The brachial plexus is a network of nerves that runs from your spinal cord in the neck, through your shoulder, and into your arm. It originates from five spinal nerve roots (C5 through T1) and is responsible for virtually all movement and sensation in your shoulder, arm, and hand. Every time you lift a cup, grip a steering wheel, or feel someone touch your fingertips, signals are traveling through this network.
How the Brachial Plexus Is Organized
The brachial plexus follows a branching structure that moves from simple to complex as it travels from the neck toward the arm. It starts with five nerve roots exiting the spinal cord between the vertebrae in your lower neck. These roots merge into three trunks, which then split into six divisions, regroup into three cords, and finally branch into five terminal nerves. Each level of this network combines and redistributes nerve fibers so that the final nerves carry exactly the right mix of motor and sensory signals to their target areas.
In rare cases, the plexus receives contributions from additional spinal levels. A small percentage of people have fibers from C4 (called a prefixed plexus) or T2 (postfixed), and in fewer than 0.1% of people, both C4 and T2 contribute. These variations matter during surgery or nerve block procedures in the neck and shoulder region, but they don’t typically cause symptoms on their own.
The Five Terminal Nerves and What They Control
The five major nerves that emerge from the brachial plexus each serve a distinct region of your arm and hand. Together, they cover every muscle and patch of skin from your shoulder down to your fingertips.
- Musculocutaneous nerve (C5-C7): Powers the muscles in the front of your upper arm, including the biceps. It’s why you can bend your elbow and rotate your forearm palm-up. It also provides sensation along the outer forearm down to the base of the thumb.
- Axillary nerve (C5-C6): Controls the deltoid muscle, which lifts your arm out to the side, and the teres minor, which rotates the shoulder outward. It provides feeling over the outer surface of the shoulder.
- Radial nerve (C5-T1): Runs along the back of the arm and forearm, controlling all the muscles that straighten your elbow, extend your wrist, and open your fingers. It provides sensation over the back of the arm, forearm, and the top of the hand near the thumb side.
- Median nerve (C6-T1): Handles most of the muscles in the front of your forearm and the fleshy pad at the base of your thumb. It allows you to pronate your forearm (turn it palm-down), flex your wrist, curl your fingers, and oppose your thumb to your other fingers. Sensation covers the palm side of the thumb, index, middle, and half the ring finger.
- Ulnar nerve (C8-T1): Controls the small muscles of the hand that spread and close your fingers, plus the muscle that pulls the thumb inward. It provides feeling to the little finger and the inner half of the ring finger. This is the nerve responsible for the tingling you feel when you hit your “funny bone.”
Common Causes of Brachial Plexus Injuries
Brachial plexus injuries happen when the nerves are stretched, compressed, or torn. In adults, high-energy trauma is the most common cause: motorcycle and car accidents, sports collisions, or falls that force the head and shoulder apart violently. The stretch can range from mild (a “stinger” or “burner” that resolves in seconds) to severe, where nerve roots are ripped from the spinal cord itself.
In newborns, injury can occur during delivery, particularly when the baby’s shoulder gets caught behind the mother’s pubic bone. A prospective study from 2021 to 2024 found that 14.5 out of every 1,000 live births resulted in a brachial plexus injury diagnosed at birth. Most of these resolve on their own: only 3.6 per 1,000 still showed symptoms beyond two months of age.
Three Grades of Nerve Damage
Not all brachial plexus injuries are equally serious. The Seddon classification system divides nerve injuries into three categories based on how deeply the nerve is damaged.
Neuropraxia is the mildest form. The nerve is bruised or temporarily disrupted but not physically torn. Nerve signals are blocked at the injury site, yet the nerve conducts normally below that point. Recovery typically takes hours to a few months, and the nerve heals fully on its own.
Axonotmesis is a moderate injury where individual nerve fibers inside the nerve are broken, but the surrounding protective layers remain intact. Because those outer layers act as a guide for regrowing fibers, the nerve can recover, though the process is slow. Nerves regenerate at roughly one millimeter per day, so recovery after this type of injury is measured in months and depends on how far the regenerating fibers need to travel to reach their target muscles.
Neurotmesis is the most severe grade, a complete disruption of the nerve and all its surrounding tissue. Without surgery, the severed ends form a nonfunctional lump of scar tissue called a neuroma, and no recovery occurs. Surgical repair is necessary.
How Brachial Plexus Injuries Are Diagnosed
Diagnosis starts with a physical exam. Your doctor will test individual muscles in your shoulder, arm, and hand to map which ones are weak or paralyzed, then check sensation in specific skin zones. This pattern of deficits points to which nerves, cords, or roots are involved.
Electromyography (EMG) and nerve conduction studies are the primary tools for confirming the type and severity of injury. In neuropraxia, conduction is blocked at the injury site but normal below it, a distinctive finding. In more severe injuries, EMG picks up spontaneous electrical activity in denervated muscles, called fibrillation potentials. Over time, the appearance of new motor unit potentials on EMG can signal that nerves are beginning to regenerate, though electrical recovery on testing doesn’t always translate to meaningful strength in real life.
MRI can visualize the nerve roots and surrounding structures, which is particularly useful for detecting root avulsions, where the nerve has been pulled out of the spinal cord entirely. This distinction matters because avulsed roots cannot be repaired directly and require different surgical strategies.
Treatment and Recovery
Mild injuries (neuropraxia) are managed with observation and physical therapy while the nerve recovers on its own. Rehabilitation starts with passive range-of-motion exercises to prevent joints from stiffening while muscles are paralyzed. As strength returns, therapy progresses to active-assisted movements, then resisted exercises. Electrical stimulation is often used alongside these exercises to promote nerve healing and keep denervated muscles from wasting.
For moderate injuries with intact protective layers, the same rehabilitation approach applies, but the timeline stretches longer. Doctors typically monitor progress with serial physical exams and EMG studies over three to six months before deciding whether surgery is needed.
Severe injuries require surgical reconstruction. The two main approaches are nerve grafting, where a donor nerve (often taken from the leg) bridges the gap between severed ends, and nerve transfer, where a functioning but less critical nearby nerve is rerouted to power the paralyzed muscle. A study comparing these techniques for high ulnar nerve injuries found that nerve transfer produced better results: 83% of patients regained functional strength, compared to 57% with nerve grafting. Grip strength recovery was also significantly higher in the nerve transfer group. Sensory recovery was similar between both approaches, with more than half of patients in each group regaining useful sensation.
After surgery, the operated area is immobilized briefly, then passive motion of nearby joints begins early to prevent stiffness. Full recovery after surgical reconstruction takes one to two years in many cases, because regenerating nerve fibers must grow from the repair site all the way to the target muscles. The closer the injury is to the spinal cord and the farther from the hand, the longer recovery takes.
Newborn Brachial Plexus Injuries
Rehabilitation for infants follows the same general principles as adult treatment, scaled for a baby’s size and development. Gentle passive range-of-motion exercises start early to maintain joint flexibility. Sensory stimulation and electrical stimulation may also be introduced. Most birth-related injuries involve the upper roots (C5 and C6), which affects the shoulder and elbow but spares the hand. The majority resolve within the first few months as swelling around the nerves subsides and mild stretch injuries heal. Infants who don’t show improvement by three to six months are evaluated for surgical intervention.