Accessory Nerve

Description

Arising from the upper cervical spinal roots (classically C1 to C5), the spinal accessory nerve (cranial nerve XI) ascends toward the jugular foramen and then descends obliquely across the lateral neck. Along its superficial course it enters the deep surface of the sternocleidomastoid muscle, then re-emerges near the muscle’s posterior border and traverses the posterior triangle on the prevertebral fascia. It continues inferoposteriorly to reach the trapezius, where terminal branches distribute along the muscle’s deep surface. The sternocleidomastoid lies anteromedial to the nerve’s posterior triangle segment, while the trapezius forms the posterolateral muscular target. This lateral neck trajectory is the one surgeons worry about. During posterior cervical triangle lymph node biopsy, selective neck dissection, or placement of a central line high in the neck, the accessory nerve can be injured where it crosses the posterior triangle, leading to trapezius denervation, scapular winging, shoulder droop, and impaired arm abduction above 90 degrees due to loss of upward rotation. A small nerve with big consequences. In teaching settings, the layout supports head and neck anatomy labs, cranial nerve modules, and operative anatomy primers that need a clean explanation of why CN XI palsy produces shoulder dysfunction rather than facial deficits. It also fits otolaryngology and plastic surgery references that map safe dissection planes relative to sternocleidomastoid and trapezius, and it pairs well with exam checklists for assessing accessory nerve function by resisted shoulder shrug and head rotation. Anatomical accuracy verified by SciePro's Medical Advisory Board.