Human Anatomy
Posterior Triangle of the Neck
Posterior triangle of the neck for MBBS: boundaries, accessory nerve, cervical and brachial plexus, subclavian vessels and applied anatomy, mapped to NMC codes AN29.1 to AN29.4.
MedNext Academy | 3 min read
Posterior Triangle of the Neck
Posterior triangle of the neck for MBBS: boundaries, accessory nerve, cervical and brachial plexus, subclavian vessels and applied anatomy, mapped to NMC codes AN29.1 to AN29.4.
The posterior triangle of the neck chapter defines a surgically important space bounded by sternocleidomastoid, trapezius and the clavicle. It covers the accessory nerve, the cervical and brachial plexus roots, the subclavian vessels and the reasons this triangle is entered so often in the clinic.
High-yield: Posterior Triangle of the Neck
- The posterior triangle is bounded in front by sternocleidomastoid, behind by trapezius and below by the middle third of the clavicle.
- Its roof is the investing layer of deep cervical fascia and its floor is the prevertebral fascia over the scalene and other muscles.
- The accessory nerve is the most superficial nerve crossing the triangle and is at risk during lymph node biopsy.
- Injury to the accessory nerve weakens trapezius, causing drooping of the shoulder and difficulty in raising the arm above the head.
- The roots and trunks of the brachial plexus emerge between scalenus anterior and scalenus medius in the lower part of the triangle.
- The subclavian artery, third part, lies on the first rib and is the lowest large vessel in the triangle.
- The external jugular vein crosses the triangle superficially and drains into the subclavian vein.
- The cutaneous branches of the cervical plexus, the lesser occipital, great auricular, transverse cervical and supraclavicular nerves, emerge at the posterior border of sternocleidomastoid.
- The point where these cutaneous nerves emerge is called the nerve point or Erb's point of the neck.
- The subclavian triangle, or supraclavicular fossa, is the lower part where the subclavian vessels and lower brachial plexus lie.
- The occipital triangle is the upper part crossed by the accessory nerve.
- The inferior belly of omohyoid crosses the lower triangle and subdivides it.
- Lymph nodes along the accessory nerve and the transverse cervical vessels drain the scalp and neck.
- A cervical rib may compress the lower trunk of the brachial plexus and the subclavian artery, producing thoracic outlet syndrome.
Boundaries and key contents
- **Boundaries:** Anterior: sternocleidomastoid. Posterior: trapezius. Base: middle third of clavicle.
- **Nerves:** Accessory nerve, cutaneous branches of the cervical plexus, roots and trunks of the brachial plexus.
- **Vessels:** Third part of the subclavian artery, external jugular vein, transverse cervical and suprascapular arteries.
- **Clinical:** Accessory nerve injury at biopsy, cervical rib causing thoracic outlet syndrome.
NMC competencies in this chapter
- **AN29.1:** Boundaries, roof and floor of the posterior triangle
- **AN29.2:** Contents of the posterior triangle
- **AN29.3:** Course and distribution of the accessory nerve
- **AN29.4:** Applied anatomy of the posterior triangle
Frequently Asked Questions
Why is the accessory nerve at risk in the posterior triangle?
It runs superficially across the triangle, deep only to the investing fascia, so it can be damaged during lymph node biopsy, leading to trapezius weakness and a drooping shoulder.
What is the nerve point of the neck?
It is the point near the middle of the posterior border of sternocleidomastoid where the four cutaneous branches of the cervical plexus emerge, also called Erb's point of the neck.
How can a cervical rib cause symptoms?
It can compress the lower trunk of the brachial plexus and the subclavian artery as they cross the first rib, producing the neurovascular features of thoracic outlet syndrome.
How does this chapter help in NEET-PG?
The accessory nerve, brachial plexus roots and subclavian vessels feature in surgery and orthopaedics questions, so the triangle's anatomy transfers directly to PG preparation.
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