Trapezius muscle

Origin and insertion

The trapezius muscle has several origin points along the midline of the posterior neck and back. 

• The superior fibers attach on the medial third of the superior nuchal line, the external occipital protuberance of the occipital bone and the nuchal ligament, that is attached to the spinous processes of the C1-C6 vertebrae. These fibers have a descending course towards their insertion point, hence why this part of the trapezius is referred to as the descending part.
• The middle fibers originate from the spinous processes of T1-T4 (some sources state C7-T3) vertebrae and their intervening supraspinous ligaments. These fibers are directed horizontally, running laterally towards the shoulder. Thus, these fibers represent the transverse part of the trapezius.
• The inferior fibers originate from the spinous processes of the T4-T12 vertebrae and their respective supraspinous ligaments. These fibers course superiorly and laterally towards their insertion point and hence, represent the ascending part of the trapezius. 

Along their course, all fibers of the trapezius converge laterally on the superior angle of the scapula to attach on their respective insertion points. 

• The superior fibers insert on the posterior border of the lateral third of the clavicle.
• The middle fibers insert onto the medial margin of the acromion of the scapula, as well as the superior crest of the scapular spine. 
• The inferior fibers insert via an aponeurosis on a tubercule at the lateral apex of the medial end of the scapular spine.

Structure and relations

According to their attachments, course and location discussed earlier, we can divide the trapezius into three functional parts. Each part is implicated in different movements, as we’ll explore in more detail later in this article. The parts of the trapezius include:

• The descending (upper) part, composed of superior fibres
• The transverse (middle) part, composed of middle fibres
• The ascending (lower) part, composed of inferior fibres

Along with latissimus dorsi, the trapezius is the most superficial of the superficial extrinsic back muscles. It overlies the remaining muscles of this group, namely the rhomboids and levator scapulae. Deep to the trapezius muscle we can also find the serratus posterior superior, which belongs to the intermediate layer of extrinsic muscles of the back. 

In addition, the trapezius covers several muscles of the superficial layer of the intrinsic muscle group, such as the splenius capitis, splenius cervicis, spinalis, longissimus and iliocostalis muscles. The upper part of the trapezius also overlies the suboccipital region.

The anterior margin of the trapezius muscle forms the posterior border of the posterior triangle of the neck. Moreover, its free inferior margin forms the medial boundary of the triangle of auscultation, an area of the chest wall not obscured by the scapula and only covered by a thin layer of muscle.

Innervation

The trapezius muscle is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, motor innervation to the trapezius is conveyed by the accessory nerve (CN XI) as well as the anterior rami of the C3 and C4 spinal nerves, which also contain proprioceptive/sensory fibers from the muscle.

Test your knowledge on the superficial muscles of the back!

Blood supply

The arterial supply to trapezius varies depending on the level. 

• The descending (upper) part 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 transverse (middle) part of the muscle is supplied by the superficial cervical artery, or by a branch from the transverse cervical artery.
• The ascending (lower) part is supplied by muscular branches of the dorsal scapular artery, which arises from the subclavian artery.

Function

The main function of the trapezius is stabilizing the scapula in its anatomical place, as well as controlling it during movements of the shoulder and upper limb. The throwing action is a common manoeuvre in which the trapezius muscle is active, along with the deltoid muscle and the rotator cuff muscles. In addition, the trapezius is also involved in the movements of the head and neck. 

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The actions of the trapezius muscle is largely dependent on the direction of the fibres that are contracting.