9 Nerves of the Upper Extremity

Peter Hahn and Ranjan Gupta

9 Nerves of the Upper Extremity

The great German philosopher Immanuel Kant once stated, “The hand is the visible part of the brain.” With our hands, we have the ability to build, fabricate, and manifest our creativity. Without them, humanity ceases to evolve. Our hands, the active agents of the cerebral system, and by extension, the human soul (taken from the Karma Indriya), are in turn controlled by the complex neural network of the upper extremity. This network of nerves is named the brachial plexus and bridges the gap between the cerebrum and the hand, thereby allowing the translation of thought into gestures and meaningful function.

The brachial plexus is regularly formed by ventral rami from C5–T1, with contributions from C4 and T2 nerve roots intermittently, ¹ ^, ⁵ and is responsible for the innervation of the entire upper limb. The plexus formation begins at the level of the scalene muscles, continues under the clavicle, and ends at the level of the axilla. The portion of the plexus within the posterior triangle of the neck is referred to as the supraclavicular region (▶Fig. 9.1), whereas the portion distal to the axilla is known as the infraclavicular region (▶Fig. 9.2). The brachial plexus is divided into roots, trunks, divisions, cords, and terminal branches (▶Fig. 9.3). ¹ ^, ⁵ These roots are derived from the five ventral rami of the spinal nerves from C5–T1, after they have given off segmental supply to the muscles of the neck. They emerge from the interval between the anterior and middle scalene muscles. Near the posterior portion of the first rib, the ventral rami merge to form trunks. C5 and C6 rami unite to from the superior trunk, C7 rami continue to form the middle trunk, and C8 and T1 rami unite to form the inferior trunk. Near the lateral region of the first rib, each trunk splits into an anterior and a posterior division, giving rise to three anterior divisions and three posterior divisions. Ultimately, the anterior divisions give rise to nerves that innervate the anterior compartments of the upper limb, whereas the posterior divisions innervate the posterior parts of the limb.

**Fig. 9.1** Supraclavicular branches of the brachial plexus are located in the posterior triangle of the neck.

**Fig. 9.2** Infraclavicular branches of brachial plexus are located in the axilla.

**Fig. 9.3** Distribution of the brachial plexus.

At the level of the proximal third of the clavicle, the six divisions regroup to form three cords. The anterior divisions of the superior and middle trunks unite to form the lateral cord. The anterior division of the inferior trunk continues as the medial cord, while the three posterior divisions of each trunk join to form the posterior cord. These cords traverse with and span the first and second parts of the axillary artery; their names indicate their position with respect to the axillary artery. At the level of the axilla, the cords recombine and divide to form terminal branches. The lateral cord gives rise to the musculocutaneous nerve. The medial cord gives off the ulnar nerve and medial cord of the median nerve. The remaining portions of the medial and lateral cords then combine to form the median nerve, whereas the posterior cord ultimately branches into the axillary and radial nerves.

Branches of the Brachial Plexus

Several branches stem throughout the brachial plexus. They are divided into the supraclavicular and infraclavicular branches.

9.1 Supraclavicular Branches of the Brachial Plexus

At the posterior triangle of the neck, the supraclavicular nerves of the brachial plexus arise from the rami and trunks of the brachial plexus (▶Fig. 9.1).

The first nerve from the brachial plexus that arises proximally is the dorsal scapular nerve , ² and it originates from the fifth cervical ventral ramus in the majority (75%) of cases within the posterior cervical triangle deep to the prevertebral fascia. It courses through the middle scalene muscle and travels posteriorly between the posterior scalene muscle and the serratus posterior superior and levator scapulae muscles to innervate the rhomboid major and minor muscles and, occasionally, the levator scapulae muscles.

