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The cervical plexusFormation and location of the cervical plexusThe cervical plexus is formed by union of the anterior branches of the first four cervical nerves (C1-C4) and a portion of C5 (Fig.18-2). It lies deep to the sternocleidomastiod muscle, on the levator scapulae and scalenus medius. Branches of the cervical plexus are divided into superficial (cutaneous) and deep branches. Branches of the cervical plexusSuperficial branchesThey emerge around the middle of the posterior border of the sternocleidomastoid muscle and supply the skin of the neck and scalp (Fig 18-3). In a cervical plexus block, an anesthetic agent is injected at several points along the posterior border of the sternocleidomastoid muscle, mainly at its midpoint. The superficial branches are as follows:The lesser occipital nerve(C2) It ascends along the posterior border of the sternocleidomastoid muscle and supplies the skin of the neck and scalp posterosuperior to the auricle.The great auricular nerve(C2-3) It ascends across the sternocleidomastoid muscle onto the parotid gland and supplies the skin over the parotid gland and around the auricle.The transverse nerve of neck, or transverse cervical nerve(C2-3) It curves around the middle of the posterior border of the sternocleidomastoid muscle and passes forward across it. It supplies the skin on the anterior and lateral surfaces of the neck.The supraclavicular nerves(C3-4) They emerge from beneath the posterior border of the sternocleidomastoid muscle and descend across the side of the neck. They are divided into the medial, intermediate and lateral supraclavicular nerves and supply the skin over shoulder as well as the upper portion of the chest.Deep branchesThey supply the deep muscles of the neck, levator scapulae, infrahyoid muscles and diaphragm. The most important branch is the phrenic nerve.Fibers from the third, fourth, and fifth cervical nerves unite to become the phrenic nerve(Fig18-4). The nerve descends across the front of the scalenus anterior, then enters the thorax by passing in front of the subclavian artery. Within the thorax, it passes in front of the root of the lung and descends between the pericardium and the mediastinal pleura to the diaphragm. The phrenic nerve possesses motor and sensory fibers. The motor fibers innervate the diaphragm, while the sensory fibers supply the peritoneum on diaphragmatic undersurface, the pleura on the upper surface of the diaphragm, the mediastinal parietal pleura and the pericardium. The sensory fibers from right phrenic nerve also supply the liver, the gallbladder and the extrahepatic bile ducts.A branch from C1 joins the hypoglossal nerve. Some of these C1 fibers later leavethe hypoglossal nerve as the descending branch, which is joined by the descending cervical nerve (C2 and C3) from the cervical plexus, to form a loop called the ansa cervicalis, which gives off branches to supply the infrahyoid muscles.Nerves of the cervical plexus are summarized in Table 18.1The brachial plexusFormation and location of the brachial plexusThe brachial plexus is formed by the union of the anterior branches of the four lower cervical nerves (C5-8) and great part of the anterior branch of the first thoracic nerve (T1). From its emergence, the brachial plexus extends downward and laterally through the interval between the scalenus anterior and scalenus medius muscles, then passes over the first rib behind the clavicle, and enters the axilla. Each brachial plexus innervates the entire upper extremity of one side, as well as a number of shoulder and neck muscles.Structurally, the brachial plexus is divided into roots, trunks, divisions,and cords (Figs. 18-5). The roots of the brachial plexus are simply continuations of the anterior branches of the cervical nerves. The anterior branches of C5 and C6 converge to become the superior trunk, the C7 branch continues as the middle trunk, and the anterior branches of C8 and T1 converge to become the inferior trunk. Each of the three trunks immediately divides into an anterior division and a posterior division. The divisions then converge to form three cords. The posterior cord is formed by the convergence of the posterior divisions of the upper, middle, and lower trunks. The medial cord is a continuation of the anterior division of the lower trunk. The lateral cord is formed by the convergence of the anterior division of the