Anatomy and supply
The Coracobrachialis is a long, slender muscle of the shoulder joint. As the name suggests, it originates from the coracoid process of the scapula, where its' tendon is partly blended with the short head of the biceps. The insertion of this muscle happens at the medial surface of the humeral shaft (between the brachialis muscle and the medial head of the triceps). Both the coracobrachialis and the humerus form the lateral border of the axilla, where it is also the easiest to palpate the muscle. The nervous supply comes from the musculocutaneous nerve (C5-C7), a branch from the lateral cord of the brachial plexus. This nerve penetrates the coracobrachialis on a middle level.
The contraction of the coracobrachialis leads to two movements at the shoulder joint. On one hand, it bends the arm (flexion), and on the other hand it pulls the arm towards the trunk (adduction). To a smaller extent, it also turns the humerus inwards (inward rotation). Another important function is the stabilization of the humeral head within the shoulder joint, especially when the arm is hanging freely straight down.
The overuse of the coracobrachialis can lead to a hardening of the muscle. Common causes include, among others, bench pressing with extremely heavy weights and carrying heavy loads with hanging arms. Typical symptoms are pain in the arm and shoulder, radiating down to the back of the hand. In more severe cases the musculocutaneous nerve, which goes through the coracobrachialis, can even get trapped (entrapment). Clinically the affected patients show skin sensation disturbances on the radial part of the forearm and a weakened flexion in the elbow, as the nerve also supplies the biceps brachii and brachialis muscles. In contrast, an actual rupture of the coracobrachialis is extremely rare and almost only occurs in serious accidents.