Posterior division of middle trunk of brachial plexus

At a glance

English

Posterior division of middle trunk of brachial plexus

Latin (TA2)

Divisio posterior trunci medii plexus brachialis

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the posterior division of middle trunk of brachial plexus

The middle trunk of the brachial plexus is unusual in being formed by a single root: it is simply the continuation of the ventral ramus of the seventh cervical spinal nerve. Lying between the upper and lower trunks in the posterior triangle of the neck, it passes down towards the clavicle and, at about the level of that bone, splits into an anterior and a posterior division. The posterior division of the middle trunk is the branch that turns backwards.

This division is relatively large, because a substantial share of C7 fibres is bound for the extensor side of the limb. It joins the matching divisions of the upper and lower trunks behind the axillary artery, and the three together form the posterior cord. The anterior division of the middle trunk takes a different route, joining the anterior division of the upper trunk to form the lateral cord.

As with the other divisions, the split into anterior and posterior divisions sorts nerve fibres by the side of the limb they serve. Posterior divisions carry the supply for the muscles that develop on the dorsal side of the limb bud, mainly the extensors and the muscles at the back of the shoulder. Fibres of C7 in this division end up mostly in the radial nerve, where they reach triceps and the wrist and finger extensors, and in the thoracodorsal nerve to latissimus dorsi. Sensory fibres from the skin on the back of the arm and forearm return the same way.

The seventh cervical root is the main supply of triceps, so injury to the middle trunk or its posterior division weakens elbow extension and reduces the triceps reflex. Isolated damage to the middle trunk is uncommon; it usually occurs with injury to the neighbouring trunks behind the clavicle. Because C7 runs largely on its own in this trunk, surgeons sometimes use part of it as a donor in nerve transfers.

Course and supply

Arises from

The middle trunk of the brachial plexus (ventral ramus of C7), dividing behind the clavicle.

Ends in

Joins the matching divisions of the upper and lower trunks to form the posterior cord behind the axillary artery.

Supplies

Carries C7 motor fibres to the posterior cord, chiefly for the radial nerve (triceps and the wrist and finger extensors) and the thoracodorsal nerve, and returns sensory fibres from the back of the limb.