Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Section
Peripheral Nervous System (Systema nervosum periphericum) – General Overview
Spinal Nerves – Brachial Plexus

The Brachial Plexus, plexus brachialis, is formed by the anterior rami of the four lower cervical nerves (C_V-C_VII) and the greater part of the fibers of the anterior ramus of the first thoracic nerve (Th_I) (Fig. 190). Occasionally, the plexus is formed by C_IV-Th_I (cranial type) or C_V-Th_II (caudal type). It is divided into supraclavicular and infraclavicular parts, pars supraclavicularis et infraclavicularis. The former lies within the lateral triangle of the neck, while the latter is located in the axillary fossa (cavum axillare).

The supraclavicular part of the plexus consists of three trunks, trunci plexus, which lie in the interscalene space (spatium interscalenum) above the Subclavian Artery. The upper trunk, truncus superior, is formed by the union of the anterior rami of the fifth and sixth (occasionally C_IV-C_V) cervical nerves; the middle trunk, truncus medius, is a continuation of the anterior ramus of the seventh cervical nerve; and the lower trunk, truncus inferior, is formed by the union of the anterior ramus of the eighth cervical nerve and the fibers of the anterior ramus of the first thoracic nerve (occasionally C_VIII, Th_I-Th_II). All three trunks branch into anterior and posterior divisions, divisiones anteriores et posteriores, which give rise to the infraclavicular part.

The infraclavicular part of the plexus consists of three cords. The lateral cord, fasciculus lateralis, lies lateral to the axillary artery (a. axillaris). It is formed by the union of the anterior Divisions of the upper and middle trunks. The medial cord, fasciculus medialis, lies initially inferior to and then medial to the axillary artery. It is a continuation of the anterior division of the lower trunk. The posterior cord, fasciculus posterior, lies posterior to the axillary artery and is formed by the union of all the posterior divisions.

The plexus gives rise to short and long branches. The short branches, with the exception of the axillary nerve, arise from the supraclavicular part, whereas the long branches and the axillary nerve originate from the infraclavicular part.

Short Branches of the brachial plexus (Figs. 190, 191):

1. Dorsal scapular nerve, n. dorsalis scapulae (C_V), passes posteriorly and innervates the levator scapulae Muscles (mm. levator scapulae).

2. Long thoracic nerve, n. thoracicus longus (C_V-C_VII), descends inferiorly posterior to the trunks of the plexus onto The surface of the serratus anterior Muscle, which it innervates.

3. Subclavian nerve, n. subclavius (C_V), innervates the muscle of the same name.

4. Suprascapular nerve, n. suprascapularis (C_V-C_VI), passes through the scapular notch (incisura scapulae) into the supraspinous fossa, wraps around the Base of the acromion to enter the infraspinous fossa, and innervates the supraspinatus and infraspinatus muscles (mm. supra- et infraspinatus) as well as the capsule of the shoulder joint.

5. Subscapular nerve, nervus subscapularis (C_V-C_VIII), extends to the subscapularis, teres major, and latissimus dorsi muscles, which it innervates. The branch to the last-named muscle is called the thoracodorsal nerve, nervus thoracodorsalis, which may arise independently from the brachial plexus.

6. Medial and lateral pectoral nerves, nervus pectoralis medialis et lateralis (C_V-Th_I), pass anteriorly, pierce the clavipectoral fascia, and innervate the pectoralis Major and minor muscles.

7. Axillary nerve, nervus axillaris (C_V-C_VII), originates from the posterior cord and, together with the posterior circumflex humeral artery (a. circumflexa humeri posterior), passes through the quadrangular space into the subdeltoid space. Here it branches to supply the shoulder joint, the deltoid and teres minor muscles, as well as the Skin of the posterolateral surface of the deltoid region. The latter branch is called the superior lateral cutaneous nerve of the arm, nervus cutaneus brachii lateralis superior.

Long branches of the brachial plexus (Figs. 192, 193):

1. Radial nerve, nervus radialis (C_V-C_VIII), the thickest of the brachial plexus nerves, originates from the posterior cord. Initially, it runs posterior to the brachial artery, then passes through the humeromuscular canal (canalis humeromuscularis) alongside the deep brachial artery. Emerging from the lower opening of the canal, it lies in the lateral anterior cubital sulcus (sulcus cubitalis anterior lateralis), where it divides into deep and superficial branches.

