The Trapezius is part of the superficial posterior thoracoappendicular muscles (which are the extrinsic shoulder muscles) and the left and right Trapezius muscles form a diamond shape or Trapezoid shape from which the muscle got its name. The Trapezius is a large flat muscle and the most superficial of the upper part of the back which together with the Deltoid muscle gives the contour of the shoulder. Deep to the Trapezius, there are three muscles that attach the scapula to the vertebral column, these muscles include rhomboid major, rhomboid minor and levator scapulae; together, these three muscles and Trapezius and also the anterior muscles of the scapula help to position the scapula on the trunk.
Trapezius Origin and Insertion
The Origin of Trapezius is very extensive, arising in the midline from the skull to lower thorax and then converges on the outer part of the pectoral girdle. The muscle extends from the medial third of the superior nuchal line to the spine of C7 vertebra. The Trapezius inserts on the lateral third of clavicle; below this point, this origin extends along the spinous processes and supraspinous ligaments of all the 12 thoracic vertebrae. The upper fibers are inserted into the posterior border of the lateral third of the clavicle at its posterior border. The middle fibers are inserted 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 lower six thoracic spines is inserted by a narrow recurved tendon into the medial end of the spine
Trapezius Nerve supply
Trapezius is innervated by the spinal part of the accessory nerve (C1 to C5) and branches from the cervical plexus (C3 and C4) the branches from the cervical plexus are usually only proprioceptive, although in some cases they contain motor fibers as well. These nerves cross the posterior triangle to enter the deep surface of trapezius. The sternocleidomastoid muscles helps to differentiate the accessory nerve from the cervical branches because the accessory nerve emerges from within the substance of sternocleidomastoid but the cervical nerves emerge from behind sternocleidomastoid.
Blood supply of Trapezius
The Trapezius is supplied by the superficial branch of transverse cervical artery or superficial cervical artery a branch of the thyrocervical trunk.
The whole fibers of Trapezius can act at once to retract the scapula, while the action of the upper and lower fibers are for rotation of the scapula, tilting the glenoid cavity upwards, an essential component of abduction of the shoulder. The descending part of the Trapezius which is the superior part elevates the scapula, the ascending (inferior) part depresses; and middle part(or all parts together) help to retract scapula. The upper fibers can elevate the whole scapula (shrug the shoulder) or prevent its depression (as when carrying something heavy). They can also produce lateral flexion of the neck, but acting with the upper fibers of the opposite side they can extend the neck.
Test for Trapezius
To test the Trapezius, the shoulder is shrugged against resistance and the upper border of the muscle is seen and felt.