SYNOPSIS OF THE MAJOR NERVES OF THE

SYNOPSIS OF THE MAJOR NERVES OF THE

BRACHIAL PLEXUS

O.W. Henson, Jr.

University of North Carolina at Chapel Hill

Median nerve

1. Formed by the contribution of fibers associated with spinal cord segments C5 through Tl; thus, a lesion of any one of these roots will affect fibers in the nerve.

2. The roots of the median nerve embrace the axillary artery; thus, trauma to, or diseases of, the artery may affect the roots of the nerve.

3. In the arm the median nerve accompanies, and lies on the medial surface of, the brachial artery. In a bloody field the pulsations of the artery are transmitted to the nerve and the nerve may look like an artery. It is bad practice to ligate nerves.

4. At the elbow the median nerve lies on the surface of the brachialis muscle and deep to the bicipital aponeurosis (“thank God fascia”). The nerve is thus protected during cubital vein puncture by the aponeurosis.

5. As it enters the forearm it penetrates (passes between the two heads) and innervates the pronator teres.

6. In the forearm the median nerve innervates all muscles on the volar (flexor) surface except the 1.5 muscles innervated by the ulnar nerve. It should be obvious that lesions of the median nerve lead to profound atrophy of the muscles of the anterior half of the forearm.

7. Note that both pronator muscles are innervated by the median nerve. Thus, after a lesion of the median nerve pronation is very weak or absent and the hand is held in a supinated position.

8. The median nerve supplies all wrist flexors except the 1.5 muscles innervated by the ulnar nerve. Thus, with median nerve injuries wrist flexion will show an ulnar deviation.

9. Note that the index and middle fingers are dependent on the median nerve for flexion at the DIP (flexor digitorum profundus) and the PIP joints (flexor digitorum superficialis). After median nerve lesions these joints are held in an extended position and cannot be flexed.

10. In the forearm the median nerve lies between the two major muscles it innervates (the radial half of the flexor digitorum profundus and the entire flexor digitorum superficialis); here it is well protected against trauma.

11. About 5 cm proximal to the wrist it becomes superficial and lies between the tendons of the flexor digitorum superficialis and flexor carpi radialis. Here it is very vulnerable to injury by a wrist slash.

12. At the wrist the median nerve passes deep to the flexor retinaculum and lies within the carpal tunnel. Here the nerve is in a tight space and can readily be compressed when the space is narrowed (carpal tunnel syndrome).

13. In the hand the main intrinsic muscles innervated are the thenar muscles which are superficial to the tendon of the flexor pollicis longus. Thus, median nerve injuries lead to atrophy of the thenar eminence and by virtue of the loss of the opponens pollicis muscle the thumb cannot be placed in a position of function.

14. Note the superficial and thus vulnerable position of the recurrent (motor) branch of the median nerve in the hand.

15. The palmar cutaneous branch passes superficial to the flexor retinaculum and thus would not be involved in a carpal tunnel syndrome.

16. The distal tips (including the nail beds) of the index and middle fingers are entirely median nerve innervated, thus, a major problem with median nerve lesions is the inability to feel what is grasped between the fingers and thumb.

17. Note that the median nerve innervated lumbrical muscles arise from the tendons of the median nerve innervated flexor digitorum profundus (compare with ulnar nerve innervated lumbrical muscles).

18. A useful exercise is to compare the effect of a median nerve injury at the elbow vs. wrist.

Radial nerve

1. Contains fibers that are associated with spinal cord segments C5 through Tl. Thus, trauma to any root of the brachial plexus will affect some of the fibers of the radial nerve. Compare with the median nerve.

2. The fibers making up the posterior divisions and the posterior cord of the brachial plexus contribute the great majority of their fibers to the radial nerve. Lesions of these parts of the plexus will thus affect the radial nerve and the structures it innervates.

3. The radial nerve innervates all of the extensor muscles which act on the forearm, hand and proximal phalanges (i.e., elbow, wrist and MP joints). After complete (high) radial nerve lesions, the elbow, hand and proximal phalanges rest in a flexed position.

4. Note that the radial nerve innervates the supinator muscle and thus the hand is not only flexed but also pronated after a radial nerve lesion. “Wrist drop” is the most characteristic feature of a radial nerve lesion.

5. With your forearm flexed, and the hand in the wrist drop position, note the weakness in your finger flexion capacity. This is not due to paralysis of any of the finger flexors but to the loss of mechanical advantage of these flexors due to the position of the wrist. You should realize that the position of function of the hand is extension. In a person with a radial nerve lesion the finger flexors work well when the hand is supported in an extended position.

