Musculocutaneous Nerve

Transformation of the medial and lateral cords of the brachial plexus into their terminal branches is M-shaped. The lateral leg of the letter M is the musculocutaneous nerve ( ▶ Fig. 4.1).

The first muscle that the musculocutaneous nerve innervates is the coracobrachialis muscle. It assists the anterior deltoid with shoulder flexion (lifting the arm forward in front of one’s body), and it also stabilizes the humerus during elbow flexion. The coracobrachialis cannot be isolated or readily palpated. Therefore, it is not examined clinically.

After passing through and then deep to the coracobrachialis, the musculocutaneous nerve innervates the brachialis muscle and the biceps brachii muscle ( ▶ Fig. 4.2).

The biceps brachii, with the assistance of the brachialis and brachioradialis (innervated by the radial nerve), flexes the elbow. The biceps brachii is also a strong supinator of the forearm when the elbow is flexed. To test the biceps brachii and brachialis, have the patient flex a fully supinated forearm against resistance. Contribution from the brachioradialis (radial nerve) is minimized by performing this test with the patient’s forearm in full supination ( ▶ Fig. 4.3).

The brachialis muscle receives some innervation from the radial nerve (in addition to its main innervation from the musculocutaneous nerve). However, this innervation is usually not enough to flex the arm in the presence of musculocutaneous nerve palsy.

Distal to the branches to the biceps brachii and the brachialis muscle, the musculocutaneous nerve continues as the lateral antebrachial cutaneous nerve. The territory of this sensory nerve includes, as the name implies, the lateral half of the forearm ( ▶ Fig. 4.4).

Isolated musculocutaneous palsies are rare, but can occur following shoulder trauma or dislocation. These patients present with numbness on the surface of their anterolateral forearm, along with elbow flexion weakness. These findings need to be clinically differentiated from a biceps tendon rupture, as well as from a C6 radiculopathy. Following a tendon rupture, the biceps still contracts and can be felt rolling up the arm. A C6 radiculopathy is identified not only because of the radicular pain, but also because of possible weakness in other, nonmusculocutaneous innervated C6 muscles, including the brachioradialis and latissimus dorsi. Furthermore, C6 radiculopathies usually cause numbness confined to the thumb and index finger, whereas the sensory coverage of the lateral antebrachial cutaneous nerve stops at the wrist. Focal damage to the lateral antebrachial cutaneous nerve can occur during venipuncture in the antecubital fossa.

4.3 Median Nerve

The median nerve is derived from the lateral and medial cords of the brachial plexus, with the lateral cord providing mostly sensory axons from C6 and C7, and the medial cord mostly motor axons from C8 and T1 ( ▶ Fig. 4.1).

The median nerve remains slightly lateral and superficial to the brachial artery as it travels down the arm. About halfway down the upper arm, the median nerve crosses over the top of the brachial artery, to lie just medial to it by the time it passes under the bicipital aponeurosis (lacertus fibrosis) in the proximal forearm. The median nerve travels down the center of the forearm deep to the flexor digitorum superficialis, but superficial to the underlying flexor digitorum profundus (FDP; ▶ Fig. 4.5).

About one-third to halfway down the forearm, an important branch of the median nerve exits: the anterior interosseous nerve (AIN). Once formed, the AIN passes deeper within the forearm and terminates in the distal forearm deep to the pronator quadratus. As the median nerve continues down the forearm, it becomes superficial about 5 cm proximal to the wrist. Before entering the hand, the median nerve gives out a pure sensory branch—the palmar cutaneous branch—which runs superficial to the carpal tunnel and ramifies over the thenar eminence.

The median nerve passes through the center of the wrist within the carpal tunnel. After it passes through the carpal tunnel, the median nerve gives a branch off on its radial side: the thenar motor branch (or recurrent thenar motor branch). Next, in the deep palm, the median nerve splits into two divisions: radial and ulnar. The radial division divides into the common digital nerve to the thumb and the proper digital nerve to the radial half of the index finger. The common digital nerve to the thumb subsequently divides into the two proper digital nerves to the thumb. The ulnar division of the median nerve divides into the common digital nerves of the second and third web spaces, which also subsequently divide into proper digital nerves.

