brachial plexus
Meaning of brachial plexus: Everything the arm can do or feel passes through one braided tangle behind the collarbone, which is why damage there spreads so widely.
Definition of brachial plexus
The brachial plexus is the braided network of nerves carrying motor and sensory signals between the spinal cord and the shoulder, arm and hand. It is built from the spinal nerves of the lower neck and the top of the chest, which merge, divide and recombine on their way through the neck and armpit before emerging as the named nerves of the arm.
How is it organized?
| Stage | What it consists of |
|---|---|
| Roots | The spinal nerves of the lowest four neck levels and the first chest level |
| Trunks | Three bundles formed as the roots join, sitting above the collarbone |
| Divisions | Each trunk splits into a front and back portion behind the collarbone |
| Cords | Three bundles named for their position around the artery to the arm |
| Branches | The named nerves, including the radial, median and ulnar nerves |
The reshuffling has a purpose. Fibers arriving from several spinal cord levels are redistributed so that each final nerve carries fibers from more than one level, and each muscle receives supply from more than one level too.
Why does injury here spread so widely?
Almost the entire nerve supply of one arm is funneled through a confined space between the neck, the collarbone and the armpit. Damage before the branches separate therefore affects many muscles and skin regions at once, whereas damage to a single named nerve further down the arm produces a much narrower loss. The plexus is stretched when the arm is pulled away from the body or the head is forced away from the shoulder, which is the mechanism behind motorcycle injuries and some difficult births. It can also be compressed or invaded by a tumor at the top of the lung, and scarring from radiation treatment in that region can affect it slowly and long afterward, a condition described as brachial plexopathy.
What do the injury patterns look like?
Damage to the upper part of the plexus weakens the shoulder and the bending of the elbow, leaving the arm hanging at the side and rotated inward, with sensation reduced over the outer shoulder and upper arm. Damage to the lower part weakens the small muscles of the hand and grip and reduces sensation along the inner forearm and little finger. When the injury sits very close to the spine it may also disturb nearby nerve fibers heading for the face, producing a drooping eyelid and a small pupil on the same side, a sign that the damage is at root level. Whether the roots were torn from the cord itself or damaged beyond it governs what recovery is possible.
Used in a sentence
Imaging was ordered to evaluate for a possible brachial plexus injury after the patient's fall.
Good to know
An injury to the brachial plexus can affect an entire limb because the network is crowded into a small space near the shoulder, before its fibers have separated into the individual nerves supplying specific muscles further down the arm.
Word origin
Where “brachial plexus” comes from
Formed from Latin brachium, meaning arm, and plexus, from plectere, meaning to braid or weave, describing the interwoven arrangement of nerves.