6. The sensory fibers of the radial nerve innervate the skin over the radially innervated muscles (for the most part). There is, however, much overlap with cutaneous branches of other nerves and the only “autonomous zone” is a small area over the first dorsal interosseous muscle (an ulnar innervated structure). Can you localize the position of this muscle on yourself?

7. The close relationship of the radial nerve to the humerus is important to remember - it explains its common involvement after fractures of the humerus, after prolonged pressure from a crutch (crutch palsy), or when a person falls asleep or passes out with their arm thrown over the back of a chair (Saturday night palsy). Which branches of the radial nerve have already been given off at the point where the nerve passes behind the humerus?

8. Weak finger extension at the PIP and DIP joints is possible after a radial nerve lesion. This is explained by the insertion of the ulnar and median nerve innervated intrinsic muscles (interossei and lumbricales) into the extensor hood.

Ulnar nerve

1. Arises primarily from spinal cord segments C8 and Tl but contributions from C7 are often present. Thus, it is similar in origin to the musculocutaneous nerve except that it arises from the lower rather than the upper part of the plexus. It should be obvious that the fibers of this nerve will be interrupted when the lower roots of the brachial plexus are damaged. (Klumlpke's paralysis).

2. Note its position between the axillary artery and vein in the axilla; in the upper part of the arm it accompanies the brachial artery but in the lower part it pierces the intermuscular septum to reach the posterior compartment. In the upper part it lies in close relation to the nerve to the medial head of the triceps muscle which is sometimes called the ulnar collateral nerve. Here the two nerves may be injured together.

3. At the elbow the nerve passes through the interval between the olecranon and the medial epicondyle, in company with the superior ulnar collateral artery. This is the point where the nerve is most vulnerable to trauma.

4. The nerve enters the forearm by passing between the two heads of the flexor carpi ulnaris (which it penetrates and innervates). At the elbow it lies on the posterior and oblique parts of the ulnar collateral ligament. It is thus closely related to the joint and may be affected by joint disease.

5. In the upper half of the forearm it lies between the flexor digitorum profundus and the flexor carpi ulnaris, i.e., between the only two muscles which it innervates in the forearm. Note that the flexor carpi ulnaris lies along the medial side of the forearm and its atrophy (after an ulnar nerve lesion) will produce a flattening of the contour of this part of the forearm.

6. In the lower part of the forearm it is accompanied by the ulnar artery.

7. In the hand the ulnar nerve supplies all of the sensation to the little finger. Note, however, that the dorsal branch arises 5 cm from the wrist and thus a lesion of the ulnar nerve at the wrist will not result in a total sensory loss to the little finger.

8. Note the relationship of the ulnar nerve to the pisiform bone of the wrist - a landmark which can be easily palpated.

9. The ulnar nerve terminates by dividing into superficial and deep branches. The superficial branch is cutaneous while the deep branch innervates most of the muscles of the hand (all interossei, 3rd and 4th lumbricals, all thenar muscles deep to the flexor pollicis longus tendon). In the hand, ulnar nerve lesions produce a “claw hand” deformity which is expressed primarily by hyperextension at the metacarpophalangeal joints. This is due to the unopposed action of the radially innervated extensor muscles. Note that the visible and palpable first dorsal interosseous muscle is the last muscle to be innervated by the ulnar nerve; checking the integrity of this muscle is of diagnostic value.

l0. The ulnar nerve innervated lumbrical muscles function to flex the MP joint and extend the PIP joint. Thus, after a lesion of the ulnar nerve the 4th and 5th fingers will tend to be flexed at the PIP joint to a greater degree than the other two fingers.

-- SUMMARY OF ULNAR NERVE LESION EFFECTS --

A. Lesion proximal to first forearm branch:

1) Paralysis of flexor carpi ulnaris and ulnar half of flexor digitorum profundus (inability to flex distal phalanx of 4th and 5th digits; ring finger may be only partially affected).

2) Subsequent atrophy of above muscles with flattening of medial surface of forearm.

3) Complete loss of sensation to little finger.