4.3.1 Motor Innervation

The median nerve innervates no muscles in the upper arm. However, it innervates numerous muscles in the forearm and hand that control forearm pronation, wrist flexion, flexion of the digits (especially the first three), and thumb opposition and abduction. To facilitate memorization, these muscles can be separated into four groups: proximal forearm, anterior interosseous, thenar motor, and terminal group.

The Proximal Forearm Group

Four muscles form this group: the pronator teres, flexor carpi radialis, flexor digitorum superficialis, and palmaris longus.

The pronator teres is the main pronator of the forearm and the first muscle innervated by the median nerve. Branches to the pronator teres exit the median nerve in the lowest part of the upper arm, before the median nerve passes between the two heads of the pronator teres. To test this muscle, the elbow should be extended with the forearm fully pronated. The patient is then instructed to resist forced supination by the examiner ( ▶ Fig. 4.6a).

The flexor carpi radialis is the more important wrist flexor. Wrist flexion is done through contraction of the flexor carpi radialis (median nerve) and flexor carpi ulnaris (ulnar nerve). Loss of flexor carpi radialis function severely limits wrist flexion, but not toward the ulnar side. Test the flexor carpi radialis by having the patient flex the wrist toward the anterior aspect of the forearm ( ▶ Fig. 4.6b). During wrist flexion, the flexor carpi radialis tendon can be observed and palpated proximal to the wrist.

The palmaris longus corrugates the palmar skin. This muscle is not readily examined for muscular strength; in fact, it is absent in roughly 15% of the population.

The flexor digitorum superficialis flexes all the fingers, except the thumb, at their proximal interphalangeal joint. To assess proximal interphalangeal joint flexion, each finger is tested separately. Placing your fingers between the single finger to be tested and the remaining fingers isolates this movement ( ▶ Fig. 4.6c). This position places the finger to be tested in mild flexion at the metacarpal–phalangeal joint and stabilizes the remaining fingers in extension, a position that allows for isolation of the flexor digitorum superficialis.

The Anterior Interosseous Group

The AIN innervates three forearm muscles: the FDP (to the second and third digits), the flexor pollicis longus, and the pronator quadratus. Although the AIN gives sensory innervation to the distal radioulnar, radiocarpal, intercarpal, and carpometacarpal joints, it provides no cutaneous innervation.

The median and ulnar nerves innervate the FDP. The median nerve controls flexion of the distal interphalangeal joint of the second and, partly, the third digits; the ulnar nerve controls this muscle’s action upon the third (partly), fourth, and fifth digits. Distal interphalangeal joint flexion of the third (or long) digit has variable dominance, in terms of innervation from the median versus ulnar nerve. Therefore, to assess median innervation of the FDP in isolation, one must concentrate on the index finger. To do so, hold the metacarpophalangeal and proximal interphalangeal joints immobile and have the patient flex the distal phalanx against resistance ( ▶ Fig. 4.7a).

The flexor pollicis longus flexes the distal phalanx of the thumb at the interphalangeal joint. Assess the flexor pollicis longus by immobilizing the thumb, except for the interphalangeal joint, and asking the patient to flex the distal phalanx against resistance ( ▶ Fig. 4.7b). A quick way to assess both FDP and flexor pollicis longus innervation from the AIN is to ask the patient to make an okay sign by touching the tips of the thumb and index finger together. When these muscles are weak, the distal phalanges of the thumb and index finger cannot flex; consequently, instead of the fingertips touching, the volar surfaces of each distal phalanx make contact ( ▶ Fig. 4.7d).

The third muscle innervated by the AIN is the pronator quadratus. This is a significantly weaker forearm pronator than the pronator teres. In fact, weakness of the pronator quadratus is often not readily apparent when the pronator teres is strong. However, fully flexing the forearm at the elbow removes the mechanical advantage of the pronator teres; and, in this position, weakness of the pronator quadratus should be detectable when compared against the normal arm ( ▶ Fig. 4.7c).

The Thenar Group

The thenar motor branch of the median nerve innervates three muscles: the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis.

