Anatomy

Anatomy (Figure 21.4)

1. The shoulder, axilla, and upper extremity are innervated by the brachial plexus (BP). Skin of the medial aspect of the upper arm is innervated by the intercostobrachial nerve formed by T2 and the medial cutaneous nerve of the arm. The supraclavicular nerve, which is formed by C3-C4, innervates the skin of the top of the shoulder (see Section VI.A.1).
   

   Figure 21.4 Diagram of the brachial plexus and its terminal branches.

   (From Horn  JL , Nasiri  M . Regional blocks—A brief overview. In: Jaffe  RA , Schmiesing  CA , Golianu  B , eds. Anesthesiologist’s Manual of Surgical Procedures. 6th ed. Wolters Kluwer; 2020: 1763- 1799. )

2. The BP is formed from the ventral rami of the spinal nerves from C5 to C8 and T1, with frequent contributions from C4 and T2. It is traditionally divided into five parts: roots, trunks, divisions, cords, and branches.
3. Each root or ventral ramus exits posterior to the vertebral artery and travels laterally in the trough of its cervical TP, sandwiched between the fascial sheaths of the anterior and middle scalene muscles.
4. The trunks (superior, middle, and inferior) pass over the first rib through the space between the anterior and middle scalene muscles in association with the subclavian artery, which shares the same fascial sheath.
5. As the trunks pass over the first rib and under the clavicle, they split into the anterior (flexor) and posterior (extensor) divisions, which then reorganize to form the three cords (lateral, medial, and posterior, named for their usual positions relative to the axillary artery) of the plexus.
6. Branches of the lateral and medial cords form the median nerve. The lateral cord also gives off a branch that forms the musculocutaneous nerve. The medial cord also forms the ulnar,medial antebrachial cutaneous, and medial brachial cutaneous nerves. The posterior cord becomes the axillary and radial nerves (Figure 21.4). In the axilla, the median nerve classically lies lateral to the axillary artery, the radial nerve posterior and over the conjoint tendon (of the teres major and latissimus dorsi muscles), and the ulnar nerve medial, but variation in these relative positions may occur. The musculocutaneous and axillary nerves exit the sheath high up in the axilla. The musculocutaneous nerve travels through the coracobrachialis muscle before becoming subcutaneous below the elbow, and the axillary nerve travels through the quadrilateral space (bordered by the humeral shaft, long head of the triceps, and teres major and minor muscles) before dividing into its terminal branches.
7. The dermatome distribution of the nerves of the body is summarized in Figure 21.3. Cutaneous innervation does not necessarily correlate with deep structures, including the bones; therefore, knowledge of the osteotomes can be very useful in predicting the ultimate success of any regional technique.
8. The major motor functions of the five nerves are as follows: axillary (circumflex nerve): shoulder abduction; musculocutaneous: elbow flexion; radial: wrist and finger extension; median: forearm pronation, wrist and finger flexion, thumb flexion and opposition; and ulnar: wrist flexion and adduction, thumb adduction.