Trapezius

The trapezius muscle (Latin: musculus trapezius) is a large, flat and broad diamond-shaped muscle of the back that resembles a trapezoid. Together with the latissimus dorsi muscle, it lies in the most superficial layer of the group. The trapezius extends from the occipital bone to the lower thoracic vertebrae and laterally to the scapula. This muscle moves and stabilizes the scapula. The trapezius is composed of three different functional parts:

• Descending (superior) part - fibers of this part descend towards the insertion site, they elevate and rotate the scapula upwards;
• Transverse (middle) part - fibers of this part are directed horizontally and run towards the shoulders, they retract the scapula;
• Ascending (inferior) part - fibers go in the superior direction to reach the insertion point, they medially rotate and depress the scapula.

Origin

The trapezius muscle originates from the nuchal ligament and, therefore, the spinous processes of all seven cervical vertebrae (C1 - C7) lying below the ligament and the following twelve thoracic vertebrae (Th1 - Th12). Also, it arises from the external occipital protuberance and the medial third of the superior nuchal line. Both landmarks are found on the occipital bone of the skull.

The descending part of the muscle originates from the landmarks of the occipital bone, the transverse part arises from the nuchal ligament and spinous processes of all cervical and first three thoracic vertebrae (C1 - Th3), while the ascending part originates from the spinous processes of the thoracic vertebrae below the Th3 level (Th4 - Th12).

Insertion

Overall, the trapezius inserts on the acromial end of the clavicle, acromion and the spine of the scapula. The descending part inserts on the lateral third of the clavicle, the transverse part ends at the acromion and scapular spine, while the ascending part inserts on the spine of the scapula.

Action

The trapezius muscle retracts, elevates, depresses and rotates the scapula upon activation. Each muscle part provides different functions. The descending part elevates the scapula and draws it superomedially. The superior fibers act not only at the scapulothoracic joint but also at the atlantooccipital and atlantoaxial joints. Thus, the descending part also extends and bends the head and neck to the ipsilateral side and provides rotation of the head to the contralateral side. The transverse part retracts the scapula, while the ascending part depresses and medially rotates the scapula.

Innervation

Several nerves participate in the innervation of the trapezius. The trapezius is innervated by the accessory nerve (CN XI) and anterior rami of the third and fourth cervical spinal nerves (C3 - C4) that arise from the cervical plexus. The accessory nerve provides motor innervation, while the cervical spinal nerves are responsible for motor and sensory (pain and sense of position in space) innervation.

Blood supply

The trapezius muscle receives arterial blood supply from the transverse cervical, dorsal scapular and posterior intercostal arteries. The transverse cervical artery is a branch of the thyrocervical trunk, while the dorsal scapular artery usually branches off the subclavian artery. Finally, the posterior intercostal arteries are branches of the thoracic aorta.