Nerves of the Brachial Plexus That Supply Intrinsic Muscles of the Thoracic Limb

The suprascapular nerve (n. suprascapularis) arises primarily and occasionally entirely from the sixth cervical nerve (Fig. 17-14). It often has a contribution from the seventh, but rarely from the fifth, cervical nerve. According to Sharp et al. (1991), this nerve arose from both C-6 and C-7 in six of six dogs. In this nerve, 65% of the efferent axons arise from C-6, and 34% from C-7. The nerve enters the distal end of the intermuscular space between the mm. supraspinatus and subscapularis from the medial side. It is accompanied by the suprascapular artery and vein. The suprascapular nerve is primarily a muscle nerve to the mm. supraspinatus and infraspinatus. It passes over the scapular notch, innervates the supraspinatus and continues across the neck of the scapula distal to the end of the spine to enter the infraspinatus. Prior to passing distal to the spine the nerve sends a small branch to the lateral part of the shoulder joint (Fig. 17-15). The suprascapular nerve does not have any cutaneous branches in the dog (Kitchell et al., 1980).

The subscapular nerve (n. subscapularis) is usually a single, but occasionally double, nerve that arises from the union of a branch from the sixth and seventh cervical nerves, or if the nerve is double, one part usually arises from the seventh cervical nerve directly (Fig. 17-14). A contribution from the sixth cervical nerve may also be present. It may arise completely or nearly completely from either the seventh or the eighth cervical nerve (Allam et al., 1952). According to Sharp et al. (1991), its efferent supply arises from C-6 and C-7 in six of six dogs; 59% of the efferent axons in this nerve arise from C-6, and 41% from C-7. It usually divides into cranial and caudal parts on entering the medial surface of the distal fifth of the subscapular muscle. The subscapular nerve is approximately 5 cm long in a medium-sized dog. This permits the extensive sliding movement of the scapula on the thorax during locomotion without nerve injury. The subscapular nerve does not have any cutaneous branches (Kitchell et al., 1980).

The axillary nerve (n. axillaris), like the subscapular nerve, is much longer than the distance between its origin and its peripheral fixed end. It arises as a branch from the combined seventh and eighth cervical nerves (Fig. 17-14). A contribution from the sixth cervical nerve may also be present. It may arise completely or nearly completely from either the seventh or the eighth cervical nerve (Allam et al., 1952). According to Sharp et al. (1991), its efferent supply included C-6 in six of six dogs, C-7 in five of six, and C-8 in two of six; 59% of the efferent axons in this nerve arise from C-6, and 41% from C-7, with fewer than 1% from C-8. Bailey et al. (1982) report that C-6 contributed afferent axons to its cutaneous branches in 8 of 10 dogs; C-7, 10 of 10; C-8, 2 of 10. The axillary nerve leaves the axillary space caudodistal to the subscapular muscle and proximal to the teres major muscle. It supplies mainly the muscles of the shoulder joint as it curves around the caudoventral border of the subscapular muscle near its distal end. In its intermuscular course proximocaudal to the shoulder joint, it divides basically into two portions; one part sends branches to caudal fascicles of the subscapular muscle and completely supplies the m. teres major. The other portion, accompanied by the caudal circumflex humeral vessels, runs laterally to supply the laterally lying teres minor and deltoideus. Before entering the teres minor, a branch enters the caudal part of the shoulder joint capsule (Fig. 17-15).

The cranial lateral cutaneous brachial nerve (n. cutaneus brachii lateralis cranialis) leaves the axillary nerve just prior to the entry of this nerve into the deltoid muscle. Therefore it arises lateral to the space between the origins of the lateral and long heads of the triceps muscle. It runs distally on the lateral head of the triceps muscle, where it is covered by the deltoid muscle. It appears subcutaneously caudal to the main portion of the cephalic vein, where it is associated with the cutaneous branches of the caudal circumflex humeral artery and vein (Fig. 17-16). The cutaneous area of this nerve lies on the lateral surface of the brachium (Fig. 17-17), overlapping in its distribution cranial aspects of the cutaneous area of the intercostobrachial nerve (lateral cutaneous branch of thoracic spinal nerve II) caudally and overlapping cranially the caudal aspects of the cutaneous areas of the fifth cervical nerve, dorsally, and the cutaneous branch of the brachiocephalicus nerve, ventrally (Kitchell et al., 1980). On entering the forearm, the cranial lateral cutaneous brachial nerve is named the cranial cutaneous antebrachial nerve (n. cutaneus antebrachii cranialis). It terminates in the skin of the proximocraniolateral aspect of the forearm, where its cutaneous area is completely overlapped by the cutaneous areas of the radial and musculocutaneous nerves (Kitchell et al., 1980). At the elbow joint or just distal to it, it often joins the medial branch of the superficial radial nerve (Fig. 17-16), and by means of this nerve its fibers are carried to the skin of the cranial two thirds of the length of the antebrachium (Kitchell et al., 1980).

