Brachial plexus: A bundle of nerves that begins in the back of the base of the neck and extends through the armpit. It is formed by the union of portions of the fifth through eighth cervical spinal nerves and the first thoracic spinal nerve. Damage to the brachial plexus can affect nerves responsible for muscle function and sensation of the arm and chest.
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Last Editorial Review: 5/13/2016