Inferior trunk of brachial plexus

At a glance

English

Inferior trunk of brachial plexus

Latin (TA2)

Truncus inferior plexus brachialis

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the inferior trunk of brachial plexus

The brachial plexus is the network of nerves that serves the upper limb. Its roots, the ventral rami of C5 to T1, come together in the neck to make three trunks. The inferior, or lower, trunk is made when the ventral ramus of the eighth cervical nerve joins that of the first thoracic nerve. This union takes place behind the scalenus anterior, where the trunk rests on the upper surface of the first rib, behind the subclavian artery and close to the dome of the pleura.

Like the other two trunks, it divides behind the clavicle into an anterior and a posterior division. The anterior division of the inferior trunk does not merge with any other division; it simply carries on as the medial cord, so the medial cord carries fibres of C8 and T1 only. The posterior division joins the posterior divisions coming from the superior and middle trunks to form the posterior cord. In anatomical position, each anterior division lies in front of its posterior partner, and the cords that follow are named for where they lie in relation to the axillary artery.

Through these divisions the fibres of the lower trunk reach the ulnar nerve, the medial pectoral nerve, the medial cutaneous nerves of the arm and forearm, the medial root of the median nerve and, via the posterior cord, parts of the radial nerve. In practical terms it carries much of the motor supply to the small muscles of the hand and the long finger flexors, and the feeling from the inner side of the forearm and hand. Sympathetic fibres for the head reach the sympathetic chain close to T1.

Its low position makes the trunk vulnerable in particular ways. A forceful upward pull on the arm, such as grabbing to stop a fall, can stretch or tear the C8 and T1 roots, giving Klumpke's palsy with a weak, clawed hand and numbness along its inner border. A cervical rib or fibrous band can press on the trunk where it crosses the first rib, one form of thoracic outlet syndrome. A tumour at the apex of the lung can invade it, causing pain down the inner arm and, if the sympathetic chain is involved, Horner's syndrome.

Course and supply

Arises from

Formed by union of the ventral rami of C8 and T1, behind scalenus anterior and on the first rib.

Ends in

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

Supplies

Carries C8 and T1 fibres to the medial cord and posterior cord, and through them motor supply to the intrinsic hand muscles and long finger flexors and sensation from the medial forearm and hand.