The musculocutaneous nerve (n. musculocutaneus) gives muscular branches to the coracobrachialis, biceps brachii, and brachialis (Figs. 17-11 and 17-13). It continues in the forearm as the medial cutaneous antebrachial nerve (Figs. 17-18 and 17-19). The musculocutaneous nerve is irregular in its formation, arising mainly from the seventh cervical nerve but also receiving contributions from C-6 and C-8 (Fig. 17-14). It receives branches from the first and second thoracic nerves in rare instances. According to Sharp et al. (1990) it was formed by contributions from C-6, C-7, and C-8 in six of six dogs and T-1 in two of six; 57% of the efferent axons in the nerve came from C-7, 26% from C-6, and 16% from C-8, with fewer than 1% from T-1. Bailey et al. (1982) found that its cutaneous nerves have a much wider origin, arising in part from C-6 in six of nine dogs, C-7 in nine of nine, C-8 in six of nine, and T-1 in two of nine. Throughout its course in the brachium it lies between or deep to the cranially lying m. biceps brachii and the brachial vessels caudally (Fig. 17-12). There are three muscular branches. Proximally, a small branch goes to the coracobrachialis (Fig. 17-12). This branch is small, and often instead of arising from the musculocutaneous nerve directly it may exist as a separate branch that comes from the eighth cervical or first thoracic nerve, or both. In reaching the coracobrachialis, it follows the cranial circumflex humeral vessels over a portion of its course. A large branch, the proximal muscular branch (ramus muscularis proximalis), often called the muscular nerve to the m. biceps brachii, enters the deep surface of the biceps brachii approximately 4 cm from its origin and near its caudomedial border. In the distal third of the brachium, a communicating branch (ramus communicans cum n. mediano) passes distocaudad, usually medial to the brachial vessels, and joins the median nerve, which with the ulnar nerve lies caudal to the brachial vessels (Figs. 17-12, 17-18 to 17-20). The communicating branch carries both cutaneous (afferent) and efferent axons to the median nerve (Kitchell et al., 1980). The cutaneous afferents from the communicating branch supply a cutaneous area located on the palmar aspect of the forepaw (Fig. 17- 21). This cutaneous area is also supplied by the median nerve axons (described later). The efferent axons go to some of the antebrachial muscles that are supplied by the median nerve. Sharp et al. (1990) reported that efferent axons from the median nerve also go through the communicating branch to supply the brachial muscle. (This communication between the musculocutaneous and the median nerves in the dog is not homologous to the ansa axillaris, according to the Nomina Anatomica Veterinaria [NAV] [1973]). As the musculocutaneous nerve winds deep to the terminal part of the biceps brachii from the medial side, it terminates by dividing into the distal muscular branch (ramus muscularis distalis), also called the muscle nerve to the brachialis muscle (Figs. 17-18 and 17-19), which enters the distal medial portion of the brachialis muscle, and the small medial cutaneous antebrachial nerve (n. cutaneus antebrachii medialis) (Figs. 17-18 and 17-19). This cutaneous branch crosses the lateral side of the tendon of the biceps brachii and enters the cranial surface of the antebrachium from the flexor angle of the elbow joint. As the nerve crosses the cranial surface of the elbow joint, it sends a small branch to the craniolateral part of it (Fig. 17-19). It freely branches in its course distad in the forearm. Its cutaneous area lies on the craniomedial portion of the antebrachium (Fig. 17-21). The cutaneous area of the medial cutaneous antebrachial nerve does not extend into the carpus. The cutaneous area that it supplies is overlapped medially by the cutaneous area supplied by the cutaneous branches of the medial branch of the superficial radial nerve and by cutaneous branches of the axillary nerve and laterally by the cutaneous areas of the caudal cutaneous antebrachial nerve proximally and the cutaneous areas of the median and the dorsal branch of the ulnar nerve distally.

