Arm muscles, computer artwork. The uppermost muscle, wrapping around the shoulder, is the deltoid. Below it are the biceps. The brachioradialis muscle is at left in the forearm. To its right are the flexor carpi radialis, palmaris longus, flexor digitorum superficialis and flexor carpi ulnaris muscles. All these muscles act to flex (bend) the upper and lower arm, hand and fingers around the elbow, wrist and knuckle joints. Muscles on the other side of the arm cause the opposite action (extension) to straighten the joints. At right is the brachial plexus, the network of nerves (yellow) that originate in the neck, pass through the shoulder and innervate the arm.
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