The brachial plexus is a network of nerves that originates from five spinal nerve roots in your neck (C5 through T1) and branches out to supply movement and sensation to your entire arm, from shoulder to fingertips. Think of it as the electrical wiring system for your upper limb. Every time you lift your arm, grip a cup, or feel someone touch your hand, signals are traveling through this network.
Where the Brachial Plexus Sits in Your Body
The plexus begins deep in the neck, where five nerve roots exit the spinal column and travel through a narrow gap between two muscles called the anterior and middle scalene muscles. From there, the nerves pass under the collarbone and into the armpit, running alongside major blood vessels the entire way. The carotid artery sits just to the inside of the nerve roots in the neck, and the axillary artery becomes the central landmark once the nerves reach the armpit. The phrenic nerve, which controls the diaphragm, runs just in front of and above the brachial plexus roots, which is why injuries or surgical procedures in this area sometimes affect breathing.
How the Network Is Organized
The brachial plexus follows a specific branching pattern: roots, trunks, divisions, cords, and terminal branches. Medical students often memorize this sequence because each level has clinical significance when pinpointing where an injury has occurred.
The five nerve roots (C5, C6, C7, C8, and T1) merge into three trunks. The upper trunk combines C5 and C6, the middle trunk continues from C7 alone, and the lower trunk joins C8 and T1. These trunks run just above and behind the collarbone, near the subclavian artery. Each trunk then splits into front and back divisions, which regroup in the armpit into three cords named for their position relative to the axillary artery: lateral, posterior, and medial.
Those cords finally give rise to five major terminal nerves that travel down the arm.
The Five Terminal Nerves and What They Control
Each terminal branch has a distinct job, which is why damage at different points in the plexus produces very different symptoms.
- Musculocutaneous nerve: Powers the biceps and brachialis muscles, giving you the ability to bend your elbow and flex your arm. It also provides sensation along the outer forearm.
- Axillary nerve: Controls the deltoid muscle, which lifts your arm out to the side. It also supplies sensation to a patch of skin on the outer shoulder sometimes called the “regimental badge” area.
- Radial nerve: Runs down the back of the arm and is responsible for straightening your elbow (triceps), extending your wrist, and opening your fingers. It provides sensation across the back of the hand and the outer three and a half fingers on that side.
- Median nerve: Supplies most of the forearm flexor muscles and the thumb muscles that allow you to grip and pinch. Sensation covers the palm side of the thumb, index finger, middle finger, and half of the ring finger. Compression of this nerve at the wrist is what causes carpal tunnel syndrome.
- Ulnar nerve: Controls the small intrinsic muscles of the hand that allow fine finger movements and a strong grip. It provides feeling to the little finger and the inner half of the ring finger. This is the nerve responsible for the “funny bone” sensation when you bump your elbow.
Common Brachial Plexus Injuries
Brachial plexus injuries range from mild stretches to complete tears. The Seddon classification breaks nerve damage into three severity levels. In neuropraxia, the mildest form, the nerve’s insulating coating is temporarily disrupted but the nerve fiber itself stays intact. Recovery is typically complete within days to weeks. Axonotmesis involves actual damage to the nerve fibers, though the surrounding connective tissue tubes remain intact enough to guide regrowth. This type occurs in crush and stretch injuries. Neurotmesis is the most severe form, where the nerve fibers, insulation, and surrounding tissue are all destroyed. Recovery from neurotmesis is nearly impossible without surgery.
Erb’s Palsy
Erb’s palsy results from damage to the upper nerve roots, C5 and C6. It’s most commonly seen in newborns after a difficult delivery but can also happen in adults. The classic presentation is called the “waiter’s tip” position: the shoulder hangs down and rotates inward, the elbow is straight, and the forearm is turned palm-down. The person loses the ability to lift their arm at the shoulder, rotate it outward, or bend the elbow effectively because the deltoid, biceps, and several rotator cuff muscles all lose their nerve supply.
Klumpke’s Palsy
Klumpke’s palsy affects the lower roots, C8 and T1, and produces a very different picture. The hand’s small muscles lose function, creating a “claw hand” where the fingers hyperextend at the knuckles and curl at the fingertip joints. The wrist may bend sharply backward because the wrist extensors are still working but the flexors are not. This injury is less common than Erb’s palsy and often results from the arm being pulled forcefully overhead.
How Injuries Are Diagnosed
Doctors use a combination of physical examination and imaging to determine exactly where and how badly the plexus is damaged. Electromyography (EMG) involves inserting a thin needle electrode into different muscles to measure their electrical activity, both at rest and during contraction. Nerve conduction studies, usually done at the same time, measure how fast electrical signals travel through the nerves. Together, these tests reveal which nerves are affected and whether the damage is mild or severe.
MRI provides detailed images of the plexus itself and can show the extent of damage. Newer high-resolution techniques like magnetic resonance neurography produce even sharper pictures of individual nerves. When MRI doesn’t give enough information, CT myelography may be used. This involves injecting contrast dye into the spinal fluid to visualize the nerve roots where they connect to the spinal cord, which is especially useful for detecting avulsions (roots torn completely away from the cord).
Treatment and Recovery
Mild injuries often heal on their own. When the nerve fiber is intact and only the insulation is disrupted, remyelination restores function within weeks. More significant injuries where the nerve fibers themselves are damaged depend on regrowth, and nerves regenerate slowly, roughly one inch per month (1 to 3 millimeters per day). Because the brachial plexus starts in the neck and the hand is a long way from there, recovery of hand function after an upper injury can take many months, and the muscle at the end of the nerve may waste away before the regrowing fibers reach it.
Surgery becomes the best option for serious injuries. The three main approaches are direct nerve repair, where the cut ends are reconnected; nerve grafting, where a piece of nerve from another part of the body bridges a gap; and nerve transfer, where a healthy but less critical nerve is rerouted to take over the job of the damaged one. Nerve transfers have a key advantage: the surgeon can connect the donor nerve close to the paralyzed muscle, shortening the distance the regenerating fibers need to travel and speeding recovery. They also tend to deliver a greater number of nerve fibers to the target muscle compared to long grafts. Avulsions, the most severe injuries where roots are torn from the spinal cord, can only be treated with nerve transfers since the original root can never be reattached.
Not every surgical repair succeeds, and outcomes depend heavily on the type and location of injury, how quickly surgery is performed, and the patient’s age. Physical therapy plays a critical role throughout recovery, keeping joints mobile and muscles as active as possible while waiting for nerve regeneration to reach its targets.