The radial nerve (n. radialis) (Figs. 17-11 to 17-13) arises from the seventh and eighth cervical and the first and second thoracic nerves (Fig. 17-14). According to Sharp et al. (1991) C-7, C-8, and T-1 contributed to the radial nerve in six of six dogs. In five of six, C-6 also contributed. In three of six dogs, T-2 gave some fibers to this nerve. In the radial nerve, 45% of the efferent axons came from C-8, 29% from T-1, 21% from C-7, and 1% from T-2. Bailey et al. (1982) reported that the cutaneous afferent axons in the radial nerve came in part from C-6 in 7 of 10 dogs, from C-7 in 10 of 10, from C-8 in 10 of 10, and from T-1 in only 2 of 10; none came from T-2. The radial nerve is the largest nerve of the brachial plexus. It supplies all the extensor muscles of the elbow, carpal, and digital joints and also the supinator, brachioradialis, and abductor digiti I longus muscles. The skin on the cranial portion of the antebrachium and the dorsal surface of the paw is also supplied by axons of the radial nerve (Fig. 17-22). As the radial nerve approaches the brachium by traversing the axillary space, it lies lateral to the axillary vein and medial to the axillary artery. On crossing the medial surface of the conjoined tendons of the teres major and latissimus dorsi, it lies caudal to the brachial vessels that are the continuation of the axillary vessels after these have crossed the conjoined tendon. It exits from the axilla distal to the conjoined tendons of the teres major and the latissimus dorsi. On entering the interval between the medial and the long heads of the triceps muscle, the radial nerve gives off a small branch to the tensor fascia antebrachial muscle, then divides into a branch that runs proximolateral and is distributed to the long head of the triceps. The second branch runs distolateral and represents the main continuation of the radial nerve. It supplies a branch to the accessory and medial heads of the triceps muscle before it makes contact with the brachialis muscle. Accompanied by the nutrient artery of the humerus, it follows this muscle in a spiral manner around the humerus. On contacting the lateral head of the triceps, it sends a branch to it, and shortly thereafter it bifurcates into deep and superficial branches. Prior to this bifurcation the radial nerve gives off the caudal lateral brachial cutaneous nerve (n. cutaneus brachii lateralis caudalis), which supplies the skin covering the lateral head of the triceps muscle. The deep branch of the distal radial nerve runs deep to the proximocranial border of the extensor carpi radialis. According to Sharp et al. (1990), the deep branch of the radial has a higher percentage of its efferent axons coming from the more cranial roots of the brachial plexus than do the more proximally located muscles supplied by the radial nerve. At the place of bifurcation of the radial nerve, a minute branch runs to the deep surface of the brachioradialis. The superficial branch pursues a more cranial course and becomes superficial between the distocranial border of the lateral head of the triceps and the lateral surface of the deeply lying brachialis muscle.

The deep branch (ramus profundus) supplies all of the extensor muscles of the carpus and the digits (Fig. 17-16). On the lateral aspect of the elbow joint it passes deep to the extensor carpi radialis near its origin from the lateral supracondyloid crest and sends a branch into it (Fig. 17-16). As the deep branch crosses the flexor surface of the elbow joint, it sends an articular branch to the craniolateral part of it (Fig. 17-19). The remaining part of the deep branch then passes deep to the supinator muscle, which it supplies. On emerging from deep to this muscle, it immediately divides into branches that supply the common and lateral digital extensors and a small branch that closely follows the lateral border of the radius and runs distad to innervate the abductor digiti I longus and extensor digiti I longus et digiti II. The distal end of the deep branch of the radial nerve supplies the antebrachiocarpal joint.