The long thoracic nerve ³ innervates the serratus anterior muscle; arises from the posterior aspect of the ventral rami of C5, C6, and C7; and travels through the apex of the axilla posterior to the brachial plexus. The first two roots (C5 and C6) pierce the substance of the scalenus medius muscle and then unite and join the C7 root on the surface of the scalenus medius muscle. It then travels beneath the posterior aspect of the trunks of the brachial plexus and stays within the fascia. After its supraclavicular course, the long thoracic nerve approaches the superior margin of the first rib at the middle axillary line, and continues obliquely in an anterosuperior to posteroinferior direction in the axilla. At the posterior angle of the second rib, the nerve becomes embedded in the fascia of the serratus anterior muscle and descends inferiorly between the middle and posterior axillary lines, giving off two or three branches to each digitation of the muscle as it progresses.

The suprascapular nerve ⁴ ^, ⁵ (▶Fig. 9.4) is a mixed motor and sensory nerve originating from the ventral rami of spinal nerves C4, C5, and C6 or at the upper trunk of the brachial plexus. It innervates the supraspinatus and infraspinatus muscles as well as the glenohumeral joint. The suprascapular nerve initially passes through the scapular notch inferior to the transverse scapular ligament. From there, it courses obliquely and laterally in the supraspinous fossa along the inferior surface of the supraspinatus muscle to the base of the scapular spine. It then curves medially to innervate the infraspinatus muscle. The muscular branch to the supraspinatus muscle arises behind the transverse scapular ligament. The sensory branch of the suprascapular nerve comes off immediately after passing the suprascapular notch, turns laterally along the base of the coracoid process, and eventually innervates the acromioclavicular joint and the superior aspect of the subacromial bursa. Additionally, just before entering the infraspinous fossa, a branch of the suprascapular nerve runs laterally and enters the posterior inferior aspect of the capsule deep to the tendons of the infraspinatus muscle.

**Fig. 9.4** Suprascapular nerve. 1, sternocleidomastoid muscle; 2, anterior scalenus; 3, suprascapular nerve; 4, suprascapular vessels; 5, clavicle.

The nerve to the subclavius muscle ¹ arises anterior from the superior trunk of the brachial plexus. With contributions primarily from C5 and occasionally from C4 and C6, it descends posterior to the clavicle but in front of the brachial plexus and supplies the subclavius muscle.

Branches of Cords of Brachial Plexus

The infraclavicular branches of the brachial plexus arise from the cords of the brachial plexus at the level of the clavicle (▶Fig. 9.2).

Branches of the Lateral Cord

The lateral cord is formed from the anterior divisions from the upper and middle trunks with contributions from C5 to C7. It gives rise to the lateral pectoral nerve ; the musculocutaneous nerve , which continues as the lateral antebrachial cutaneous nerve in the forearm; and the lateral root of the median nerve (Table 9.1).

**Table 9.1** Branches of the lateral cord
Nerve	Contributing spinal nerves	Course	Innervated structures	Surgical note
Lateral pectoral	C5/C6/C7	Pierces clavipectoral fascia; sends branch to medial pectoral nerve	Pectoralis major
Lateral antebrachial cutaneous	C5/C6/C7; continuation of musculocutaneous nerve	Pierces brachial fascia proximal to lateral epicondyle and closely associates with cephalic vein	Sensation to lateral forearm	At risk during invasive procedures involving access to cubital fossa; particularly when cephalic vein is punctured just lateral to biceps tendon and crossed by nerve
Lateral cord of median	C6/C7	See medial cord	See medial cord	See medial cord
Musculocutaneous	C5/C6/C7	Runs anterior and lateral to axillary artery and exits axilla after piercing coracobrachialis and travels in between biceps and brachialis; continues as lateral antebrachial cutaneous nerve	Coracobrachialis, biceps brachii, brachialis	Not commonly injured, but may become entrapped with relocation of tendon of long head of biceps
Source: Data from Zhang et al. ²⁴

The lateral pectoral nerve ¹ derives its name from its origin in the lateral cord of the brachial plexus. It contains nerve fibers from the anterior divisions of C5 to C7 and pierces the clavipectoral fascia to supply the pectoralis major muscle. It also sends a branch to the medial pectoral nerve, which supplies the pectoral minor muscle.

