In some individuals, the middle cervical ganglion is often absent and the inferior cervical ganglion is often fused with the first thoracic ganglion, as a result is known as the cervicothoracic ganglion. In addition to this, the superior and middle cervical ganglia are commonly connected together.
Superior Cervical Ganglion
The superior cervical ganglion is located posteriorly to the carotid artery, and anterior to the C1-4 vertebrae. Several important post-ganglionic nerves originate from here:
- Internal carotid nerve – hitch-hikes along the internal carotid artery, forming a network of nerves. Branches from the internal carotid plexus innervate structures in the eye, the pterygopalatine artery and the internal carotid artery itself.
- External carotid nerve – hitch-hikes along the common and external carotid arteries, forming a network of nerves. It innervates the smooth muscle of the arteries.
- Nerve to pharyngeal plexus – combines with branches from the vagus and glossopharyngeal nerves to form the pharyngeal plexus.
- Superior cardiac branch – contributes to the cardiac plexus in the thorax.
- Nerves to cranial nerves II, III IV, VI and IX.
- Gray rami communicantes – distributes sympathetic fibres to the anterior rami of C1-C4.
Middle Cervical Ganglion
The middle cervical ganglion is absent in some individuals. When present, it is located anteriorly to the inferior thyroid artery and the C6 vertebra. Its postganglionic fibres are:
- Gray rami communicantes – distributes sympathetic fibres to the anterior rami of C5 and C6.
- Thyroid branches – travel along the inferior thyroid artery, distributing fibres to the larynx, trachea, pharynx and upper oesophagus.
- Middle cardiac branch – contributes to the cardiac plexus in the thorax.
Inferior Cervical Ganglion
The inferior cervical ganglion is situated anteriorly to the C7 vertebra. It is occasionally fused with the first thoracic vertebrae, forming the cervicothoracic ganglion. There are three post-ganglionic fibres that arise from this ganglion:
- Gray rami communicantes – distributes sympathetic fibres to the anterior rami of C7, C8 and T1.
- Branches to the subclavian and vertebral arteries – These innervate the smooth muscle present in the arteries.
- Inferior cardiac nerve – contributes to the cardiac plexus in the thorax.
Clinical Relevance: Horner’s Syndrome
The sympathetic fibres can be stretched or damaged along their course to the head and neck. If these nerves are unilaterally disturbed, it produces a triad of main symptoms known as Horner’s syndrome:
Partial Ptosis – drooping of the upper eyelid. This is due to paralysis of the superior tarsal muscle, which acts to help open the eyelid.
- Miosis – constriction of the pupil. This is due to paralysis of the dilator pupillae, a muscle located within the eye that acts to dilate the pupil.
- Anhydrosis – decreased sweating (affecting the same side of the face as the lesion). This is due to a loss of innervation to the sweat glands of the face.
Horner’s syndrome has a multitude of causes. These include spinal cord lesions, traumatic injury and a Pancoast tumour (a cancer affecting the apex of the lung, which can involve the ganglia).
|Ganglia||Vertebral Level||Arteries Involved||Effector Organ(s)|
|Superior cervical ganglion||C1-C4||Common, external and internal carotid arteries|
|Middle cervical ganglion||C6||Inferior thyroid artery|
|Inferior cervical ganglion||C7||Vertebral and subclavian arteries|