S2 Posterior Neck - Learning Objectives
1. What muscles (and other structures) form the boundaries of the posterior (=lateral) triangle (and sub-triangles)? What are the high-yield contents of these triangles? What are the actions of these muscles? What are the innervations of these muscles?
The posterior cervical triangle is bounded anteriorly by the posterior border of the sternocleidomastoid m., posteriorly by the superior part of the trapezius m., and inferiorly by the clavicle.
The posterior cervical triangle may be subdivided into two triangles. Both triangles are bordered anteromedially by the sternocleidomastoid m., posterolaterally by the upper fibers of the trapezius m., and these triangles are divided from one another by the inferior belly of the omohyoid m. They are the:
- occipital triangle: the superior-most triangle; typically contains the nerve point of the neck (Erb’s point) and its branches and the accessory n. (CN XI).
- subclavian (omohyoid) triangle: the inferior-most triangle; typically contains the subclavian v. and supraclavicular lymph nodes.
Trapezius and SCM mm. are both efferently (motor) innervated by the accessory n. (CN XI). The accessory n. is strictly efferent; therefore, afferent supply to these muscles comes from ventral primary rami of cervical nerves (C2-C4). Typically, C2-C3 (sometimes C4) afferently serve the SCM, whereas C3-C4 afferently serve the trapezius. It is likely that C2-C4 also carry some motor fibers.
2. 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 (common carotid aa., jugular vv., subclavian aa. & vv., vagus nn., and trunks of the brachial plexus).
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) and subclavian a. (posterior),
- 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..
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.
The vagus n. (CN X) is the major parasympathetic 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 into 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.
3. How are the parts of the subclavian artery (SCA) delimited? What are the major branches (arteries) from each part of the SCA, and what do these branches supply? When does the SCA become the axillary artery?
The subclavian a. is conceptually divided into three parts, with respect to the vessel’s relationship to the anterior scalene m. The first part of the subclavian a. is found medial to the anterior scalene, the second part of the subclavian a. is posterior to the anterior scalene m., and the third part is lateral to the anterior scalene m.
The first part of the subclavian a. typically hosts three major branches:
- vertebral a. (supplies the brain & spinal cord)
- thyrocervical trunk,
- inferior thyroid a. (supplies the thyroid and parathyroid glands)
- ascending cervical a. (supplies deep muscles of the neck)
- suprascapular a. (supplies supra- and infraspinatus mm.)
- transverse cervical a. (supplies trapezius m. & often gives rise to the dorsal scapular a.)
- internal thoracic a. (supplies anterior thoracic & abdominal walls & the diaphragm).
The second part of the subclavian a. typically hosts one major branch, the:
- costocervical trunk
- deep cervical a. (supplies deep neck muscles)
highest (supreme) intercostal a. (supplies 1st & 2nd intercostal spaces).
The third part of the subclavian a. typically hosts one major branch, the:
- dorsal scapular a. (supplies rhomboid mm. and levator scapulae m.). The dorsal scapular a. may also be a branch of the transverse cervical a.
Beyond the lateral border of the first rib, the subclavian a. transitions into the axillary a.
4. What is the ‘big picture’ structure of a single spinal nerve, and what are the various functions of the parts of a spinal nerve?
Spinal nerves exit the spinal cord at regular intervals as dorsal and ventral rootlets, which coalesce into dorsal and ventral roots, which join to form the trunk of the spinal nerve. The dorsal root is most obvious as it is associated with a mass of cell bodies known as the dorsal root ganglion (DRG). Dorsal roots (and rootlets) are entirely afferent (sensory), whereas ventral roots (and rootlets) are efferent (motor). The trunk of the spinal nerve exits the vertebral canal laterally via an intervertebral foramen and almost immediately bifurcates into a dorsal primary ramus (DPR) and a ventral primary ramus (VPR). These rami may further bifurcate along their courses.
5. Which spinal nerves participate in the cervical and the brachial plexus? Generally, how are these plexuses organized, and what structures do they serve?
The cervical plexus consists of contributions of ventral primary rami (VPR) of cervical spinal nerves (C1,2,3,4) that are responsible for cutaneous (sensory of skin) innervation of the neck and posterior scalp, and efferent (motor) innervation of the infrahyoid mm., diaphragm (and portions of the pericardium), and anterior & middle scalene mm.
The brachial plexus consists of contributions from the ventral primary rami (VPR) of cervical spinal nerves (C5,6,7,8,) & a thoracic spinal nerve (T1) that are responsible for innervation (afferent and efferent) of the upper limb and pectoral girdle (excluding trapezius and SCM). The brachial plexus may also include contributions from C4 & T2. The roots (ventral primary rami) and trunks of the brachial plexus may be found between the anterior and middle scalene mm.
The trunks of the brachial plexus are named according to their relative anatomical positions. The superior trunk typically consists of coalescing ventral primary rami (VPR) of C5 & C6. The middle trunk is typically the continuation of the VPR of C7. The inferior trunk typically consists of coalescing VPRs of C8 & T1.
6. What are the major vascular (artery & vein) pathways of the head & neck and upper limbs?
The arch of the aorta (aortic arch) connects the ascending aorta to the descending aorta. Typically, three branches - brachiocephalic trunk (giving rise to the R. subclavian a. & R common carotid a.), L. common carotid a., & L. subclavian a. - originate from the arch of the aorta to supply blood to the head, neck, upper limbs, and thorax.
The common carotid a. bifurcates into the internal and external carotid aa.
The internal carotid has no branches in the neck. It travels to the cranium, where it is transmitted through the carotid canal, and supplies blood to the brain, orbit, and forehead.
The external carotid a. is the primary source of blood to the face and superficial head. The external carotid has eight branches:
- superior thyroid a.
- ascending pharyngeal a.
- occipital a.
- lingual a.
- facial a.
- posterior auricular a.
- maxillary a.
- superficial temporal a.
The subclavian a. (SCA) supplies the neck, cranial cavity (& brain), anterior wall of the thorax, and upper limbs. See learning objective 3 above for a more detailed treatment of the SCA.
The superior vena cava transmits blood from the head, neck, upper limbs, and thorax to the right atrium of the heart. The superior vena cava is formed by the confluence of the brachiocephalic vv., which are each formed by the confluence of the internal jugular (IJ) vv. and subclavian vv..