The musculocutaneous nerve (▶Fig. 9.5, 9.6) ⁵ ^– ⁷ is one of the two terminal branches of the lateral cord and is composed of fibers derived from C5, C6, and C7. The main nerve trunk arises from the lateral cord of the brachial plexus at the lower border of the pectoralis minor muscle and runs anterior and lateral to the axillary artery. As it runs laterally, it sends a primary branch 6 cm distal to the coracoid process and enters the deep surface of the coracobrachialis muscle. The primary motor branch for the biceps muscles branches 10 to 13 cm distal to the coracoid process and bifurcates into two secondary branches. The proximal branch innervates the short head of the biceps, and the distal branch innervates the long head of the biceps. The motor branch innervating the brachialis muscle exits from the main trunk of the musculocutaneous nerve 14 to 18 cm distal to the coracoid process, with a single or two primary branches supplying the brachialis. As the musculocutaneous nerve continues toward the elbow joint, it emerges from beneath the biceps muscle and becomes the lateral antebrachial cutaneous nerve (Fig. 9.6).

**Fig. 9.5** Branches of the brachial plexus through the course of the arm. 1, coracobrachialis; 2, medial cutaneous nerve of arm; 3, ulnar nerve; 4, brachial artery; 5, median nerve; 6, biceps; 7, musculocutaneous nerve; 8, medial antebrachial cutaneous nerve.

**Fig. 9.6** Medial brachial cutaneous nerve. 1, latissimus dorsi; 2, medial brachial cutaneous nerve; 3, musculocutaneous; 4, biceps; 5, brachial artery; 6, median nerve; 7, ulnar nerve.

The lateral antebrachial cutaneous nerve (▶Fig. 9.7) ⁸ ^, ⁹ provides sensory innervation to the lateral aspect of the forearm and is a continuation of the musculocutaneous nerve. It pierces the brachial fascia 3 cm proximal to the lateral epicondyle and emerges from the subcutaneous fat distal to the lateral border of the biceps tendon and is closely associated with the cephalic vein. At the elbow, it is located medial to the lateral epicondyle as it crosses the interepicondylar line and travels parallel and volar to the cephalic vein. It is never farther than 1 cm from cephalic vein proximal to the crossover of the abductor pollicis longus and extensor pollicis brevis with the extensor carpi radialis longus and extensor carpi radialis brevis.

**Fig. 9.7** Lateral antebrachial cutaneous nerve. 1, lateral antebrachial cutaneous nerve; 2, cephlic vein; 3, lacertus fibrosus; 4, extensor muscles; 5, biceps.

The other terminal branch of the lateral cord is the lateral root of the median nerve . It joins the medial root of the median nerve off of the medial cord, lateral to the axillary artery, to form the median nerve (see median nerve hereafter).

Branches of the Medial Cord

The medial cord is a continuation of the anterior division of the lower trunk with contributions from C8 and T1. It gives rise to the medial pectoral nerve, medial antebrachial cutaneous nerve, and medial brachial cutaneous nerve. After the medial cord gives off the medial root of the median nerve, it continues distally as the ulnar nerve (see ulnar nerve hereafter; Table 9.2).

