Like sternocleidomastoid, omohyoid, and levator scapulae, this muscle, though attached to the pectoral girdle, is not derived from myotomes of the limb bud. In other words, it is not supplied from the brachial plexus.
The muscle arises in the midline from skull to lower thorax and converges on the outer part of the pectoral girdle, which it rotates upwards. Its origin extends from the medial third of the superior nuchal line to the spine of C7 vertebra, finding attachment to the ligamentum nuchae between the external occipital protuberance and the vertebral spine. Below this the origin extends along the spinous processes and supraspinous ligaments of all 12 thoracic vertebrae. Opposite the upper thoracic spines the muscle shows a triangular aponeurotic area, which makes a diamond with that of the opposite side.
The occipital fibers are inserted into the lateral third of the clavicle at its posterior border; from above downwards the fibers can be traced into their insertion along the medial border of the acromion and the superior lip of the crest of the scapular spine. The part of the muscle which arises from the lowest half-dozen thoracic spines is inserted by a narrow recurved tendon into the medial end of the spine; this tendon slides over the bare area at the base of the spine of the scapula, a bursa intervening.
Nerve supply of trapezius:
From the spinal part of the accessory nerve (C1-5 or 6) and branches from the cervical plexus (C3 and 4), the latter normally being only proprioceptive. These nerves cross the posterior triangle to enter the deep surface of trapezius, although in some cases they do appear to contain motor fibers. The accessory nerve can be distinguished from the cervical branches by the fact that it emerges from within the substance of sternocleidomastoid; the cervical nerves emerge from behind sternocleidomastoid.
Action of trapezius:
Rotation of the scapula, so that the glenoid fossa faces upwards, is produced by both upper and lower fibers. The upper fibers elevate the acromion, the lower depress the medial end of the scapular spine; each factor is complementary to the other in producing rotation of the scapula, like turning a butterfly nut. This action is strongly assisted by the lowest four digitations of serratus anterior, which are inserted into the inferior angle of the scapula. The antagonists of the trapezius are the rhomboids and levator scapulae, much weaker because gravity assists them. The muscles of the anterior and posterior axillary folds (pectoralis major and latissimus dorsi) should also be counted as opponents, since by adducting the arm they indirectly rotate the scapula back to its position of rest.
Test. The shoulder is shrugged against resistance and the upper border of the muscle is seen and felt. If paralyzed, the combined actions of levator scapulae and serratus anterior may compensate.