Anatomy, shoulder and upper extremity, forearm nerves (2023)

introduction

The nerves of the forearm are complex due to the various nerve branches and muscles that are found in the upper extremities. The forearm consists laterally of the radius bone and medially of the ulna bone. The four major joints of the forearm are the humeroulnar joint, the humeroradial joint, and the proximal and distal radioulnar joints.[1]Ein fibröses Syndesmosegelenk verbindet Speiche und Ulna und teilt den Unterarm in ein vorderes Beuge- und ein hinteres Streckkompartiment. Muskelkomponenten im vorderen Kompartiment des Unterarms sind der Pronator teres, Flexor carpi radialis, Flexor carpi ulnaris, Palmaris longus, Flexor digitorum superficialis, Flexor digitorum profundus, Flexor pollicis longus und Pronator quadratus. Muskelkomponenten im hinteren Kompartiment des Unterarms sind Brachioradialis, Extensor carpi radialis longus, Extensor carpi radialis brevis, Extensor digitorum, Extensor digiti minimis, Extensor carpi ulnaris, Abductor pollicis longus, Extensor pollicis brevis, Extensor pollicis longus, Extensor Index und Unterdrücker[2]The nerves in the forearm originate from branches of the brachial plexus and medial antebrachial cutaneous nerve. The five branches of the brachial plexus are the musculocutaneous, axillary, median, ulnar, and radial nerves.[3]With the exception of the axillary nerve, all contribute to the innervation of the forearm. The branches of these four major nerves are the anterior interosseous, posterior interosseous, lateral cutaneous nerve of the forearm, deep branch of the radial nerve, superficial branch of the radial nerve, dorsal cutaneous branches of the ulnar and median nerves, and palmar cutaneous branches of the median and ulnar nerves .

structure and function

The entire upper limb consists of the shoulder, arm, forearm and hand. The function of the forearm is to serve as a bridge between the movement of the arm and the wrist and hand. The nerves of the forearm are ultimately responsible for innervating the muscles of the forearm. In addition to motor function, the nerves of the forearm provide afferent skin sensation on the forearm, wrist, and hand.

The nerves carry a lot of information: electrical, biochemical (immune substances, growth factors and hormones). The nerves participate in the metabolism of the tissue in which they run.

Embryology

The entire human body originates from either ectoderm, mesoderm, or endoderm. Nerves specifically originate from the ectoderm. The neural tube and neural crest cells form from the ectoderm. These structures eventually become the brain, spinal cord, and peripheral nerves.[4]

Towards the end of the first month of embryonic life, the spinal and cranial nerves begin to develop. During this time, the neuroblasts of the basal lamina of the spinal cord shed neurites and turn to the developing myotomes. These are the original motor fibers that are meant to form the anterior roots of the spinal nerves. The sensory component begins to form around the second embryonic month.

blood supply and lymph vessels

blood supply

The radial and ulnar arteries supply most of the blood to the forearm. These two arteries branch off the brachial artery just below the antebrachial fossa.[5]The radial artery is lateral, the ulnar artery is medial. After the A. radialis branches off from the A. brachialis, it migrates laterally via the Musculus pronator teres to the Flexor carpi radialis. The radial artery gives off the radially recurrent and muscular branches distally. After the ulnar artery branches from the brachial artery, it travels laterally under the ulnar head of the pronator teres to the adjacent ulnar nerve. The ulnar artery branches into the anterior ulnar recurrent, posterior ulnar recurrent, common interosseous, and muscular branches. The venous drainage of the forearm occurs via the cephalic, basilic, radial and ulnar veins. The cephalic vein gives off the branch of the median cubital vein, which merges with the basilic vein. The cephalic vein meets the axillary vein and forms the subclavian vein. The radial and ulnar veins drain into the basilic vein. The basil vein empties into the axillary vein. The blood supply to the nerves in the forearm comes from the vessels that run closest to each nerve.

Lymphatic

Most of the lymphatic drainage from the upper extremity goes to the axillary lymph nodes. There are five groups of axillary lymph nodes; central, lateral, posterior, anterior and apical nodes. Drainage from the upper extremity goes primarily to the lateral nodes, also known as the brachial and humeral nodes.

Annoy

The brachial plexus carries most of the innervation to the forearm; However, a small portion of the sensory innervation comes from the medial antebrachial cutaneous nerve.[6]The brachial plexus derives from the ventral branches of the C5 to T1 spinal nerves. The brachial plexus is divided into five sections in the distal course; five nerve roots, three trunks, six subdivisions, three cords and finally into five branches. This article focuses on the branches of the brachial plexus that deal specifically with the forearm.

Medial antebrachial cutaneous nerve

The medial antebrachial cutaneous nerve arises as a branch of the medial cord from the C8-T1 nerve roots. This nerve travels subcutaneously in front of the medial epicondyle and gives sensation to the medial forearm.[7]

Musculocutaneous Nerve

The musculocutaneous nerve is formed from nerve roots C5-C7.[8]It travels in the subcutaneous tissue overlying the brachioradialis muscle. This nerve provides motor supply to the biceps, brachialis, and coracobrachialis. The lateral cutaneous nerve of the forearm arises from the musculocutaneous nerve when it runs lateral to the biceps tendon. This nerve supplies sensation to the radial distribution of the forearm.

