Descending part of trapezius muscle

At a glance

English

Descending part of trapezius muscle

Latin (TA2)

Pars descendens musculi trapezii

System

Muscle

Region

Back

Side

Paired, left and right

About the descending part of trapezius muscle

Trapezius is the broad, flat, diamond-shaped muscle that covers the nape and upper trunk on each side, and anatomists divide it into three parts by the direction of their fibres. The descending part, also called the upper or superior fibres, forms the thick ridge between the neck and the point of the shoulder. Its fibres run down and outward, which is why it is called descending.

These fibres arise from the nuchal ligament, the external occipital protuberance and the medial part of the superior nuchal line, with the uppermost fibres from the spine of C7 in many descriptions. At the occiput the muscle is fixed by a thin fibrous sheet that is closely bound to the skin. The fibres converge on the back edge of the outer third of the clavicle. With sternocleidomastoid, which lies in front, it shares an investing layer of deep fascia, and between the two muscles lies the posterior triangle of the neck.

With the neck fixed, the descending part lifts the scapula and the outer end of the clavicle, as in shrugging, and carries some of the weight of the arm. Working with the ascending part and serratus anterior, it rotates the scapula so that the glenoid faces upward, which is needed to raise the arm above shoulder height. With the shoulder fixed, the two sides together extend the head and neck, and one side bends the neck to its own side and turns the face away.

Motor supply comes from the accessory nerve, which runs across the posterior triangle just under the skin and can be injured during operations on lymph nodes there. The result is a drooping shoulder, difficulty raising the arm overhead and pain in the neck and shoulder. Fibres from C3 and C4 bring sensation, including position sense, from the muscle. Holding a weight out in front of the body makes the upper fibres stand out between the neck and shoulder.

Attachments, action and supply

Origin

External occipital protuberance, medial part of the superior nuchal line, and nuchal ligament, with fibres from the spine of C7.

Insertion

Back edge of the outer third of the clavicle.

Action

Elevates the scapula and carries the weight of the arm; helps rotate the scapula upward; extends the head and neck on both sides and bends the neck to its own side on one side.

Nerve supply

Accessory nerve (CN XI) for movement; C3 and C4 for sensation.

Blood supply

Superficial branch of the transverse cervical artery, with branches of the occipital artery.