The brachial plexus is a network of nerves that serves as the communication pathway between the spinal cord and the entire upper limb. Originating in the neck and shoulder area, this structure is formed by the joining of the ventral rami, or roots, from the lower four cervical spinal nerves (C5-C8) and the first thoracic spinal nerve (T1). Its purpose is to deliver both motor signals, which control muscle movement, and sensory information, which allows for feeling, to the shoulder, arm, forearm, and hand, ensuring the full range of motion, strength, and sensation.
Understanding the Brachial Plexus Structure
The organization of the brachial plexus follows a systematic pattern, proceeding from the spine outward toward the arm. The five spinal nerve roots (C5-T1) merge shortly after exiting the vertebrae to form three Trunks: the Superior (C5-C6), the Middle (C7), and the Inferior (C8-T1). Each Trunk subsequently divides into an Anterior and a Posterior Division.
These Divisions then reorganize to form three Cords, named based on their location relative to the axillary artery. The three Posterior Divisions unite to form the Posterior Cord. The Anterior Divisions of the Superior and Middle Trunks form the Lateral Cord. The Anterior Division of the Inferior Trunk continues as the Medial Cord.
The Five Major Terminal Nerves
The five primary nerves that supply the majority of the upper extremity emerge from the three Cords.
The Musculocutaneous Nerve arises from the Lateral Cord and innervates the main flexor muscles of the arm, including the biceps brachii, brachialis, and coracobrachialis. It also provides sensory innervation to the skin along the lateral side of the forearm. The Axillary Nerve branches off the Posterior Cord, innervating the deltoid muscle, which allows the arm to be lifted away from the body. It also supplies the teres minor muscle, which assists in external rotation, and provides sensation to the skin over the lower part of the deltoid.
The Radial Nerve is the continuation of the Posterior Cord and carries fibers from all five spinal roots (C5-T1). It is primarily the nerve of extension, controlling the triceps muscle in the arm and the muscles in the posterior compartment of the forearm responsible for extending the wrist and fingers. Sensory distribution includes the posterior surface of the arm and forearm, as well as the lateral dorsal surface of the hand.
The Median Nerve is formed by contributions from both the Lateral and Medial Cords, creating a loop around the axillary artery. Its motor function controls most of the flexor muscles in the forearm and the muscles at the base of the thumb, allowing for fine motor movements like opposition. Sensory fibers supply the skin of the palm and the first three and a half fingers on the thumb side.
The Ulnar Nerve is the continuation of the Medial Cord and travels down the medial side of the arm, passing behind the medial epicondyle of the elbow (the “funny bone”). It governs many of the intrinsic muscles within the hand, which are responsible for movements such as spreading the fingers apart and bringing them together. The sensory component supplies the skin of the medial side of the hand, including the little finger and the medial half of the ring finger.
The Collateral Nerves of the Brachial Plexus
Several smaller nerves branch off the brachial plexus earlier in its structure, primarily supplying muscles closer to the trunk and shoulder girdle. These collateral nerves arise from the Roots, Trunks, and Cords, and their role is to stabilize and move the scapula and proximal arm.
The Dorsal Scapular Nerve branches directly from the C5 root and innervates the rhomboid muscles, which retract the shoulder blade toward the spine. The Long Thoracic Nerve originates from the roots (C5, C6, and C7) and supplies the serratus anterior muscle. This muscle holds the scapula flat against the chest wall and assists in upward rotation of the arm during overhead movements.
The Suprascapular Nerve arises from the Superior Trunk and provides motor control to the supraspinatus and infraspinatus muscles, which stabilize the shoulder joint. The Cords also give off collateral branches, such as the Medial and Lateral Pectoral Nerves, which supply the pectoralis major and minor muscles used for shoulder adduction and flexion. The Thoracodorsal Nerve, arising from the Posterior Cord, controls the latissimus dorsi muscle, which assists in extending and adducting the arm.
Clinical Consequences of Nerve Damage
Damage to the brachial plexus can result in significant functional loss, with symptoms varying depending on the location and severity of the injury. Injuries often result from trauma, such as high-impact accidents or excessive stretching of the neck and shoulder. Compression, such as that caused by Thoracic Outlet Syndrome, is also a common cause.
Specific patterns of injury are recognized based on which part of the plexus is affected. Erb’s Palsy, an upper plexus injury, involves damage to the C5 and C6 roots, leading to weakness or paralysis in the muscles of the shoulder and upper arm. Affected individuals often present with the arm hanging loosely at the side, internally rotated, in a position described as a “waiter’s tip” posture.
Klumpke’s Palsy is a lower plexus injury, typically affecting the C8 and T1 roots. This damage results in a loss of function primarily in the forearm and hand muscles, often causing a characteristic deformity known as a “claw hand,” where the fingers are flexed and the intrinsic hand muscles are paralyzed.