The brachial plexus is a neural plexus (a grouping and branching of nerves) located deep in the neck, shoulder, and maxilla region that is responsible for the proper innervation and control of the muscles of the shoulder, upper chest, and arms (upper limbs). Because of the complexities of branching nerve roots, trunks, and cords of the brachial plexus, injuries to the brachial plexus region often cause loss or impairments of function at distant muscle groups.
The nerves forming the brachial plexus come from spinal nerves, specifically the last four cervical and first thoracic spinal nerve. The cervical nerves are designated C5, C6, C7, and C8. Although there are only seven cervical vertebrae, there are eight cervical nerves. The thoracic spinal nerve is designated as T1. Spinal nerves result from the unification of dorsal and ventral spinal roots in the intervertebral foramen. The spinal nerves then divide into anterior and posterior primary rami. The spinal verves comprising the brachial plexus are from the anterior rami divisions.
The spinal nerves C5 and C6 fuse to form the upper trunk of the brachial plexus. Spinal nerve C7 becomes a middle trunk of the brachial plexus. Spinal nerves C8 and T1 join to form a lower trunk of the brachial plexus.
At about the level of the clavicle, the trunks of the brachial plexus divide to form anterior and posterior nerves. The anterior nerve divisions from the upper and middle trunks form the lateral cord. The anterior branch nerves from the lower trunk form a medial cord. The posterior divisions band together to form the posterior cord of the brachial plexus. As the cords continue, they come to lie lateral, medial, and posterior to the axillary artery, and it is this anatomical relationship from which each cord derives its name.
The brachial plexus has many branches from its root, trunk, and cord regions.
Above the level of the clavicle are the supraclavicular branches that branch directly from the spinal nerve roots. These nerves include the long thoracic nerve (also known as the nerve of Bell) that forms from C5, C6, and C7 spinal nerves. The long thoracic nerve innervates the serratus anterior muscle. Injuries to this nerve may result in an inability to push an object because of the loss of function to the agonist muscles in the scapular region. Also branching from the brachial plexus in the supraclavicular region are the dorsal scapular nerve, and nerves to the subclavius.
The medial brachial cutaneous nerve, and the median pectoral nerve have their origins in the inferior trunk of the brachial plexus.
The thorcodorsal nerve and middle subscapular nerves branch from the posterior cord. The axillary nerve and the upper and lower subscapular nerves also trace back to the posterior cord. The axillary nerve ultimately continues on as the radial nerve.
The medial root of the median nerve comes from the medial cord of the brachial plexus. The medial nerve ultimately continues down the arm as the ulnar nerve.
Injury to the axillary nerve branching from the brachial plexus can result in a paralysis of the deltoid and a loss of sensation in the skin in the scapular area. Injuries to the deltoid region can also result in the ability to abduct the arm.
Injury to the median nerve of the brachial plexus can cause a loss of flexion of the fingers. This loss of flexion results in a loss of the critical ability to oppose the thumb with individual fingers. Median nerve impairment can also result in a loss of range of motion of the arm. Individuals who sustain median nerve injury causing loss of index finger flexion may develop an index finger that "points" or remains extended. Because the median nerve ultimately passes through the carpal tunnel of the wrist, injuries or inflammation of the wrist (e.g., carpal tunnel syndrome) can result in pain and loss of feeling well away from the wrist itself.
Injuries to the radial nerve that branches from the brachial plexus, injuries commonly associated with injuries to the humerus bone of the arm, often develop into an excessive or permanent flexion of the wrist or flaccid wrist drop.
Impairment of the ulnar nerve derived from the brachial plexus can result in paralysis or wrist extension. Severe ulnar nerve injuries can result in the hand taking on a claw-like appearance with the fingers spread out and unable to flex.
A protective neurovascular sheath made up of fascia protects the brachial plexus. Microscopic and imaging examinations establish that the connective tissues invest and help partition the brachial plexus.
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