Accessory Nerve, Posterior View

Description

Arising from the upper cervical spinal cord and ascending through the foramen magnum to exit the skull via the jugular foramen, the spinal accessory nerve (CN XI) is traced on the posterior male neck as it crosses the posterior triangle. From a point just deep to the sternocleidomastoid region, its course angles inferolaterally toward the trapezius, running superficial to the levator scapulae and deep to the investing fascia, with the external ear and occipital region providing cranial orientation. Beneath the trapezius, the splenius capitis and splenius cervicis form a layered muscular bed over the posterior cervical spine, while portions of the deltoid and scapular contour anchor the shoulder girdle landmarks. Posterior exposure clarifies the relationship between CN XI and the cervical plexus, where proprioceptive fibers from C2 to C4 commonly join the nerve as it enters the trapezius. This is the corridor implicated in iatrogenic accessory nerve injury during posterior triangle lymph node biopsy or neck dissection, a lesion that produces shoulder droop, lateral scapular winging, and weak overhead abduction from trapezius denervation even when the deltoid remains intact. Surface anatomy matters here. The nerve becomes vulnerable as it traverses the posterior cervical triangle toward the anterior border of trapezius. Faculty can place this plate directly into head and neck anatomy labs to teach the posterior triangle contents, fascial planes, and the motor pattern linking sternocleidomastoid to trapezius. It also supports surgical texts and ENT or plastics teaching files that cover levels II to V lymphadenectomy, accessory nerve preservation, and postoperative shoulder syndrome. Anatomical accuracy verified by SciePro's Medical Advisory Board.