There are two types of thumb abduction: palmar abduction away from the plane of the palm (mediated by the abductor pollicis brevis) and radial abduction away from the line of the forearm (mediated by the abductor pollicis longus). Therefore, even with complete palsy of the abductor pollicis brevis, radial abduction of the thumb can still occur. To test the abductor pollicis brevis, resist movement of the thumb away from the plane of the palm (palmar abduction) while the hand is immobilized ( ▶ Fig. 4.8a).

The flexor pollicis brevis has both a deep and superficial head. The superficial head is innervated by the median nerve and the deep head by the ulnar nerve. This muscle flexes the thumb at the metacarpophalangeal joint. To test the flexor pollicis brevis, immobilize the thumb’s interphalangeal joint and have the patient flex at the metacarpal–phalangeal joint ( ▶ Fig. 4.8b). Make certain that the distal interphalangeal joint is blocked for flexion because, if it is allowed, substitution by the flexor pollicis longus may occur. Also use your other hand to immobilize the first metacarpal to reduce substitution by the opponens pollicis. Because the flexor pollicis brevis is dually innervated, some thumb flexion can still occur with median nerve palsy.

To assess the opponens pollicis, have the patient forcibly maintain contact between the fingertips of the thumb and fifth digit while you try to pull the first metacarpal away from the fifth digit ( ▶ Fig. 4.8c). Although the median nerve independently controls thumb opposition, a combination of thumb adduction (adductor pollicis; ulnar nerve) and thumb flexion (flexor pollicis brevis; deep head, ulnar nerve) may mimic thumb opposition when median nerve palsy is present.

The key to exploring motor function in the thumb is to compare it against the normal hand. This is because, even after complete loss of median nerve function, some movement of the thumb may occur either secondary to true muscle action via radial and ulnar innervations or through substitution by adjacent muscles.

The Terminal Group

The terminal group consists of the first and second lumbricals, which are innervated by the terminal radial and ulnar divisions of the median nerve, respectively. To examine the first lumbrical, stabilize the index finger in a hyperextended position at the metacarpophalangeal joint and then provide resistance as the patient extends the finger at the proximal interphalangeal joint ( ▶ Fig. 4.9).

4.3.2 Sensory Innervation

The median nerve carries cutaneous sensory information from the radial two-thirds of the palm and the volar surfaces of the first, second, third, and radial half of the fourth digits ( ▶ Fig. 4.10). Dorsal fingertip sensation is also carried by the median nerve, including the dorsum of the ulnar half of the distal phalanx of the thumb. Explore sensation over the thenar eminence to assess the palmar cutaneous branch, and sensation over the distal portion of the second and third digits to assess the sensory fibers that are carried by the median nerve through the carpal tunnel.

4.3.3 Martin-Gruber and Riche-Cannieu Anastomoses

Anastomoses between the ulnar nerve and either the median nerve or its anterior interosseous branch may occur in the hand and in the forearm. Many variations are possible, and minor and major shifts in motor innervation of the hand may occur through these two potential routes of communication: the Martin-Gruber and Riche-Cannieu anastomoses.

In up to 15% of limbs, a Martin-Gruber anastomosis is present that involves the median nerve–innervated thenar muscles (opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis). With this variation, nerve fibers destined for these three muscles run down the anterior interosseous branch and are transferred to the ulnar nerve. Within the palm, these fibers are finally transferred back to the thenar motor branch, innervating their respective muscles. This distal communication between the deep ulnar branch and the thenar motor branch in the palm is termed the Riche-Cannieu anastomosis.

Therefore, in patients with a low median nerve injury in the wrist or distal forearm in whom a Martin-Gruber anastomosis is present, thenar motor function can paradoxically be spared. In the same way in these patients, damage to the ulnar nerve near the wrist can cause more severe deficits in intrinsic hand function than typically expected.

It is important to remember that, whenever strange deficit patterns are evident following a median or ulnar nerve injury, one should always consider these potential anastomoses.

4.3.4 Clinical Findings

The Upper Arm

Severe injury to the median nerve in the upper arm affects the entire distribution of the nerve, with sensory loss and lost function in all of the muscles innervated.

The branch to the pronator teres (the first muscle innervated by the median nerve) often arises above the elbow. Loss of pronator function suggests injury to the median nerve at or above the elbow. Involvement of the flexor carpi radialis also suggests median nerve injury at or above the elbow.

