Middle Trunk of the Brachial Plexus

truncus medius plexus brachialis

The middle trunk is simply the continuation of the C7 root, the only root that does not join another to form its trunk. It has no named branches of its own and divides behind the clavicle into an anterior division (to the lateral cord) and a posterior division (to the posterior cord).

Anatomical Data

Course & Distribution

Origin	Continuation of the C7 root
Course	Passes laterally between the superior and inferior trunks in the posterior triangle and divides behind the clavicle into anterior and posterior divisions
Motor	Triceps and the wrist and finger extensors (C7 fibres in the radial nerve) Pronator teres and flexor carpi radialis (C7 fibres in the median nerve)
Sensory	Middle finger (C7 dermatome)

Clinical Relevance

Clinical Notes

Because C7 fibres are shared between the radial and median nerves, a middle trunk lesion mainly weakens elbow, wrist and finger extension, with sensory loss over the middle finger. When an upper plexus injury extends to C7, the Erb posture is joined by wrist and finger flexion. The healthy opposite C7 root can be transferred across the neck as a donor in contralateral C7 transfer, used in severe plexus injuries and studied for arm spasticity after stroke.

Pathology

Common Injuries & Conditions

Extended Erb Palsy (C5-C7)

Traction injury involving the superior and middle trunks adds triceps and wrist extensor weakness to the classic Erb pattern, so the wrist hangs flexed. It has a poorer outlook for spontaneous recovery than pure C5-C6 injury.

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