**Table 9.2** Branches of the medial cord
Nerve	Contributing spinal nerves	Course	Innervated structures	Surgical note
Medial pectoral	C8–T1	Runs through deep surface of pectoralis minor and lies lateral to lateral pectoral nerve	Pectoralis major
Medial brachial cutaneous	C8–T1	Runs along medial side of axillary and brachial veins	Medial surface of arm and proximal part of forearm	Susceptible to injury during cubital tunnel surgery
Medial antebrachial cutaneous	C8–T1	Initially runs with ulnar nerve, pierces brachial fascia with basilic vein, and enters subcutaneous tissue, dividing into ulnar and volar branches	Medial surface of forearm	Nerve divides into two main branches anterior and posterior to medial epicondyle; posterior branch susceptible to injury during cubital tunnel surgery and anterior branch susceptible during venipuncture
Ulnar	C8–T1; occasionally C7	Descends medially in the arm posterior to the medial epicondyle of humerus onto the ulnar aspect of forearm; passes through Guyon’s canal in hand prior to branching into superficial and deep branches in hand	Flexor carpi ulnaris, medial half of flexor digitorum profundus, intrinsic muscles of hand except for radial two lumbricals and thenar eminence, skin of hand medial to ulnar side of ring finger to little finger	Closed reduction and K-wiring of displaced supracondylar fractures frequently cause iatrogenic injury; medial pin can damage ulnar nerve during insertion or after insertion by constricting the cubital tunnel
Medial root of median	C8–T1; contribution from lateral root of median(C6–C7)	Lateral and medial roots merge lateral to axillary artery to form the median nerve; descends with brachial artery in arm; anterior interosseous nerve branches off of median nerve deep to pronator teres prior to passing in between two heads of pronator teres; median nerve continues on deep surface of flexor digitorum superficiales; gives off superficial palmar branch prior to entering carpal tunnel under the flexor retinaculum	Muscles of anterior forearm compartment (except for flexor carpi ulnaris, and ulnar half of flexor digitorum profundus, 5 intrinsic muscles of hand and the ulnar two lumbricals), palmar skin from radial side of ring finger to thumb. The median nerve supplies the radial two lumbricals through the common digital branches.	Median nerve especially vulnerable to injury during carpal tunnel release, during both open and endoscopic procedures; variant anatomy may increase risk of iatrogenic injury
Source: Data from Zhang et al. ²⁴

The medial pectoral nerve ¹ supplies the deep portion of the pectoralis major. It runs through the deep surface of the pectoralis minor muscle and lies lateral to the lateral pectoral nerve.

The medial brachial cutaneous nerve (▶Figs. 9.5, 9.6) ¹⁰ ^, ¹¹ is a tenuous nerve that supplies skin over the medial surface of the arm and the proximal part of the forearm. It comes off the medial cord of the brachial plexus and courses with the basilic vein and medial to the brachial artery. At the midportion of the arm, there is an arborization of several cutaneous branches through the muscular fascia across the ulnar nerve to become subcutaneous to the skin of the medial arm.

The medial antebrachial cutaneous nerve (▶Figs. 9.5, 9.8) ¹² supplies the skin over the medial surface of the forearm. It runs in between the axillary artery and vein and then runs distally anterior and medial to the brachial artery. At the midforearm level, it emerges from under brachial fascia adjacent to the basilic vein and divides into a volar and ulnar branch. The volar branch passes in front of the median basilic vein and descends on the ulnar side of the forearm. The ulnar branch passes obliquely downward on the medial side of the basilic vein in front of the medial epicondyle and descends on the ulnar side of the medial forearm.

**Fig. 9.8** Medial antebrachial cutaneous nerve. 1, basilic vein; 2, medial antebrachial cutaneous nerve.

The Median Nerve

The median nerve (▶Fig. 9.3) is formed by two terminal branches, one from the medial cord (C8, T1) and the other from the lateral cord (C5, C6, C7). These two branches travel around the axillary artery and unite either anterior or lateral to the artery so as to form the median nerve.

