Arm muscles, computer artwork. The uppermost muscle, wrapping around the shoulder, is the deltoid (dark green). Below it are the triceps. In the forearm at right and centre are the extensor carpi radialis longus and brevis, extensor digitorum, and extensor carpi ulnaris muscles. All these muscles act to extend (straighten) the upper and lower arm, hand and fingers around the elbow, wrist and knuckle joints. In the forearm at left is the flexor carpi ulnaris muscle (pink), which causes the opposite action (flexion) to bend the joints. At left 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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