upper and middle trunks. The posterior cord is consisted of three posterior divisions. All the three cords surround the axillary artery.The entire upper extremity can be anesthetized in a procedure called a brachial block or brachial anesthesia. The site for anesthetic injection is located midway between the base of the neck and the shoulder, posterior to the clavicle. At this point, the anesthetic can be injected close to the brachial plexus.Branches of the brachial plexusBranches above the clavicle (Fig18-6)Long thoracic nerve(C5-7) It arises from the roots of the brachial plexus and enters the axilla behind the axillary vessels, then descends over the lateral surface of the serratus anterior muscle, which it supplies. The damage of this nerve results in paralysis of the serratus anterior muscle and causes the “wing of the scapula”.Dorsal scapular nerve (C4-5) It pierces scalenus medius, desends deep to levator scapulae, and enters deep surface of rhomboids which it supplies.Suprascapular nerve (C5-6) It passes laterally across posterior triangle of neck, through scapular notch, and supplies supraspinatus and infraspinatus.Branches below the clavicle (Fig18-6)Subscapular nerves (C5-7) They arise from the posterior cord of the brachialplexus and supply the subscapularis and teres major muscles.Medial pectoral nerve(C8-T1) It arises from the medial cord of the brachial plexus, passes between axillary artery and vein and passes through pectoralis minor, supplying it and pectoralis major.Lateral pectoral nerve(C5-7) It arises from the lateral cord of the brachial plexus, pierces clavipectoral fascia to reach deep surface of pectoral major muscle, and supplies it.Thoracodorsal nerve (C6-8) It arises from the posterior cord of the brachial plexus and runs downward in company with thoracodorsal artery to supply latissimus dorsi.Axillary nerve (C5-6) It is one of the terminal branches of the brachial plexus. It turns backward and passes to posterior aspect of arm through the quadrangular space with the posterior circumflex humeral vessels (Fig18-8), and winds round the surgical neck of the humerus. It supplies teres minor and deltoid, and skin over the deltoid.Injury to the axillary nerve is usually caused by the fracture of the surgical neck of the humerus or dislocation of the shoulder joint, results in paralysis of deltoid muscle and causes the “quadrate” shoulder.Musculocutaneous nerve(C5-7) It arises from the lateral cord of the brachial plexus, exits the axilla by piercing the coracobrachialis and descends between the biceps brachii and brachialis, supplying the three muscles. It then continues as the lateral cutaneous nerve of forearm (Fig18-7).Median nerve (C6-T1) It is formed by the union of the lateral and medial roots from the lateral and medial cords of the brachial plexus, respectively. It initially runs on the lateral side of the brachial artery until it reaches the middle of the arm, where it crosses to the medial side. Then it descends to the cubital fossa, where it lies deep to the bicipital aponeurosis. The median nerve enters the forearm by passing between two heads of the pronator teres, runs deep to the flexor digitorum superficialis and continues distally through the middle of the forearm. Near the wrist, the median nerve becomes superficial by descending between the tendons of the flexor digitorum superficialis and the flexor carpi radialis, deep to the palmaris longus tendon. It enters the hand through the carpal tunnel, deep to the flexor retinaculum (transverse carpal ligament) (Fig18-7).The median nerve has no branches in the arm. In the forearm, it supplies the anterior group of muscles of forearm except the flexor carpi ulnaris and the ulnar half of flexor digitorum profundus. Near the wrist, the median nerve gives off a palmar cutaneous branch which supplies the skin over the lateral part of the palm. In the hand, it gives off a recurrent branch and three common palmar digital nerves. The recurrent branch supplies the thenar muscles except the adductor pollicis. Each common palmar digital nerve is divided into two proper digital nerves, which supply the skin of the palmar surface of the lateral three and one-half fingers and the distal half of the dorsal surface of these fingers. It also supplies the 1st and 2nd lumbricales (Fig18-9 to 11).The laceration of the wrist often causes median nerve injury. This results in paralysis of the thenar muscles and the