Radialnerv

The radial nerve forms from the C5-T1 nerve roots. It supplies the anconeus, brachioradialis and extensor carpi radialis longus muscles with motor.[9]After entering the forearm between the brachioradialis and brachialis, the radial nerve divides into superficial and deep branches. After piercing the supinator muscle, the deep branch branches into the posterior interosseous nerve (PIN). The PIN gives feel to the rear forearm and dorsal wrist. It provides motor supply to the deep and superficial extensors of the posterior compartment and the extensor carpi radialis brevis.

median nerve

The median nerve is formed from the C5-T1 nerve roots. The median nerve runs under the biceps fascia between the two heads of the pronator teres. There is no sensory component provided by the median nerve. The median nerve innervates the pronator teres, flexor carpi radialis, palmaris longus and flexor digitorum superficialis. The only muscle in the anterior compartment that is not innervated by the median nerve is the flexor digitorum profundus. This muscle is innervated by the ulnar nerve.[10]The anterior interosseous nerve (AIN) branches off from the median nerve and passes under the arch of the flexor digitorum profundus.[11]AIN provides sensation to the volar wrist capsule and motor innervation of the deep flexors in the anterior compartment.

Ulnarnerv

The ulnar nerve forms from the C8-T1 nerve roots. It travels down the arm and across the medial epicondyle. There is no sensory distribution through the ulnar nerve in the forearm. The ulnar nerve supplies the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus with motor.[12]The nerve travels backwards and enters the forearm through the cubital tunnel. Dorsal and palmar skin nerves branch off from the ulnar nerve about 5 cm proximal to the wrist.[12]The ulnar nerve continues into Guyon's canal, where it divides into the deep and superficial branches.

Physiological variants

The musculocutaneous nerve may be absent on both sides; The median nerve can innervate muscles normally innervated by the musculocutaneous nerve or part of the lateral column of the brachial plexus. The musculocutaneous nerve can anastomose with the median nerve.

The radial nerve (superficial branch of the radial nerve) may remain superficial on its way to the brachioradialis muscle and may be confused with the medial or lateral antebrachial cutaneous nerves. The radial nerve can anastomose bilaterally or only unilaterally with the ulnar nerve.

The ulnar nerve, like the other nerves of the forearm, has various anatomical variations in its course and with anastomosis with the radial and median nerves.

Surgical Considerations

In surgery, knowledge of the anatomy of the forearm nerves is crucial when it comes to repairing or reconstructing bony or muscular defects. Proper technique and understanding of the structures in the forearm will help ensure no motor or sensory damage occurs. Some common forearm procedures include ORIF of fractures, osteotomy, tendon repair, and carpal tunnel release. When surgically accessing the forearm through a volar approach, the intranervous plane is a landmark to aid in dissection. The proximal internervous plane lies between the brachioradialis and the pronator teres. Distally, the internervous plane lies between the brachioradialis and the flexor carpi radialis.[13]Knowledge of the specific location and course of nerves is important to all physicians, but it is especially important to orthopedic surgeons attempting to realign a fracture.

Clinical Significance

Damage to nerves supplying the forearm can occur anywhere along the brachial plexus. Brachial plexus damage, known as brachial plexopathy, can be caused by trauma, inflammation, tumor, radiation, or hemorrhage.[3]This would present itself clinically as a combination of pain, loss of sensation and motor weakness.

Cubital tunnel syndrome results from compression and damage to the ulnar nerve in the cubital tunnel. The cubital tunnel is the space on the dorsal medial side of the elbow. This syndrome causes tingling along the ulnar side of the forearm and into the 5th finger and the ulnar half of the 4th finger. Almost half of patients with cubital tunnel syndrome show improvement with conservative treatment.[14]Conservative treatment includes splinting and analgesia. Patients who fail conservative management may require surgical decompression of the cubital tunnel.

Compartment syndrome is another notable surgical consideration that could have an adverse impact on forearm function. This syndrome occurs when swelling in a specific area compresses vessels and nerves in the same region. The most common etiology of compartment syndrome in the forearm is fractures of the bones of the forearm. Signs and symptoms of this condition are recorded by the five Ps: pain disproportionate to examination, pallor, paresthesia, palpitations, and paralysis. A change in diastolic and compartment pressures of less than 30 mmHg indicates compartment syndrome.[2]If the patient has clearly positive findings on physical examination, the surgeon can dispense with measuring compartment pressure and begin treatment. Compartment syndrome is a surgical emergency that requires immediate fasciotomy. A fasciotomy is a surgical procedure in which an incision is made in the skin and a blunt dissection is performed to access the fascia that surrounds all compartments of the forearm. The fascia is then cut to relieve the tension caused by the swelling.

Understanding the innervation of the forearm muscles and the action of the forearm muscles is clinically important. When a nerve is injured, the normal function of the muscles that that particular nerve innervates is abnormal. This abnormality is deducible through proper history and physical examination, which allows the doctor to provide effective treatment.

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references

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(Video) Muscles of the Upper Limb

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