Overview of Upper Limb - LO 7

7. Describe the nerve supply of the muscular compartments of the upper limb, and consider the effect a lesion would have on these nerves and the muscular compartments they serve.

Nearly all nerves in the upper limb arise from the brachial plexus, a network of nerve fibers. The nerve fibers come from the ventral primary rami (VPR) of C5-T1 levels to form the roots of the brachial plexus. These five roots variably contribute their nerve fibers into three trunks, which then split into six divisions (3 anterior, 3 posterior). The divisions recombine into three cords which eventually terminate in five main terminal branches (nerves) that supply the upper limb. A picture is worth a thousand words here. The key concept is that the spinal nerves (C5-T1) form a network that allows the terminal branches/nerves to consist of nerve fibers from multiple spinal cord levels.

Each compartment of muscles is primarily supplied by a single nerve (although the nerve has multiple spinal cord levels represented). Below is a basic summary – exceptions and deeper level details are not included here.

Shoulder:

Deltoid muscle – axillary n.

Arm:

Anterior (flexor) compartment – musculocutaneous n. The musculocutaneous n. pierces through the coracobrachialis muscle, gives motor supply to the anterior compartment of the arm, and then continues on into the forearm as a cutaneous n. of the lateral forearm.

Extensor (extensor) compartment – radial n. The radial n. runs deep in the posterior region directly on the posterior humerus. The radial n. runs with the deep artery of the arm in the radial groove of the humerus, where it is vulnerable to injury if the shaft of the humerus is injured.

Forearm:

Anterior (flexor/pronation) compartment – median n. & ulnar n. The median n. supplies all of the muscles of the anterior compartment of the forearm except for 1.5 muscles: the flexor carpi ulnaris m. and half of the flexor digitorum profundus m. (which are innervated by the ulnar n.). The median n. continues into the hand (crossing the wrist through the carpal tunnel, where it is vulnerable to compression from inflammation of the other structures in the “tunnel”) and supplies the thenar muscles of the hand. The ulnar n. supplies 1.5 muscles in the anterior forearm – the flexor carpi ulnaris m. (ulnar-side wrist flexor) and the ulnar half of the flexor digitorum profundus m. (deep flexors of the digits). The ulnar n. continues into the hand to supply most of the hand muscles. Note the location of the ulnar nerve as it runs through behind the medial epicondyle of the humerus leaves is vulnerable to compression against the bone; you know this spot as your “funny bone”.

Posterior (extensor/supination) compartment – radial n. The radial n. continues from the posterior compartment of the arm across the lateral epicondyle of the humerus (on the anterior surface) where it branches into a deep muscular branch supplying the muscles of the posterior forearm (extensors of the wrist, MC, PIP, & DIP) and a superficial cutaneous branch where it supplies the skin of the dorsum of the hand and fingers.