4) Paralysis of all intrinsic muscles of the hand with subsequent atrophy. Metacarpal bones will become more prominent due to atrophy of dorsal interossei. Hyperextension (clawing) at all MP joints (except thumb). Inability to abduct or adduct fingers. Inability to adduct thumb. Hypothenar but not thenar atrophy. Clawing creates an inefficient roll-up of the fingers when trying to grasp an object (recall the demonstration of this in the movie on the hand)

5) Greater degree of flexion at PIP joints of 4th and fifth fingers than at PIP joints of other fingers (due to loss of lumbrical muscles of these digits)

6) Weakened ulnar abduction and flexion of wrist due to loss of flexor carpi ulnaris.

B. Lesion at wrist:

1) Incomplete loss of sensation to little finger due to preservation of dorsal branch. Loss expected on opposing surfaces of the 4th and 5th digits.

2) No loss in ability to flex the DIP joints of the little or ring fingers; no weakening of ulnar abduction or wrist flexion because the innervation of these muscles is in the forearm.

3) Loss of all intrinsic muscles of the hand except the thenar muscles superficial to the long flexor tendon (see #4 and #5 above).

4) Loss of the 3rd and 4th lumbrical muscles. These muscles flex the MP joint and extend the PIP joint. Thus, the positions of ring and little fingers (without lumbricals) will differ from those of the index and middle fingers (with lumbricals)

5) Inability to hold an object (such as a piece of paper) strongly against the palm with thumb due to loss of the adductor muscles.

Axillary nerve

1. Arises from spinal cord segments C5 and C6.

2. Is a branch of the posterior cord of the brachial plexus. Thus, lesions involving the posterior cord or the C5 and C6 roots and/or upper trunk would interrupt the afferent and efferent fibers which comprise this nerve.

3. At the lower border of the subscapularis muscle the nerve passes behind the surgical neck of the humerus in close association with the lowest part of the articular capsule of the shoulder joint. Thus, the nerve is most susceptible to injury when the surgical neck is fractured or when the shoulder is dislocated and the articular capsule is stretched or damaged.

4. The nerve passes through the quadrangular space and is the principle structure affected in a “Quadrangular space syndrome”.

5. As the nerve passes behind the humerus it is accompanied by the posterior humeral circumflex vessels. Thus, the nerve and vessels may be traumatized together.

6. The nerve has an anterior branch which lies on, and courses anteriorly on, the deep surface of the deltoid muscle and it innervates this muscle. Thus, longitudinal incisions (i.e., those parallel to the fibers of the deltoid) will likely denervate all portions of the muscle anterior to the line of the incision. Such incisions may be used to approach the shoulder joint. Obviously the more anterior the incision the more muscle function can be preserved.

7. The anterior branch of the axillary nerve has sensory fibers which pass through the substance of the deltoid muscle to innervate the skin; thus, testing skin sensation over the muscle may provide valuable information about the functional capacity of the nerve.

8. The axillary nerve innervates two muscles, the deltoid (anterior branch) and teres minor (posterior branch) Deltoid paralysis and atrophy is relatively easy to demonstrate; teres minor paralysis is not very important and is difficult to demonstrate.

9. Note that the deltoid muscle is the only muscle covering the tip of the acromion and the head of the humerus laterally. Thus, when the muscle atrophies after an axillary nerve lesion these bony landmarks become prominent and the shoulder loses its soft, round contour.

Musculocutaneous nerve

1. Composed chiefly of fibers derived from the C5 and C6 spinal nerves, but with some contribution from C7. Thus, damage to the roots of these nerves, or to the other parts of the brachial plexus that give rise to the nerve, will affect the muscles and skin it innervates. The fibers of this nerve are always interrupted in Erb's palsy.

2. The nerve appears as a direct continuation of the lateral cord of the brachial plexus and it is thus closely associated with the axillary artery and the other two cords of the plexus. Lesions in this part of its course would seldom, if ever, be restricted to the musculocutaneous nerve.

3. As the nerve passes into the arm it immediately penetrates and innervates the coracobrachialis muscle and then passes obliquely between the biceps and brachialis muscles which it also innervates. Here it is well protected from trauma.

4. The role of the biceps and brachialis in flexion at the elbow is easily understood by most students and after the interruption of the nerve it should be apparent that the elbow should be extended. Do not forget, however, that the biceps is a powerful supinator and thus pronation will also be seen after a lesion of the nerve.

5. The cutaneous branch of the musculocutaneous nerve, the lateral antebrachial cutaneous, is distributed, as its name implies, to the lateral part of the forearm. Note that the normal anatomical position must be defined to establish the lateral side.

6. The cutaneous continuation of the nerve, the lateral antebrachial cutaneous, passes into the forearm just lateral to the tendon of the biceps at the elbow.