The brachial plexus is a complex network of nerves central to upper limb function. It transmits signals between the spinal cord and the muscles and skin of the shoulder, arm, forearm, and hand. This network enables both motor functions, for movement, and sensory functions, for feeling. Without this neural network, precise control and sensation in our arms would not be possible.
Understanding the Brachial Plexus Structure
The brachial plexus originates from the anterior rami, or roots, of spinal nerves C5 through T1 in the neck. These five roots then merge to form three main trunks: the superior trunk (from C5 and C6), the middle trunk (from C7), and the inferior trunk (from C8 and T1).
Each of these trunks further divides into anterior and posterior divisions, resulting in six divisions. These divisions then reorganize to create three cords, named for their relationship to the axillary artery: the lateral cord, the medial cord, and the posterior cord. The posterior cord forms from the merger of all three posterior divisions. The lateral cord is formed by the anterior divisions of the upper and middle trunks, and the medial cord is a continuation of the anterior division of the inferior trunk. This sequential organization ultimately gives rise to the various peripheral nerves that extend into the arm.
Major Nerves Arising from the Brachial Plexus
Several significant nerves emerge from the brachial plexus. The five main terminal branches are the musculocutaneous nerve, the axillary nerve, the radial nerve, the median nerve, and the ulnar nerve. These nerves travel through the arm and forearm to reach their target muscles and sensory areas.
The brachial plexus also gives rise to important collateral branches. These include the long thoracic nerve, which descends along the side of the thorax, and the dorsal scapular nerve, typically arising directly from the C5 spinal nerve.
Roles of Brachial Plexus Nerves in Body Function
Each major nerve originating from the brachial plexus contributes distinctly to the motor and sensory capabilities of the arm and hand.
Musculocutaneous Nerve
The musculocutaneous nerve primarily controls muscles that flex the elbow, such as the biceps brachii and brachialis. It also provides sensation to the outer part of the forearm.
Axillary Nerve
The axillary nerve is responsible for movement in the shoulder, specifically controlling the deltoid muscle for arm abduction and the teres minor for external rotation. It also conveys sensation from the skin over the lower part of the deltoid muscle.
Radial Nerve
The radial nerve serves a broad range of functions, controlling muscles that extend the elbow, wrist, fingers, and thumb. This nerve also provides sensory information from the back of the arm, forearm, and parts of the hand.
Median Nerve
The median nerve is crucial for many movements in the forearm and hand, including bending the wrist and fingers, and rotating the forearm. It provides sensation to the palm side of the thumb, index finger, middle finger, and half of the ring finger.
Ulnar Nerve
The ulnar nerve plays a significant role in hand movements, especially the fine motor skills of the hand and the movement of the fourth and fifth fingers. It also conveys sensory information from the pinky finger and the medial half of the ring finger, as well as the associated palm area.
Collateral Nerves
The long thoracic nerve innervates the serratus anterior muscle, which is involved in protracting the scapula, a movement necessary for pushing and reaching overhead. The dorsal scapular nerve controls the rhomboid muscles and levator scapulae, which help to retract and elevate the scapula.
Common Injuries Affecting the Brachial Plexus
The brachial plexus, despite its protected location, can be susceptible to various forms of injury. Trauma is a common cause, often resulting from high-impact events like motor vehicle accidents, particularly motorcycle collisions, or falls. Forceful stretching or pulling of the arm or shoulder can damage these nerves.
Birth injuries can also affect the brachial plexus in newborns, especially during difficult deliveries where the baby’s neck or shoulders are stretched. Other conditions such as compression from tumors or inflammatory responses can also impact the plexus. Symptoms of a brachial plexus injury can vary widely depending on the severity and specific nerves involved, but commonly include muscle weakness, numbness, loss of sensation, or pain in the affected arm, shoulder, or hand. While some mild injuries may heal on their own, more severe damage can lead to lasting weakness or limitations in movement.