The superficial branch (ramus superficialis) of the radial nerve is its more cranial branch (Fig. 17-16). On emerging from deep to the cranial part of the distal border of the lateral head of the triceps muscle, it runs obliquely craniodistad on the brachialis muscle, where it is covered by the thick intermuscular fascia. After running approximately 1 cm in this location, it perforates the thick fascia and divides unevenly into a larger lateral branch (ramus lateralis) and a smaller medial branch (ramus medialis). These branches continue to the carpus in relation to the lateral and medial branches of the cranial superficial antebrachial arteries, respectively. Thus they closely flank the medial and lateral sides of the cephalic vein as they traverse the antebrachium. From the lateral branch of the superficial branch of the radial nerve the usually double lateral cutaneous antebrachial nerve (n. cutaneus antebrachii lateralis) arises. It supplies a variable cutaneous area around and distal to the lateral epicondyle of the humerus (Fig. 17-22). The more proximal branch is the larger and is the branch that is more constantly present. It arises just distal to the flexor surface of the elbow joint, and, associated with relatively large cutaneous branches of the lateral branch of the cranial superficial antebrachial vessels, it supplies the skin of the proximal one third to two thirds of the lateral surface of the antebrachium (Fig. 17-22). The more distally located nerve to the skin of the lateral side of the antebrachium, smaller than the more proximally located nerve, also is accompanied by a cutaneous artery and vein, which serve the cutaneous area of the region. Occasionally, more than two lateral cutaneous antebrachial nerves are present.

Small branches leave both the medial and the lateral branches of the superficial radial nerves and innervate the skin on the cranial surface of the antebrachium (Fig. 17-22). The medial branches of the superficial radial often join the cranial cutaneous branches of the axillary nerve, which results in the cutaneous areas extensively overlapping; however, neither cutaneous area completely overlaps that of the other nerve. Because the medial and lateral branches of the superficial radial nerves also innervate the dorsum of the forepaw, they and their cutaneous areas are described under the description of the nerves to the forepaw.

Because the radial nerve supplies all the extensor muscles of the thoracic limb except those of the shoulder joint, injury to the proximal part of the nerve results in the inability to support weight on the thoracic limb. The most common traumatic injury that results in a radial paralysis, usually caused by a moving vehicle, is an avulsion of the spinal nerves or their roots or both that contribute to the brachial plexus. When this is complete, multiple brachial plexus nerves are affected and there is a total flaccid paralysis of the affected thoracic limb and analgesia distal to the elbow. Radial nerve injuries distal to the innervation of the triceps brachii do not result in any permanent gait abnormality. The loss of extension of the carpus and digits is rapidly compensated for by an increase in the length of protraction (swing phase) of the limb so that the forepaw is placed on its palmar surface.

The median nerve (n. medianus) (Figs. 17-11 to 17-13) arises primarily from the eighth cervical nerve by a lateral root (radix lateralis) and first thoracic nerve by a medial root (radix medialis), with small contributions from the seventh cervical and the second thoracic spinal nerves (Fig. 17-14). Before its junction with the communicating branch with the musculocutaneous nerve, Sharp et al. (1990) determined that the efferent axons in the median nerve came from C-8 and T-1 in six of six dogs; from C-7 in five of six; and from T-1 in four of six. Of the total efferent axons in the nerve, 9% came from C-7, 38% from C-8, 46% from T-1, and 6% from T-2. Reimers (1925) did not regard the nerve to be formed until it had received the communicating branch from the musculocutaneous nerve in the distal part of the brachium. He believed that through this communication the median nerve is augmented by axons from the sixth and seventh cervical nerves. Sharp et al. (1990) are in agreement with this deduction in that they determined that of the efferent axons in the communicating branch from the musculocutaneous to the median nerve, 60% came from C-7, 22% from C-8, 2% from T-1, and 17% from T-2. Bailey et al. (1982) found the afferent axons in the cutaneous branches of the median nerve to come in part from C-7 in five of eight dogs, C-8 in eight of eight, and T-1 in eight of eight. Mutai et al. (1986), using horseradish peroxidase, found that the median nerve arose from C-7 through T-1.

The median and ulnar nerves in the forearm lie caudal to the brachial artery and vein, where they are loosely joined by fascia (Fig. 17-12). The median nerve is cranial in relation to the ulnar nerve. The median nerve does not give off any branches proximal to the elbow joint. It crosses the flexor surface of the elbow joint cranial to the medial epicondyle. The median nerve passes deep to the pronator teres and enters the large caudal group of flexor muscles located in the antebrachium (Fig. 17-18). It gives muscular branches to the pronator teres, pronator quadratus, flexor carpi radialis, and flexor digitorum superficialis and the radial head of the flexor digitorum profundus. It also sends axons to the deep part of the humeral head of the flexor digitorum profundus and a small articular branch to the medial aspect of the elbow joint.

