The brachial plexus is a complex network of nerves that serves as the communication pathway between the spinal cord and the entire upper limb, including the shoulder, arm, forearm, and hand. This intricate bundle is responsible for both sensation and movement. The formation of this plexus allows nerve fibers from different spinal levels to reorganize and combine, ensuring that each major nerve carries a blend of signals to and from the various parts of the limb.
Anatomy of the Brachial Plexus
The brachial plexus is an organized structure with five distinct sections, moving from the spine outward toward the arm. It originates from the anterior branches of the spinal nerves C5 through T1, which are designated as the roots of the plexus. These five roots then combine to form three trunks: the superior trunk (C5, C6), the middle trunk (C7), and the inferior trunk (C8, T1).
Each of the three trunks immediately splits into an anterior and a posterior division, resulting in six divisions total. These divisions separate the nerve fibers that will supply the flexor muscles (anterior divisions) from those that supply the extensor muscles (posterior divisions). The divisions then merge again to create three cords, which are named based on their position relative to the axillary artery: the lateral, posterior, and medial cords.
The posterior cord is formed by the joining of all three posterior divisions. The lateral cord is created by the anterior divisions of the superior and middle trunks, while the medial cord is the continuation of the anterior division of the inferior trunk.
The Five Terminal Nerves
The brachial plexus culminates in five major terminal nerves that distribute motor and sensory information to the entire upper limb. The Musculocutaneous nerve originates from the lateral cord. It travels through the anterior compartment of the arm, eventually becoming a cutaneous nerve in the forearm.
The Median nerve is unique in its formation, as it receives contributions from both the lateral cord and the medial cord. It begins in the arm and courses down the center of the limb, ultimately supplying most of the forearm and hand. The Ulnar nerve is the continuation of the medial cord and travels along the inner aspect of the arm and forearm.
Two major nerves emerge from the posterior cord: the Axillary nerve and the Radial nerve. The Axillary nerve takes a shorter path, wrapping around the humerus near the shoulder joint. The Radial nerve travels down the posterior aspect of the arm and forearm.
Primary Motor and Sensory Functions
The Musculocutaneous nerve is responsible for the action of elbow flexion, largely by innervating the biceps brachii and brachialis muscles. Its sensory function involves providing feeling to the skin on the lateral side of the forearm.
The Axillary nerve primarily controls the shoulder joint, enabling the ability to lift the arm away from the body, known as abduction. It also innervates a small muscle that helps with external rotation of the arm. Sensation from the Axillary nerve covers the skin over the lower part of the shoulder.
The Radial nerve is the main extensor nerve of the limb, controlling the muscles that straighten the elbow, wrist, and fingers. This nerve allows for the release of objects held in the hand and provides feeling to the skin on the back of the arm, forearm, and part of the hand.
The Median nerve is largely responsible for the movements of the forearm and hand that permit fine motor tasks, particularly the muscles used for grasping and opposition of the thumb. It provides sensation to the skin on the palm side of the thumb, index, middle, and half of the ring finger.
The Ulnar nerve innervates most of the small intrinsic muscles within the hand. This control is crucial for actions like spreading the fingers apart and bringing them together. Its sensory distribution covers the skin on the medial side of the hand, including the little finger and the adjacent half of the ring finger.
Smaller Branches and Clinical Implications
Beyond the five terminal nerves, the brachial plexus gives rise to several smaller branches that supply muscles around the shoulder and chest, many of which branch off the roots and trunks. For example, the Long Thoracic nerve originates directly from the roots and is responsible for stabilizing the shoulder blade against the back of the rib cage. The Dorsal Scapular nerve and the Thoracodorsal nerve also branch earlier in the plexus to control muscles that move the scapula and assist with pulling the arm downward.
The location where the brachial plexus is injured determines the pattern of functional loss experienced by a patient. Injuries to the upper part of the plexus, such as the C5-C6 roots or superior trunk, impair the ability to raise the arm and bend the elbow. Conversely, damage to the lower part of the plexus, involving the C8-T1 roots or inferior trunk, results primarily in weakness and sensory loss in the hand and wrist.
Trauma is a common cause of brachial plexus injury, often involving excessive stretching or tearing of the nerve tissue. The nerves can also be compressed as they pass through the neck and shoulder region, a condition known as thoracic outlet syndrome.