Nerves of Upper Limb; courses and inervation

Peripheral Nerves in the Upper Quarter

The brachial plexus terminates in five primary peripheral nerves that are responsible for innervating the tissues of the upper extremity: musculocutaneous, axillary, median, ulnar, and radial nerves

Nerves in the upper lims


Axillary Nerve: C5,6

The axillary nerve emerges from the posterior cord of the brachial plexus; it passes laterally through the axilla, sends a branch to the teres minor muscle, courses behind the surgical neck of the humerus, and innervates the deltoid muscle and overlying skin. The axillary nerve is vulnerable to injury with dislocation of the shoulder and fractures of the surgical neck of the humerus. If the upper trunk of the brachial plexus is stretched or injured, it affects the function of the axillary nerve. Shoulder abduction and lateral rotation are impaired when this nerve is affected.

Musculocutaneous Nerve: C5,6

The musculocutaneous nerve  emerges from the lateral cord of the brachial plexus and crosses the axilla with the median nerve; it pierces and innervates the coracobrachialis and then travels distally to innervate the biceps and brachialis muscles. It continues between these muscles to the flexor surface of the elbow; after emerging from the deep fascia at the elbow, is becomes the lateral cutaneous nerve of the forearm. Isolated impingement of this nerve is not common; injury to the lateral cord or the upper trunk of the brachial plexus affects the musculocutaneous nerve. When affected, the patient is unable to flex the elbow with the forearm supinated and may have some instability in the shoulder.

Median Nerve: C6-8

Bundles from the medial and lateral cords of the brachial plexus unite in the uppermost part of the arm to form the median nerve. It courses the medial aspect of the humerus to the elbow, where it is deep in the cubital fossa under the bicipital aponeurosis, medial to the tendon of the biceps and brachial artery; it then moves into the forearm between the two heads of the pronator teres muscle. Hypertrophy of this muscle can compress the median nerve, producing symptoms that mimic carpal tunnel syndrome, except that the forearm muscles (pronator teres, wrist flexors, extrinsic finger flexors) are involved in addition to the intrinsic muscles. To enter the hand, the median nerve passes through the carpal tunnel at the wrist with the flexor tendons. The carpal tunnel is covered by the thick, relatively inelastic transverse carpal ligament. Entrapment of the median nerve in the tunnel, called carpal tunnel syndrome. causes sensory changes and progressive weakness in the muscles innervated distal to the wrist resulting in apehand deformity (thenar atrophy and thumb in the plane of the hand). The branch innervating the opponens muscle hooks over the carpal ligament two-thirds of the way up the thenar eminence and can be entrapped separately.

Ulnar Nerve: C8, T1

The ulnar nerve emerges from the medial cord of the brachial plexus at the lower border of the pectoralis minor and descends the arm along the medial side of the humerus. It passes posterior to the elbow joint in the groove between the medial epicondyle of the humerus and the olecranon of the ulna. The groove is covered by a fibrous sheath, which forms the cubital tunnel. The nerve possesses considerable mobility to stretch around the elbow as it flexes, although the nerve can be easily irritated at this site owing to its superficial location. It then passes between the humeral and ulnar heads of the flexor carpi ulnaris muscle, another site where impingement could occur. The only extrinsic muscles innervated by the ulnar nerve are the flexor carpi ulnaris and ulnar half of the flexor digitorum profundus. The ulnar nerve enters the hand through a trough formed by the pisiform bone and hook of the hamate bone and is covered by the volar carpal ligament and palmaris brevis muscle, forming the tunnel of Guyon. Trauma or entrapment in this region causes sensory changes and progressive weakness of muscles innervated distal to the site, resulting in partial claw-hand deformity. Injury to the nerve after it bifurcates leads to partial involvement, depending on the site of injury.

 

Radial Nerve: C6-8, T1

The radial nerve emerges directly from the posterior cord of the brachial plexus at the lower border of the pectoralis minor. As it descends in the arm it winds around the posterior aspect of the humerus in the musculospiral groove and continues to the radial aspect of the elbow. In the arm it innervates the triceps, anconeus, and upper portion of the extensor and supinator group of the forearm. Injury to this nerve may occur with shoulder dislocations and mid-humeral fractures. Also known to all therapists is “crutch palsy,” a condition of nerve compression caused by leaning on axillary crutches. “Saturday night palsy” occurs when sleeping with the person’s head on the arm that is slung over the back of a chair or open car window. The triceps is involved only if the compression or injury to the nerve occurs close to the axilla.

At the elbow the radial nerve pierces the lateral muscular septum anterior to the lateral epicondyle and passes under the origin of the extensor carpi radialis brevis; it then divides into a superficial and a deep branch. The deep branch may become entrapped as it passes under the edge of the extensor carpi radialis brevis and the fibrous slit in the supinator, causing progressive weakness in the wrist and finger extensor and supinator muscles (except the extensor carpi radialis longus, which is innervated proximal to the bifurcation). Impingement may occur here and may be erroneously called tennis elbow.

Neck of the radius and may be injured with a radial head fracture. The superficial radial nerve may undergo direct trauma that causes sensory changes in the distribution of the nerve. The radial nerve enters the hand on the dorsal surface as the superficial radial nerve, which is sensory only; therefore, injury to it in the wrist or hand does not cause any motor weakness. The influence of the radial nerve on hand musculature is entirely proximal to the wrist. Injury proximal to the elbow results in wrist drop and inability to actively extend the wrist and fingers. This affects the length–tension relationship of the extrinsic finger flexors, resulting in an ineffective grip unless the wrist is splinted in partial extension. Injury of the mid-forearm affect only the supinator muscle and extrinsic abductor and extensor pollicis muscles.

Lumbosacral Plexus

The lumbar plexus is formed by the anterior primary divisions of the nerve roots L1, L2, L3, and part of L4  the sacral plexus is formed from L4, L5, S1, and parts of S2 and S3 (Fig. 13.8B). As with the brachial plexus, the branches and divisions of the LS plexus organize the content of each of the peripheral nerves coursing into the lower extremity. In addition, the anterior primary rami of the plexus receive postganglionic sympathetic fibers from the sympathetic chain that innervate blood vessels, sweat glands, and piloerector muscles in the lower extremity. Isolated injuries to the lumbar plexus or sacral plexus are not common; symptoms more commonly arise from disc lesions or spondylitic deformities that affect one or more nerve roots or from tension or compression of specific peripheral nerves.

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