In the arm, the radial nerve gives off the inferior lateral cutaneous nerve of the arm, nervus cutaneus brachii lateralis inferior, to the skin of the posterolateral surface of the arm (Fig. 192 B); the posterior cutaneous nerve of the arm, nervus cutaneus brachii posterior, to the skin of the posterior surface of the arm; the posterior cutaneous nerve of the forearm, n. cutaneus antebrachii posterior, to the skin of the posterior surface of the forearm; and muscular branches, rami musculares, to the triceps brachii, anconeus, and brachioradialis muscles.

The deep branch, ramus profundus, pierces the supinator muscle, passes to the dorsum of the forearm between the superficial and deep muscles, and innervates all the posterior forearm muscles. The terminal continuation of the deep branch, known as the posterior interosseous nerve, nervus interosseus posterior, runs between the extensors of the thumb to the radiocarpal joint, which it innervates.

The superficial branch, ramus superficialis, descends along the radial sulcus together with the radial vessels. In the lower part of the forearm, it passes beneath the brachioradialis muscle to the dorsum of the forearm, reaches the back of the hand, and divides into five dorsal digital nerves, nervi digitales dorsales. These nerves innervate the dorsal surface of the first, second, and lateral part of the third digits (Fig. 195).

2. Musculocutaneous nerve, nervus musculocutaneus (C_V-C_VIII), originates from the lateral cord, pierces the coracobrachialis muscle, and lies between the biceps brachii and brachialis muscles. After emerging from beneath the lateral margin of the biceps brachii, it pierces the brachial fascia and becomes subcutaneous, running alongside the cephalic vein (v. cephalica), down which it descends into the forearm as the lateral cutaneous nerve of the forearm, nervus cutaneus antebrachii lateralis. In the arm, it gives off muscular branches, rami musculares, to the biceps brachii, brachialis, and coracobrachialis muscles. In the forearm, it innervates the skin of its anterolateral surface as far as the thenar eminence (Fig. 192 C).

3. Median nerve, nervus medianus (C_V-Th_I), arises by two roots. The lateral ROOT, radix lateralis, is formed by fibers from the lateral cord, while the medial root, radix medialis, is formed by fibers from the medial cord. Encircling the axillary artery, the two roots unite to form the nerve trunk (Fig. 192). In the arm, it lies within the neurovascular bundle: in the upper third of the arm, it is lateral to the brachial artery; in the middle third, anterior; and in the lower third, medial to the artery. In the region of the cubital fossa, the median nerve runs medial to the tendon of the biceps brachii, then passes beneath the pronator teres muscle. In the upper part of the forearm, it lies between the superficial and deep flexors of the digits, and further inferiorly, it runs in the medial sulcus (Fig. 143). The nerve enters the hand through the carpal tunnel (canalis carpi), where it lies adjacent to the synovial Sheath of the tendon of the long flexor of the thumb and the common synovial sheath of the flexor tendons of the digits.

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Fig. 190. Brachial plexus, plexus brachialis (sources of formation and its nerves)

Fig. 191. Brachial plexus, plexus brachialis, and Cervical plexus, plexus cervicalis.

In the region of the cubital fossa and on the forearm, the median nerve gives off muscular branches, rami musculares, to the anterior forearm muscles, with the exception of the flexor carpi ulnaris and the medial part of the flexor digitorum profundus. Furthermore, the deep muscles (flexor pollicis longus, lateral part of flexor digitorum profundus) are innervated by a branch of the median nerve known as the anterior interosseous nerve, n. interosseus anterior. It runs along the interosseous membrane together with the anterior interosseous artery. Slightly above the radiocarpal joint, the palmar branch of the median nerve, ramus palmaris n. mediani, arises and innervates the skin of the palm, predominantly in the thenar region.