first two lumbricales. The thumb movementsare limited and lie in the same plane as the other digits, it looks like the “ape” hand. Sensation is also lost over the thumb and adjacent two and half digits (Fig18-12).Ulnar nerve (C8-T1) It arises from the medial cord of the brachial plexus. The ulnar nerve descends along the medial side of the brachial artery as far as the middle of the arm. Here, it pierces the medial intermuscular septum and passes behind the medial epicondyle of the humerus. The nerve enters the forearm by passing between the heads of the flexor carpi ulnaris, then runs downwards between the flexor carpi ulnaris and the flexor digitorum profundus on the medial side of the ulnar artery to the wrist. Above the wrist, the nerve gives off a dorsal branch(cutaneous branch) which runs backwards to the dorsum of the hand. The ulnar nerve passes superficial to the flexor retinaculum to enter the palm and runs along the lateral border of the pisiform bone. It is divided into a superficial branch and a deep branch at the distal border of the flexor retinaculum (Fig18-7, 9).The ulnar nerve gives off muscular branches to supply the flexor carpi ulnaris and the ulnar part of the flexor digitorum profundus in the forearm. The dorsal branch of ulnar nerve distributes the skin on the ulnar half of the dorsum of hand and the posterior aspect of the 5th digit, and the medial half of the 4th digit. The superficial branch is distributed to the skin on the medial side of the palm and the anterior aspect of the medial one and a half digits. The deep branch supplies the hypothenar muscles, the third and fourth lumbricales, the adductor pollicis, and all the interossei (Fig18-9 to 11).Ulnar nerve injury commonly occurs where the nerve passes behind the medial epicondyle of the humerus. The injury results in the loss of flexion of the 4th and 5th digits of the distal interphalangeal joints and adduction of the thumb. The adduction of the fingers is also impaired. The hypothenar and interosseous muscles are paralysed. In addition, the metacarpophalangeal joints become hyperextended. This results in a characteristic “clawhand” appearance (Fig18-12).Radial nerve (C5-T1) It arises from the posterior cord and is the largest branch of the brachial plexus. The nerve firstly lies behind the axillary artery, then runs backward with the deep brachial vessels and winds around the back of the humerus in the groove for radial nerve, between the lateral and medial heads of the triceps. It pierces the lateral intermuscular septum above the elbow and continues downward into the cubital fossa between the brachialis and the brachioradialis muscles. Here, the radial nerve is divided into superficial and deep branches anterior to the lateral epicondyle of the humerus. In the arm, the nerve supplies the triceps and the skin of the dorsum of the arm. (Fig18-7, 8).The superficial branch is a cutaneous nerve. It descends deep to the brachioradialis in the forearm. At the junction of the middle and lower one third of the forearm, it runs backward around the radius to the dorsum of the forearm and continues to descend to the dorsum of the hand. It is distributed to the skin on the radial half of the dorsum of the hand, and proximal parts of the radial three and half digits (Fig18-7, 10 to 11).The deep branch is a muscular branch. It pierces the supinator, winds around the lateral aspect of the neck of the radius and enters the dorsum of the forearm where itcontinues as the posterior interosseous nerve, which descends between the superficial and deep extensor muscles and then deep to the extensor pollicis longus. It supplies the brachialis and the extensor muscles of the forearm (Fig18-8).A fracture to the middle or the junction of middle and lower one third of the humerus may damage the radial nerve. The principal symptom of radial nerve damage is wristdrop, in which the extensor muscles of the fingers and wrist fail to function. As a result the joints of the fingers, wrist, and elbow are in a constant state of flexion (Fig18-12).Medial brachial cutaneous nerve (C8-T1) It arises from the medial cord of the brachial plexus and is joined by the intercostobrachial nerve. It supplies the skin on the medial side of the arm.Medial antebrachial cutaneous nerve (C8-T1) It arises from the medial cord of the brachial plexus and descends in front of the axillary artery. It supplies the skin on the medial side of the forearm.。