Anatomy – What is a BPI?

The brachial plexus (Plexus: Latin ‘braid’) is a network of nerves in the neck and shoulder region. They pass from the spinal cord in the neck, under the clavicle (collar bone) into the upper arm. These nerves carry electrical signals from the brain and the spinal cord, to the muscles and skin of the shoulder, arm and hand to provide movement and sensation (feeling) in your arm.

The plexus is formed by 5 nerve roots which exit the spinal column in the neck, four named after the lower cervical (C) vertebrae (C5, C6, C7 and C8), and the fifth root from the first thoracic (T) vertebra (T1).

The roots join to form three trunks – the upper, middle and lower trunk. Each trunk then divides into an anterior and posterior division. The divisions then join to form three cords – lateral, posterior and medial, before finally splitting to form the major nerves of the arm. These nerves enable the signals that allow movement and sensation to reach the arm. If any part of the path of the nerve is injured, then signals from the brain will not reach the individual muscles in your arm and those muscles will not work.

How does the injury occur?

In adults the commonest cause of injury is traction (stretching) of the brachial plexus, usually sustained during a road traffic accident (RTA), mainly in motorcyclists. Occasionally damage occurs to the major blood vessels that carry blood to and from the arm (subclavian artery and vein).