The trapezius is innervated primarily by the spinal accessory nerve, also known as cranial nerve XI (CN XI). This nerve provides the main motor supply to all three regions of the trapezius. Cervical spinal nerves from C2 to C4 also contribute, supplying proprioceptive feedback and, in a significant number of people, additional motor fibers.
The Spinal Accessory Nerve: Primary Motor Supply
The spinal accessory nerve is the eleventh cranial nerve, and its main job is powering movement in the trapezius and the sternocleidomastoid muscle. Its spinal root originates from cervical spinal segments C1 through C5 (occasionally C6). These rootlets ascend through the spinal canal and enter the skull through the foramen magnum, where they briefly join a smaller cranial root before exiting the skull through the jugular foramen.
From there, the nerve descends through the neck. It emerges from the back edge of the sternocleidomastoid roughly one-third to halfway down that muscle, about 7.5 to 9 cm above the collarbone. It then crosses the posterior triangle of the neck on a diagonal path, running beneath the investing layer of deep cervical fascia but above the deeper muscles like the levator scapulae. The nerve enters the trapezius about 1.5 cm medial to the midclavicular line and roughly 4.5 cm above the clavicle.
Once inside the muscle, the spinal accessory nerve drives all of the trapezius’s varied actions: elevating, depressing, and rotating the scapula, extending and rotating the head, and assisting arm abduction beyond about 60 degrees by stabilizing the shoulder blade. Without this nerve, coordinated movement between the shoulder, arm, and neck breaks down.
Cervical Nerves C2 to C4: More Than Just Sensory
For decades, textbooks described the cervical nerve contributions (C2, C3, C4) as purely sensory, carrying pain and position-sense information from the trapezius back to the spinal cord. That picture turns out to be incomplete. Research using detailed nerve fiber staining has shown that C3 and C4 contain both motor fibers and a significant number of proprioceptive and sympathetic axons. In one study, a branch from C3 bypassed the spinal accessory nerve entirely and entered the upper trapezius directly, carrying about 38% motor fibers and 62% proprioceptive sensory fibers. This means the cervical nerves can independently drive some trapezius contraction while simultaneously relaying information about muscle stretch and position.
A separate study using intraoperative nerve monitoring during neck dissections found that cervical nerves triggered a measurable trapezius contraction in 45.5% of cases. The C2 root produced motor responses in 27.2% of dissections, and C3 produced them in 18.2%. Interestingly, C4 did not produce a motor response in any case in that series. So while the spinal accessory nerve is always the dominant motor nerve, roughly half of people have meaningful motor backup from upper cervical roots, particularly C2 and C3.
Why This Matters: Nerve Injury and Trapezius Dysfunction
The spinal accessory nerve’s superficial path across the posterior triangle of the neck leaves it vulnerable during surgery. Neck dissections performed for cancer treatment are the most common cause of injury, especially procedures that involve the posterior triangle (classified as level V in surgical anatomy). Radical neck dissections carry the highest risk, while more selective dissections that avoid level V tend to preserve shoulder function better. Even lymph node biopsies in this region can damage the nerve.
When the spinal accessory nerve is damaged, the consequences are predictable and often debilitating. Patients develop what’s sometimes called “shoulder syndrome,” first described in 1952. The hallmark signs include trapezius wasting, a visible drop of the shoulder on the affected side, shoulder pain, and inability to raise the arm above 90 degrees. In clinical testing, the middle and lower trapezius may score 0 out of 5 on strength testing, meaning no detectable muscle activation at all. A characteristic “scapular flip sign” also appears: the shoulder blade wings outward because the inactive middle and lower trapezius can no longer counterbalance the pull of muscles that rotate the arm bone.
The fact that cervical nerves carry independent motor fibers to the trapezius has practical implications for surgery. Cutting these cervical branches during a neck dissection, even when the spinal accessory nerve itself is preserved, can worsen shoulder outcomes. Surgeons who recognize this dual innervation pattern may take extra care to preserve the cervical nerve contributions alongside the accessory nerve.
Testing Trapezius Nerve Integrity
Because the trapezius has three functionally distinct regions, each with somewhat different fiber orientations and actions, clinicians test them separately. The upper trapezius is typically tested by having you shrug your shoulders against resistance. The middle trapezius is assessed with the arm out to the side and the shoulder blade squeezed toward the spine. The lower trapezius is tested with the arm positioned overhead and the shoulder blade drawn downward. Each position isolates the corresponding fiber group so the examiner can identify which portion of the muscle, and by extension which part of the nerve supply, is compromised.
These manual muscle tests are graded on a 0-to-5 scale. A score of 5 means full strength against resistance, while 0 means no visible contraction at all. In complete spinal accessory nerve palsy, the middle and lower portions frequently score 0, while the upper trapezius may retain some function if cervical nerve motor contributions are present. This pattern can help distinguish between a pure accessory nerve injury and a combined injury that also involves the cervical plexus.