trapezius

The trapezius is a large, flat muscle of the upper back and neck that covers the posterior aspect of the neck and shoulders. It originates from the external occipital protuberance and the nuchal ligament, as well as the spinous processes of C7 through T12, and inserts on the lateral third of the clavicle, the acromion, and the spine of the scapula. The muscle is functionally divided into three parts: the upper (descending) fibers, the middle (transverse) fibers, and the lower (ascending) fibers, which work together to move and stabilize the scapula.

Innervation and blood supply are key to its function. The trapezius receives motor innervation from the spinal accessory nerve (cranial nerve XI). Proprioceptive and pain fibers arise from the ventral rami of C3 and C4. Blood supply is mainly from the transverse cervical artery, with possible contributions from the dorsal scapular artery.

Functionally, the upper fibers elevate the scapula and assist in neck extension and lateral flexion; the middle fibers retract the scapula; and the lower fibers depress the scapula and assist in upward rotation. Together, the three parts stabilize the scapula during arm movements and contribute to overall posture and shoulder mechanics.

Clinical notes include trapezius weakness or paralysis due to spinal accessory nerve injury, which can cause shoulder drooping, impaired arm elevation, and altered scapular movement. Strains or overuse can also occur, particularly in athletes and manual laborers. Anatomical variation is common, with some individuals having differences in size or fiber composition of the trapezius.