The trapezius muscle is a large, paired, triangular shaped muscle located superficially in the back of the neck and thorax. When viewed together, the two trapezius muscles together roughly form a diamond or trapezoid shape, responsible for the trapezius name. Trapezius is one of the superficial posterior axioappendicular, or extrinsic muscle pairs. The posterior axioappendicular muscles are responsible for the attachment of the shoulder girdle to the trunk. Trapezius is largely involved in movements of the shoulder girdle, and is therefore functionally considered as a muscle of the upper limb rather than of the back.
Occipital bone, ligmamentum nuchae, spinous processes of C7 - T12
Elevation of the scapula (upper fibres), retraction of the scapula (middle fibres), depression of the scapula (lower fibres), upward rotation of the glenoid cavity
Motor: Accessory nerve (CN XI)
Sensory: C3, C4 dorsal ram
Occipital artery (upper part), superficial or transverse cervical artery (middle part), dorsal scapular artery (lower part)
The origin of the trapezius muscle is expansive and is located along the midline of the back. It attaches superiorly to the medial portion of the superior nuchal line of the occipital bone, and to the external occipital protuberance. Moving inferiorly, the fibers attach to the spinous processes of the vertebrae. They attach to the spinous processes of the C1-C6 via the ligamentum nuchae, and directly to the spinous processes of C7-T12.
The fibers of trapezius converge laterally toward the apex of its triangular shape. They insert along the spine of the scapula, to the acromion of the scapula most laterally, and onto the lateral ⅓ of the clavicle.
The action of the trapezius muscle is largely dependent on the direction of the fibres that are contracting. The different fibres act with synergist muscles to produce different motions of the scapula. The upper fibres, also called the descending fibres based on their direction, act with the levator scapulae muscle produce elevation of the scapula. The lower, or ascending fibres are responsible for depression of the scapula. The middle fibres acting with the rhomboids produce retraction of the scapula. The trapezius muscle is also responsible for upward rotation of the scapula, along with the serratus anterior muscle. This allows us to raise our arm above our heads passed the level of the shoulder.
The trapezius muscle is the only muscles of the upper limb that does not receive its innervation from the brachial plexus. The nerve conveying motor innervation to trapezius is the spinal accessory nerve, which is the 11th cranial nerve. The accessory nerve exits the skull through the foramen magnum. It emerges in the neck from the lateral border of the sternocleidomastoid muscle, which it also innervates. It crosses the posterior triangle of the neck to reach trapezius.
Sensory innervation of the trapezius muscle is through the dorsal rami of the C3 and C4 spinal nerves.
The arterial supply to trapezius varies depending on level. The upper portion of the muscle is supplied by transverse muscular branches arising from the occipital artery (branch of the external carotid), which passes along the deep surface of the muscle. The middle portion of the muscle is supplied by the superficial cervical artery, or by a branch from the transverse cervical artery. The lower portion is supplied by muscular branches of the dorsal scapular artery, which passes medial to the medial border of the scapula. It reaches trapezius by piercing the rhomboid muscles, or by passing between the rhomboid major and rhomboid minor.
The function of the trapezius muscle can be tested by placing a hand on the patients shoulder and assessing their ability to elevate or ‘shrug’ the shoulder against resistance. This test, coupled with the functionality test for sternocleidomastoid can be used to assess damage to the accessory nerve. Weakness in trapezius muscle with complete functioning of the sternocleidomastoid muscle would indicate damage to the accessory nerve at a more distal point, such as in the posterior triangle. Weakness of both the trapezius and sternocleidomastoid muscles would allude to damage closer to where the accessory nerve exits the base of the skull.
As the trapezius muscle has an extensive vascular supply, it can be used as a site for musculocutaneous tissue flap harvesting for reconstructive purposes in other areas of the body, such as for breast reconstruction.