The neck represents an important zone of transition between the head, thorax and arms. Critical neurovascular structures are densely packed and traverse this region; having a sound anatomical awareness of this region is key for performing surgery in this region, and appreciating the effect of pathological processes on the neurovascular supply. In this article we will focus on the anterior neurovascular structures, the vertebral arteries will be discussed in a separate article.
The Carotid Sheath
The key structure to first appreciate is the carotid sheath, this region of deep fascia extends from the base of the skull to the first rib and sternum. The carotid sheath is composed from dense layers of fascia from the prevertebral, pretracheal and deep investing layers of fascia.
Figure showing the different layers of deep fascia in the neck.
This structure forms a channel for neurovascular structures to ascend and descend the neck. The carotid sheaths on each side are anterolateral to the prevertebral fascia and posterolateral to the pretracheal fascia. The cervical sympathetic trunk lies behind the carotid sheath, the ansa cervicalis lies on the anterior surface of the carotid sheath. The carotid sheath is deep to the sternocleidomastoid and can be accessed by the carotid triangle or reflecting the sternocleidomastoid and deep investing layer of fascia. Or, it could be accessed by going through the floor of the posterior triangle.
The carotid sheath can be conceptually divided into a superior and inferior half, the contents differing in each half. The superior half contains structures leaving the jugular foramen, cranial nerves IX, X, XI, (Glossopharyngeal, Vagus and Accessory nerves) and the internal jugular vein (IJV). These are joined by the cranial nerve XII (hypoglossal nerve) exiting the hypoglossal canal. The internal carotid is also in the superior part of the sheath travelling in a cranial direction, into the carotid canal in the petrous part of the temporal bone. As we descend caudally, cranial nerves IX, XI and XII leave the superior half of the carotid sheath, cranial nerve X continues down into the inferior half of the sheath along with the internal jugular and common carotid, which commences at the upper border of the thyroid cartilage.
The accessory nerve
Cranial nerve XI leaves the sheath dorsally, travelling behind sternocleidomastoid and appears in the posterior triangle of the neck, 1/3 the way down the posterior border of sternocleidomastoid. It travels down across the posterior triangle, taking a diagonal path to tuck in behind the anterior border of the trapezoid, 1/3 the way up (from the clavicle) its anterior border. Remember XI innervates both sternocleidomastoid and trapezius.
The Glossopharnygeal nerve
Cranial nerve IX provides sensory branches to the carotid body and leaves the carotid sheath by first going in front of the ICA, then once it is out of the sheath it passes behind the external carotid artery (ECA) and then goes to travel between the superior and middle constrictor to become part of the pharyngeal plexus and innervate the stylopharyngeus muscle.
The Vagus nerve
Cranial nerve X travels right down the entire length of the sheath, however, it has branches which do leave the sheath. The pharyngeal branch leaves the sheath by first passing In front of the ICA, outside the sheath it passes behind the ECA. It contributes special visceral efferent fibres (branchial motor fibres) to the pharyngeal plexus. The superior laryngeal branch leaves the sheath and divides into an internal and external branch. The internal branch forms a neurovascular bundle with the superior laryngeal artery and pierces the thyrohyoid membrane to provide sensory innervation to the larynx above the vocal cords. The external branch remains on the outside of the larynx and innervates the cricothyroid muscle.
Cranial nerve X continues down the inferior part of the sheath. On the left side it travels right through the sheath to enter the superior mediastinum. Here it gives off the left recurrent laryngeal branch which hooks around the ductus arteriosus and then ascends in the groove between the trachea and oesophagus. In the base of the neck it forms a neurovascular bundle with the inferior thyroid artery. The left recurrent laryngeal innervates all the left sided intrinsic muscles of the larynx except the left cricothyroid.
On the right, as the vagus is leaving the carotid sheath to enter the superior mediastinum, it gives off the right recurrent laryngeal nerve, this hooks under the right subclavian artery and ascends forming a neurovascular bundle with the inferior thyroid artery. It ascends between the trachea and oesophagus to innervate the right sided intrinsic muscles of the larynx except the right cricothyroid. Both recurrent laryngeal nerves also provide sensation below the vocal cords in the larynx.
Figure adapted from Netter’s Atlas, showing the relationships of the vagus nerve and its branches in the neck.
The Hypoglossal nerve
Cranial nerve XII doesn’t stay in the superior aspect of carotid sheath long, it passes out and travels medial to diagastric but laterally to all other key structures (ICA ECA, facial artery, x) in the neck. It does this to reach the oral cavity. It enters the oral cavity with the lingual artery, above mylohyoid, lateral to hyoglossus and medial to the submandibular and sublingual glands.
Figure adapted from Netter’s Atlas, showing the relationships of the hypoglossal nerve.
Let’s turn our focus to the arterial contents, from the base of the neck the common carotid artery ascends in the carotid sheath, until it bifurcates at the level of C4, the upper border of the thyroid cartilage. In reality the level of bifurcation can vary. At the bifurcation, the internal carotid continues in the sheath cranially towards the carotid canal, without branching, a key features used to help distinguish it from the external carotid. The external carotid leaves the sheath and gives the following branches: Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Superficial temporal and Maxillary arteries. The branches can be recalled using the following mnemonic: Some Anatomist Like Fun Others Like SadoMasochism
Figure showing branches of the external carotid artery. Adapted from Netter’s Atlas
The internal jugular is formed by the sigmoid sinus and as it leaves the jugular foramen the inferior petrosal sinus drains into it. The internal jugular continues down the carotid sheath and is joined by the following tributaries: Facial, Lingual, Pharyngeal, Superior thyroid and Middle thyroid veins until it finally joins the subclavian vein to form the brachiocephalic vein. The IJV has the deep jugular nodes on its anterior surface and is lateral to the internal carotid and common carotid as it descends. The vagus (CN X) is typically between and behind the IJV and internal carotid.
Clinical Anatomy By Regions, Richard Snell
Last’s Anatomy By Chummy Sinnatamby