In the palm, the median nerve divides into three terminal branches—the common palmar digital nerves, nervi digitales palmares communes (Fig. 194)—which run along the first three intermetacarpal spaces, positioned beneath the palmar aponeurosis. The first of these innervates the thenar muscles* (with the exception of the adductor pollicis and the deep HEAD of the flexor pollicis brevis), as well as the First and Second lumbrical muscles (mm. lumbricales). The common palmar digital nerves branch into seven proper digital palmar nerves, nervi digitales palmares proprii, which innervate the skin of the palmar surface of the first, second, third, and the lateral part of the fourth digits (Figs. 195 A, B, C).

4. The ulnar nerve, nervus ulnaris (C_VIII-Th_I), is formed by fibers from the medial cord. Initially, it runs alongside the brachial artery, then deviates posteriorly, pierces the medial intermuscular septum (septum intermusculare mediale), and further down enters the posterior medial cubital groove (sulcus cubitalis posterior medialis). From there, the ulnar nerve passes beneath m. flexor carpi ulnaris, descends along the ulnar sulcus together with the accompanying vessels, and enters the hand through the ulnar canal (canalis carpi ulnaris).

In the region of the elbow joint and forearm, the ulnar nerve gives off several branches. The muscular branches (rami musculares) supply m. flexor carpi ulnaris and the medial part of m. flexor digitorum profundus. The dorsal branch of the ulnar nerve (ramus dorsalis n. ulnaris) passes through the space between m. flexor ulnaris and the ulna to the dorsum of the hand, where it divides into five dorsal digital nerves (nervi digitales dorsales). These innervate the skin of the dorsal aspect of the fourth, fifth, and the medial part of the third digit.

The palmar branch of the ulnar nerve (ramus palmaris nervi ulnaris) supplies the skin of the hypothenar eminence.

In the palm, the ulnar nerve divides into two terminal branches: superficial and deep.

The superficial branch (ramus superficialis) divides into two common palmar digital nerves (nervi digitales palmares communes), which give rise to three proper palmar digital nerves (nervi digitales palmares proprii). The latter innervate the skin of the PALMAR ASPECT OF the fifth and the medial part of the fourth digit (Fig. 195 A).

* Innervation of the thenar muscles is provided by 1-2 motor branches that travel transversely toward the Muscles of the thumb; therefore, incisions (e.g., for draining Phlegmon of the thenar space) should not be made in the proximal third of the thenar crease (Kanavel's "forbidden zone").

Fig. 192. Long Branches of the brachial plexus (anterior surface of the left arm)

Fig. 192-A. Nerves and Vessels of the left arm (posterior surface)

Fig. 192-B. Cutaneous NERVES OF THE right upper extremity (posterior surface) (after H. Corning).

Fig. 192-C. Cutaneous nerves of the right upper extremity (anterior surface) (after H. Corning).

Fig. 193. NERVES AND Arteries of the left forearm, anterior surface. (Superficial muscles are resected)

Fig. 194. Nerves of the right hand (palmar surface)

Fig. 195 A; B; C; Areas of cutaneous innervation of the hand: A - palmar surface;

B - dorsal surface, C - variations of hand innervation (after Leamont and Hutton)

The deep branch (ramus profundus) runs together with the deep branch of the ulnar artery and the deep palmar arch. It innervates all hypothenar muscles, all interosseous muscles (mm. interossei), the third and fourth lumbrical muscles (mm. lumbricales), as well as m. adductor pollicis and the deep head of m. flexor pollicis brevis. A slender communicating branch—ramus communicans cum nervo ulnari—runs from the median nerve to the deep branch of the ulnar nerve.

5. The medial cutaneous nerve of the arm, nervus cutaneus brachii medialis (C_VIII-Th_I), originates from the medial cord of the brachial plexus and innervates the skin of the Medial surface of the arm down to the elbow joint. Near the base of the axillary fossa, the medial cutaneous nerve of the arm connects with the perforating branch.

6. The medial cutaneous nerve of the forearm, nervus cutaneus antebrachii medialis (C_VIII-Th_I), also originates from the medial cord and innervates the skin of the medial surface of the forearm down to the wrist joint. Along the forearm, it runs alongside the basilic vein (V. basilica).

NERVES OF THE BRACHIAL PLEXUS

Summary Table