Posterior division of inferior trunk of brachial plexus

At a glance

English

Posterior division of inferior trunk of brachial plexus

Latin (TA2)

Divisio posterior trunci inferioris plexus brachialis

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the posterior division of inferior trunk of brachial plexus

The brachial plexus is built in stages: roots, trunks, divisions, cords and terminal branches. The inferior (lower) trunk forms from the union of the ventral rami of the eighth cervical and first thoracic spinal nerves, behind the subclavian artery and on the first rib. Just above or behind the clavicle each trunk splits into an anterior and a posterior division, and the posterior division of the inferior trunk is the back-facing branch of the lower trunk.

It is usually the smallest of the three posterior divisions, because most C8 and T1 fibres are destined for flexor muscles and travel instead in the anterior division, which becomes the medial cord. The posterior division of the lower trunk joins the posterior divisions of the upper and middle trunks behind the axillary artery. Together they form the posterior cord, which sits behind the second part of the axillary artery, deep to pectoralis minor.

The divisions mark a functional sorting point in the plexus. Posterior divisions carry fibres for the muscles that develop on the dorsal side of the limb bud: in the adult these are mainly the extensors of the elbow, wrist and fingers and the muscles of the back of the shoulder. Anterior divisions carry fibres for the flexor compartments. The C8 and T1 contribution reaching the posterior cord through this division ends up chiefly in the radial nerve, where it supplies the extensors of the fingers and thumb in the forearm, and to a lesser extent in the thoracodorsal and subscapular nerves.

Because the divisions lie behind the clavicle, they are injured in supraclavicular and retroclavicular trauma rather than singly. Damage to the lower trunk, from a violent upward pull on the arm or from a cervical rib or tumour at the apex of the lung, affects this division together with its anterior partner. The resulting weakness of the hand and finger muscles comes mainly from the anterior division, while any loss of finger and thumb extension reflects the posterior component.

Course and supply

Arises from

The inferior trunk of the brachial plexus (ventral rami of C8 and T1), dividing behind the clavicle.

Ends in

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

Supplies

Carries motor fibres from C8 and T1 to the posterior cord, chiefly for the radial nerve and the extensor muscles of the forearm, wrist, fingers and thumb, with sensory fibres from the back of the limb.