Median Nerve in Arm

In the upper arm, the median nerve does not give off any branches. It can be identified proximally posterior to coracobrachialis and distally in the groove between the belly of the biceps and brachialis. In the proximal half of the arm, the median nerve lies lateral to the brachial artery and its accompanying veins. In the distal half of the arm, the nerve crosses anterior to the artery and veins and assumes a medial position in relation to these structures. The median nerve lies entirely anterior to the medial intermuscular septum (▶Fig. 9.5, 9.7). ⁷ ^, ¹² ^, ¹³

Median Nerve at the Elbow

The median nerve is anterior to the intermuscular septum in the proximal part of the arm but pierces the septum in the midarm and then lies immediately posterior to the intermuscular septum within the epimysium of the triceps. The median nerve can subsequently be isolated proximal to pronator teres on the anterior aspect of the elbow. It can then be seen passing between the humeral and ulnar heads of the pronator teres and can be traced distally after sending branches to the pronator teres. The median nerve can be seen running vertically across the interval between the humeral head of pronator teres and flexor carpi radialis before passing deep to the proximal border of the flexor digitorum superficialis (▶Fig. 9.9). ¹⁴

**Fig. 9.9** Median nerve in the proximal forearm. 1, biceps muscle; 2, brachialis muscle; 3, median nerve; 4, brachialis tendon; 5, biceps tendon; 6, felxor carpi radialis; 7, brachial artery; 8, brachioradialis; 9, two heads of pronator teres; 10, radial artery.

Median Nerve in Forearm

In the proximal forearm, the median nerve passes between the humeral and ulnar heads of pronator teres (82%), whereas the ulnar artery passes deep to both the heads (▶Fig. 9.9, 9.10). ⁹ ^, ¹⁵ In about 9% of cases, the deep head of pronator teres is absent and the median nerve and ulnar artery travel deep to the superficial head. In around 7%, the median nerve travels deep to both heads of pronator teres. In around 2%, the median nerve travels through the substance of the superficial head of the pronator teres. ⁴⁴ Distal to the pronator teres, the median nerve joins the ulnar artery. It passes deep to the proximal fibrous arch of flexor digitorum superficialis and runs distally in the midline of the forearm, closely attached by the bicipital aponeurosis in the cubital fossa to the deep surface of flexor digitorum superficialis, which it supplies. Deep to the humeral head of pronator teres, the anterior interosseous nerve arises from the posterior aspect of the median nerve and runs along the interosseous membrane between the flexor digitorum profundus and flexor pollicis longus. Approximately 5 cm proximal to the flexor retinaculum, the median nerve emerges from the lateral border of flexor digitorum superficialis to become more superficial and gives off the superficial palmar branch before passing into the hand through the carpal tunnel.

**Fig. 9.10** Median nerve in the distal forearm. 1, flexor carpi radialis; 2, flexor pollicis longus; 3, brachioradialis; 4, FDS; 5, median nerve.

Anterior interosseous nerve ⁵ ^, ¹⁶ (▶Fig. 9.11) is the major branch of the median nerve to supply the deep compartment of the forearm. It typically branches immediately distal to the flexor digitorum superficialis arch and innervates the flexor digitorum profundus (index and long fingers), flexor pollicis longus, and pronator quadratus. The anterior interosseous nerve courses distally on the interosseous membrane, often radial to the anterior interosseous artery (80%), and passes dorsal to and innervating the pronator quadratus muscle. It also sends sensory branches to the distal radioulnar, radiocarpal, intercarpal, and middle and ring finger carpometacarpal joints.

**Fig. 9.11** Anterior interosseous nerve branches from median nerve. 1, flexor digitorum superficialis; 2, anterior interosseous nerve; 3, branches to flexor digitorum superficialis from median nerve; 4, median nerve; 5, continuation of median nerve after bifurcation; 6, pronator teres (superficial head); 7, branches to pronator teres from anterior interosseous nerve; 8, common flexor tendon.

Palmar cutaneous nerve of median nerve ¹⁷ ^– ¹⁹ (▶Fig. 9.12) arises from the radial side of the median nerve 5 to 7 cm proximal to the wrist joint and runs parallel with the parent nerve down along the ulnar side of the tendon of the flexor carpi radialis and crosses the base of the thenar eminence over the prominence of the tubercle of the scaphoid. It pierces the flexor retinaculum and divides into a lateral and medial branch. The lateral branch supplies the ball of the thumb, and the medial branch supplies the skin of the palm.