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The ulnar, median, and musculocutaneous nerves form a characteristic "M" shape around the axillary artery. (Note: this image is oriented 180 degrees differently to Images 6 and 7.) Roots There are five nerve roots that contribute to create the brachial plexus. They are the C5, C6, C7, C8, and T1 nerve roots (Image 6). Trunks The five nerve roots of the brachial plexus create the three trunks of the brachial plexus. They are the superior, middle, and inferior trunks. Divisions Each of the three trunks of the brachial plexus then divides to create an anterior and posterior division. Hence, there are six divisions of the brachial plexus: superior anterior, superior posterior, middle anterior, middle posterior, inferior anterior, and inferior posterior divisions. Cords The six divisions of the brachial plexus then converge to form the three cords. They are the lateral, medial, and posterior cords. It should be pointed out that the cords are named for their relative positions to the axillary artery (Image 7). Terminal Branches The three cords of the brachial plexus then diverge to form the five terminal branches. They are the musculocutaneous, axillary, median, radial, and ulnar nerves. In a cadaver lab, anatomists look to identify the cords and terminal branches of the brachial plexus by locating the characteristic "M" shape of the ulnar, median, and musculocutaneous nerves around the axillary artery. The outer legs of the "M" are formed by the medial cord giving rise to the ulnar nerve and the lateral cord giving rise to the musculocutaneous nerve. The inner legs of the "M" are formed by the medial and lateral cords contributing to the median nerve (Image 8). Once these cords and branches are located and identified, the other (posterior) cord and other two terminal (radial and axillary) branches are located by looking posterior to the axillary artery. Thus, this "M" shape is the landmark for locating all three cords and all five terminal branches of the brachial plexus. Motor Innervations of the Terminal Branches Each of the five terminal branch nerves of the brachial plexus divides to form motor neurons that innervate muscles of the upper extremity. In Image 9, each of the muscles that is innervated by a terminal branch is represented by a leaf of that branch. Note: Image 9 also shows that each of the five terminal branches also has a sensory innervation component; for more detail on these sensory innervations, please see Image 4. Motor Innervations of the Preterminal Branches The five terminal branches of the brachial plexus are named terminal because they branch from the ends of the cords. However, there are also 11 other preterminal branches that emanate from the brachial plexus before the ends of the cords, either from roots, trunks, or along the length of the cords. These preterminal branches are shown in Image 2. Leaves indicating what muscle(s) is/are innervated by each preterminal branch (or whether it is sensory) are also indicated in this image (for more detail on these sensory innervations, please Joseph Muscolino is offering his Digital Clinical Orthopedic Manual Therapy streaming subscription service at a deep discount to ABMP members. This service contains all his video content, plus new content uploaded each week. For more information, visit his website at www.learnmuscles.com/abmp. A B M P m e m b e r s e a r n F R E E C E a t w w w. a b m p . c o m / c e b y r e a d i n g M a s s a g e & B o d y w o r k m a g a z i n e 67 8