The brachial plexus

The plexus is formed by the ventral rami of C5 to T1. The roots unite to form the upper, middle and lower trunks at the root of the neck.

1Injury to the upper trunk (C5, C6) produces Erb–Duchenne paralysis, commonly from excessive separation of the head and shoulder, as in a fall on the shoulder or a difficult delivery.

The lower trunk (C8, T1) is injured by excessive abduction of the arm, producing Klumpke’s paralysis of the small muscles of the hand.

Lesions of the posterior cord paralyse the extensors of the elbow, wrist and fingers.