Superior Mediastinum and Root of Neck - LO 5

5. What is the Root of Neck (RON)? Which structures (e.g. neurovasculature, muscles, & bones) are associated with the RON, and what are the classic anatomical relationships of these structures to one another? 

The root of the neck (RON) is the nexus between the neck, thorax, and upper limbs. The RON is the proximal attachment site for many neck muscles and transmits important neurovasculature (e.g. common carotid aa., jugular vv., subclavian aa. & vv., vagus nn., and trunks of the brachial plexus).

Anterior scalene m. 

The anterior scalene m. is an important anatomical landmark for understanding the neurovasculature of the root of the neck. There are four classic anatomical relationships to understand:

• the anterior scalene m. is sandwiched between the subclavian v. (anterior to) and subclavian a. (posterior to),

• the anterior scalene m. establishes the borders for the divisions of the subclavian a.,

• the anterior scalene m. is immediately posterior to (and closely associated with) the phrenic n. as the phrenic n. descends the neck and the root of the neck, and

• the anterior scalene m. is separated from the middle scalene m. by the roots (ventral primary rami of spinal nn.) and trunks of the brachial plexus and subclavian a.

Phrenic n.

The phrenic n. (C3,4,5) descends from the cervical plexus through the root of the neck (just anterior to, and upon the anterior scalene m.) before entering the thorax between the subclavian a. & v. The phrenic n. is efferent (motor) and afferent (sensory) to the diaphragm and afferent (sensory) to the pericardium and diaphragmatic pleura.

Vagus n. (CN X)

The vagus n. (CN X) is the major parasympathetic supply and conduit to the thorax and abdomen, and is the major innervation to muscles of the larynx and pharynx, and aspects of the head.

The R. and L. vagus nn. take different routes through the thorax. Both nerves descend the neck within the carotid sheaths, and cross anteriorly over the subclavian aa., deep to the first ribs. The R. vagus n. then sends a major branch (the R. recurrent laryngeal n.) inferiorly and then posteriorly around the R. subclavian a., lateral to the trachea, on a course for the larynx. The L. vagus n. sends the L. recurrent laryngeal n. inferiorly and then posteriorly around the concavity of the arch of the aorta, lateral to the trachea, also on a course for the larynx.

Lymphatics

Lymphatic ducts and trunks are often overlooked, yet important features associated with the root of the neck. Lymphatic vessels transmit lymph, filtered blood plasma, from interstitial tissues to venous circulation in the root of the neck. Lymphatic trunks are larger, regional lymphatic vessels that may coalesce into ducts. 

The thoracic duct is the largest of all lymphatic vessels. The thoracic duct typically conveys all the lymph from inferior to the diaphragm, the left thorax (via the left bronchomediastinal trunk), the left upper limb (via the left subclavian trunk), and the left head & neck (via the left jugular trunk). The thoracic duct anastomoses with the venous system in the vicinity of the left venous angle, the point at which the subclavian v. meets the internal jugular v.

The trunks associated with the right thorax (the right bronchomediastinal trunk), the right upper limb (the right subclavian trunk), and the right head & neck (the right jugular trunk), may either coalesce into a right lymphatic trunk that anastomoses in the vicinity of the right venous angle, or the trunks may each independently anastomose in the vicinity of the right venous angle.