Middle trunk of brachial plexus

At a glance

English

Middle trunk of brachial plexus

Latin (TA2)

Truncus medius plexus brachialis

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the middle trunk of brachial plexus

The brachial plexus gathers the ventral rami of C5 to T1 into three trunks in the lower part of the neck. The middle trunk is the simplest of them: it is formed by the ventral ramus of the seventh cervical nerve alone, with no other root joining it. It emerges between the scalenus anterior and scalenus medius, lying between the superior trunk above and the inferior trunk below, and crosses the posterior triangle of the neck behind the lower part of sternocleidomastoid towards the clavicle.

Behind the clavicle the trunk divides, like the others, into an anterior and a posterior division. The anterior division joins the anterior division of the superior trunk to form the lateral cord, which therefore carries fibres from C5, C6 and C7. The posterior division joins the posterior divisions of the upper and lower trunks to form the posterior cord. The middle trunk gives off no named branch before it divides, which sets it apart from the superior trunk with its subclavian and suprascapular nerves.

Because its fibres pass into both the lateral and posterior cords, C7 ends up in several major nerves. Through the posterior cord it contributes heavily to the radial nerve, and so to triceps and the extensors of the wrist and fingers, and to the thoracodorsal nerve for latissimus dorsi. Through the lateral cord it reaches the lateral root of the median nerve and the musculocutaneous nerve, adding to the flexors of the wrist and to pronation. Its skin territory is centred on the middle finger.

An injury limited to C7 is uncommon, because the root seldom fails on its own, but it can be torn together with the upper roots in severe traction injuries of the shoulder and neck. A lesion of the middle trunk weakens elbow extension and wrist and finger extension and may blunt the triceps reflex. Surgeons sometimes take the C7 root from the healthy side and reroute it across to repair a badly damaged plexus, because losing it alone causes little lasting deficit.

Course and supply

Arises from

Formed by the ventral ramus of C7 alone, emerging between scalenus anterior and scalenus medius.

Ends in

Divides behind the clavicle into an anterior division, which joins the lateral cord, and a posterior division, which joins the posterior cord.

Supplies

Carries C7 fibres to the lateral and posterior cords, contributing to the radial, median, musculocutaneous and thoracodorsal nerves and to sensation over the middle finger.