The terminal branches of the brachial plexus (the musculocutaneous, median, ulnar, and radial nerves) provide the entire nerve supply to the limb below the shoulder. Of these, only the musculocutaneous and radial nerves distribute to the muscles of the upper arm.
The musculocutaneous nerve (C4 to C7), a branch of the lateral cord of the brachial plexus, arises opposite the lower border of the pectoralis minor muscle. This nerve is the principal motor nerve of the anterior (flexor) compartment of the arm. It continues into the forearm as a cutaneous nerve, the lateral antebrachial cutaneous nerve. The nerve lies between the axillary artery and the coracobrachialis muscle, which it perforates and supplies. Continuing downward, it runs between the biceps brachii and brachialis muscles, supplying branches to both heads of the biceps brachii muscle and most of the brachialis muscle and often communicating with the median nerve. In this part of its course, it inclines gradually toward the lateral side of the arm; at about the level of the elbow joint, it passes between the biceps brachii and the brachioradialis muscles to pierce the deep fascia and become the lateral antebrachial cutaneous nerve.
The branch supplying the coracobrachialis muscle derives its fibers from C7 and usually arises from the main nerve before that nerve penetrates the muscle. Occasionally, the branch comes directly from the lateral cord of the brachial plexus. The branches to both heads of the biceps brachii muscle and to the brachialis muscle arise from the nerve after it has emerged from the coracobrachialis muscle.
The branch supplying the brachialis muscle subdivides and descends to help in the innervation of the elbow joint; other filaments supply the brachial artery and the deep brachial artery and its nutrient humeral branch. Fibers innervating the periosteum on the distal anterior aspect of the humerus are reputedly distributed with these vascular filaments.
The lateral antebrachial cutaneous nerve passes deep to the cephalic vein and soon divides into anterior and posterior branches. The anterior branch descends along the anterior aspect of the radial side of the forearm to the wrist and ends at the base of the thenar eminence.
At the wrist, it lies in front of the radial artery and gives off branches that penetrate the deep fascia to supply this part of the artery. The terminal branches of the anterior branch of the lateral antebrachial cutaneous nerve communicate with corresponding branches from the palmar cutaneous branch of the median nerve. The posterior branch is smaller. It curves around the radial border of the forearm and breaks up into branches that supply a variable area of skin and fascia over the back of the forearm. These branches also communicate with branches of the posterior antebrachial cutaneous nerve and with the superficial terminal branch of the radial nerve.
The areas of skin supplied by the lateral antebrachial cutaneous nerve include sensory receptors, hairs, arrectores pilorum muscles, glands, and vessels. However, these terminal cutaneous branches show considerable individual variation in the territories they supply.