Superior trunk of brachial plexus

At a glance

English

Superior trunk of brachial plexus

Latin (TA2)

Truncus superior plexus brachialis

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the superior trunk of brachial plexus

The superior, or upper, trunk is the first of the three trunks of the brachial plexus. It forms when the fifth and sixth cervical ventral rami unite, usually near the lateral border of scalenus medius after both have emerged between the anterior and middle scalene muscles. Sometimes a small contribution from C4 joins it. From there the trunk runs down and laterally across the posterior triangle of the neck, where it lies fairly superficially above the clavicle, covered by skin, platysma and deep fascia.

Before it divides the trunk gives off two named nerves. The nerve to subclavius runs down in front of the plexus to the small muscle under the clavicle. The suprascapular nerve passes laterally, deep to trapezius, to the suprascapular notch of the scapula and on to supraspinatus and infraspinatus. The trunk then splits behind the clavicle into an anterior division, which joins that of the middle trunk to make the lateral cord, and a posterior division, which joins the posterior cord.

These divisions carry C5 and C6 fibres into the musculocutaneous, median, axillary and radial nerves. In functional terms the upper trunk drives abduction and lateral rotation of the shoulder through deltoid, supraspinatus, infraspinatus and teres minor, flexion of the elbow through biceps, brachialis and brachioradialis, and supination. It carries feeling from the outer side of the arm, forearm and thumb.

A sudden force that pushes the head away from the shoulder, as in a fall from a motorcycle, can stretch or tear the upper trunk. The result is Erb's palsy: the arm hangs by the side, turned inward, with the elbow straight and the forearm pronated, the so-called waiter's tip position. Feeling is reduced over the lateral arm and forearm. The trunk is also the target of the interscalene and supraclavicular nerve blocks used for surgery on the shoulder and upper arm, and these blocks often spread to the nearby phrenic nerve.

Course and supply

Arises from

Formed by union of the ventral rami of C5 and C6, near the lateral border of scalenus medius.

Ends in

Gives the nerve to subclavius and the suprascapular nerve, then divides behind the clavicle into anterior (lateral cord) and posterior (posterior cord) divisions.

Supplies

Carries C5 and C6 fibres for shoulder abduction and lateral rotation, elbow flexion and supination, and sensation from the lateral arm, forearm and thumb.