When examining for complete median nerve palsy, the following pitfalls must be considered. The brachioradialis (innervated by the radial nerve), aided by gravity, may pronate the forearm from full supination. Next, you may observe thumb opposition via the indirect actions of the flexor pollicis brevis (its deep muscle head) and the adductor pollicis (both innervated by the ulnar nerve).

The Forearm

Pronator Teres Syndrome and Sublimis Arch Syndrome

The median nerve may be compressed or pinched where it passes between the two heads of the pronator teres. The only median-innervated muscle that is not affected by this syndrome is the pronator teres itself. This is because branches from the median nerve destined for this muscle originate proximal to where the median nerve passes underneath it. Median-innervated hand sensation is often normal; and motor function may be difficult to ascertain because of pain. Nonetheless, weakness is occasionally seen during flexion of the second and third digits.

A fibrotic arch between the two heads of the flexor digitorum superficialis may also compress the median nerve as it passes underneath. This ridge has been called the sublimis arch. Clinical manifestations of this entrapment are quite similar to those of pronator teres syndrome, except that forceful flexion of the proximal interphalangeal joints of the second to fifth digits, which is mediated by contraction of the flexor digitorum superficialis muscle, may precipitate symptoms.

Anterior Interosseous Nerve Palsy

An isolated palsy affecting the AIN may occur secondary to trauma, fractures, Parsonage–Turner syndrome, anomalous muscles and/or tendons, or without any known cause. Patients usually complain of weakness or clumsiness grasping objects with their first two digits. There are usually no complaints of pain; and, because this nerve contributes nothing to cutaneous sensation, no numbness occurs. There is weakness of the FDP (involving the second and third digits), flexor pollicis longus, and pronator quadratus. This results in the inability to perform a pinch-type maneuver with the affected hand (patients have a positive “okay sign”). Weak forearm pronation will also frequently be present, but is difficult to demonstrate because the pronator teres remains functional. To confirm a pure AIN palsy, all other muscles innervated by the median nerve, as well as sensation, must be normal. Variations and incomplete syndromes are common. Consequently, many other etiologies may mimic this condition.

AIN syndrome is a pure motor nerve palsy. Inability to perform a pinch maneuver should alert the physician to this diagnosis, as this is almost pathognomonic.

The Wrist: Carpal Tunnel Syndrome

In cases of median nerve injury at the wrist, objective sensory testing over the thenar eminence should be normal, because sensation is transmitted via the palmar cutaneous branch, which does not pass through the carpal tunnel ( ▶ Fig. 4.5). However, the thenar muscular group (the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis) will be affected. Rarely, the thenar motor branch is affected selectively.

4.4 Ulnar Nerve

As stated previously, the transformation of the medial and lateral cords into their terminal branches is M-shaped, lying over the anterior aspect of the axillary artery. The lateral leg of the letter M is the musculocutaneous nerve, while the medial leg is the ulnar nerve. The ulnar nerve is an extension of the medial cord of the brachial plexus ( ▶ Fig. 4.1).

Until it reaches the forearm, the ulnar nerve gives off no branches to any muscle ( ▶ Fig. 4.11). Below the elbow, the first branches are destined for the flexor carpi ulnaris. Then, the ulnar nerve passes deep to the two proximal heads of the flexor carpi ulnaris, where it provides just a single major branch to the FDP (the ulnar portion of the FDP). Two sensory branches originate from the ulnar nerve in the distal half of the forearm—the dorsal ulnar cutaneous nerve and the palmar ulnar cutaneous nerve—which arise approximately 5 to 10 cm proximal to the wrist crease. In some cases, the dorsal ulnar cutaneous nerve may branch from the superficial sensory radial nerve.

Related posts:

Clinical Aspects of Traumatic Peripheral Nerve Lesions in the Lower Limb Surgical Repair of Nerve Lesions: Neurolysis and Neurorrhaphy with Grafts or Tubes Nerve Injuries: Anatomy, Pathophysiology, and Classification Compressive Lesions of the Lower Limb and Trunk Ultrasound in Peripheral Nerve Surgery Nerve Anatomy of the Upper Limbs

Stay updated, free articles. Join our Telegram channel

Join