brachial plexus

Anterior view of right brachial plexus.
Based on illustration from Gray's Anatomy

The brachial plexus is a collection of large nerve trunks that pass from the lower part of the cervical spine (in the neck) and the upper part of the thoracic spine (in the chest) down the arm. The nerve trunks of the brachial plexus divide into the musculocutaneous and axillary, median, ulnar, and radial nerves which both control muscles in and receive sensations from the arm and hand.

Detailed anatomy of the brachial plexus

This important plexus is formed by the anterior primary rami of the lower four cervical nerves and the greater part of the anterior primary ramus of the first thoracic nerve. The brachial plexus is reinforced, above, by a small twig of communication which passes from the fourth cervical nerve to the fifth, and, below, by a similar connecting twig that passes upward, in front of the neck of the second rib, from the second thoracic nerve to the first.

The manner in which the nerves join to form the plexus is very constant. The fifth and sixth cervical nerves unite to form an upper trunk; the seventh remains single and proceeds laterally as a middle trunk; while the eighth and first thoracic nerves join to form the lower trunk. A short distance above the clavicle each of the three trunks splits into an anterior and posterior division. The three posterior divisions unite to form the posterior cord of the plexus and the lowest or most medial of the posterior divisions is much smaller than the other two. Of the three anterior divisions, the two upper unite to form the lateral cord of the plexus, and the lower passes distally by itself as the medial cord. The three cords give off most of the branches that supply the upper limb.

The brachial plexus may be divided, therefore, into four stages:

First stage: Five separate nerves (namely, the lower four cervical and first thoracic.
Second stage: Three trunks (namely, upper, middle, and lower).
Third stage: Three anterior divisions and three posterior divisions.
Fourth stage: Three cords (namely, lateral, medial, and posterior)

The brachial plexus begins at the lateral border of the scalenus anterior muscle in the neck, behind the lower third of the border of the sternomastoid. It passes through the posterior triangle of the neck, and behind the middle third of the clavicle, into the upper part of the axilla; and it ends behind the lower border of the pectoralis minor near the coracoid process, where it beaks up into the large nerves of the upper limb. Its termination is therefore at the junction of the second and third parts of the axillary artery; consequently, the first and second parts of the artery are related to the cords of the plexus, and the third part is related to the large nerves that spring from them.

Injuries to the brachial plexus

Injuries to the brachial plexus are an important and fairly common cause of partial or complete loss of movement and sensation to the arm. Damage to the brachial plexus sometimes occurs during birth, with an increased risk in breech delivery. In adults, a common cause of brachial plexus injury is a fall from a motorcycle.

Injury is usually a forcible separation of the neck and shoulder, due to a fall pushing downwards or to a blow to the side of the neck that stretches or tears upper nerve roots in the plexus. Damage to these roots causes paralysis in muscles of the shoulder and elbow.

Injury to lower nerve roots in the plexus, causing paralysis of muscles in the forearm and hand, can result from a forcible blow that lifts the arm and shoulder upwards.

In severe injuries, both the upper and lower nerve roots of the brachial plexus are damaged, producing complete paralysis of the arm.

Paralysis may be temporary if the stretching was not severe enough to tear nerve fibers.

Treatment

Treatment of a brachial plexus injury depends on the extent and severity of nerve damage. Possible investigational procedures include EMG (electromyography) to demonstrate which nerves are still intact, and myelography (X-ray examination of the spinal cord after injection of a contrast medium).

Nerve roots that have been torn can be repaired by nerve grafting, a microsurgery procedure often performed with good results. However, is a nerve root has become separated from the spinal cord, surgical repair will not be successful.

In the event of permanent paralysis of a particular group of muscles in the arm, function can be improved by a muscle or tendon transfer operation to provide an alternative structure to perform a particular movement. Physiotherapy, with exercises continued at home, helps restore function after a successful nerve graft operation and can also help to reduce contractures in paralyzed muscles.

Other disorders

Apart from injuries, the brachial plexus may be affected by the presence of a cervical rib (extra rib), infections, tumors, or aneurysms.

Related category

• ANATOMY AND PHYSIOLOGY

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