On emerging from deep to the pronator teres, to which it sends a small branch, several muscular branches (rami musculares) leave the caudal portion of the nerve (Fig. 17-18). The shortest and most proximal of these nerves enters the flexor carpi radialis close to its humeral origin. The remaining flattened bundle of muscular branches crosses the medial surface of the brachial vessels at the place where the common interosseous artery arises, and, after running deep to the flexor carpi radialis and through the humeral head of the flexor digitorum profundus, most of them end in the superficially lying, flattened flexor digitorum superficialis. In their path to this muscle, they lie approximately 1 cm proximal and parallel to the deep antebrachial vessels. In this deep location a branch is sent to the radial head of the flexor digitorum profundus, which it completely innervates, and smaller branches enter the humeral head of this muscle, the lateral part of which is also supplied by the ulnar nerve (Sharp et al., 1990). The small interosseous nerve of the antebrachium (n. interosseus antebrachii) first runs on the proximal part of the delicate interosseous membrane. It then perforates this membrane and runs distally on approximately the proximal half of the pronator quadratus, where it appears as a fine, white streak. It enters this muscle in its distal half and innervates it.

The portion of the median nerve that continues distad in the antebrachium, after the muscular branches have arisen, is at first related to the median artery and vein. At approximately the middle of the antebrachium, the median artery gives off the radial artery, Here, the median nerve continues distad in relation to the larger median artery (Fig. 17-18). This portion of the median nerve is small, measuring approximately 0.5 mm in diameter. The remaining branches of the median nerve will be described with the nerves to the forepaw.

The ulnar nerve (n. ulnaris) (Figs. 17-11 and 17-12) arises in close association with the radial and median nerves from the eighth cervical and the first and second thoracic nerves (Fig. 17-14). Sharp et al. (1990) determined that C-8 and T-1 contributed to the ulnar nerve in six of six dogs, C-7 in one of six, and T-2 in four of the six dogs. Of the total efferent axons in the nerve, 24% came from C-8, 65% from T-1, 11% from T-2, and fewer than 1% from C-7. Bailey et al. (1982) found that the cutaneous afferents in the ulnar nerve arose from C-8 in 10 of 10 dogs, T-1 in 10 of 10, and T-2 in 8 of 10. Mutai et al. (1986), using horseradish peroxidase, found that the ulnar nerve arose from C-8 through T-2. After leaving the caudal part of the brachial plexus, the median and ulnar nerves are flanked by the brachial artery cranially and the brachial vein caudally. They are deep to a thick layer of brachial fascia and bound to each other by areolar tissue until they reach the middle of the brachium, where they diverge. The ulnar nerve, which measures approximately 3 mm in diameter, runs distad along the cranial border of the medial head of the triceps brachii and adjacent to the caudal border of the biceps brachii. It crosses the elbow caudal to the prominent medial epicondyle. On entering the caudomedial part of the antebrachium, the ulnar nerve runs deep to the thick antebrachial fascia. After crossing caudal to the medial epicondyle of the humerus just proximal to the origin of the humeral head of the superficial digital flexor, it runs deep to the ulnar head of the flexor carpi ulnaris (Fig. 17-18). Like the median nerve, no muscular branches leave the ulnar nerve as it traverses the brachium.

The caudal cutaneous antebrachial nerve (n. cutaneus antebrachii caudalis) leaves the caudal part of the ulnar nerve near the beginning of the distal third of the brachium and passes over the medial surface of the olecranon tuber into the caudomedial part of the antebrachium (Figs. 17- 12, 17-18, and 17-20). Bailey et al. (1982) determined that this branch of the ulnar nerve arose from T-1 in 1 of 10 dogs, T-1 and T-2 in 7 of 10, and C-8, T-1, and T-2 in 1 of 10. In its subcutaneous course throughout most of the area it supplies, it is accompanied by the collateral ulnar artery and vein. It freely sends branches to the skin as it winds across the proximal portion of the antebrachium from the medial to the caudolateral aspects. Ascending branches arborize in the skin of the distal part of the brachium. The cutaneous area of the caudal cutaneous antebrachial nerve lies on the proximal two thirds of the skin of the caudolateral aspect of the antebrachium (Fig. 17-23). The cutaneous area of the lateral cutaneous antebrachial branches of the superficial radial nerve overlaps the cutaneous area of the caudal cutaneous antebrachial nerve caudolaterally, and the cutaneous area of the medial cutaneous antebrachial branch of the musculocutaneous nerve overlaps its cutaneous area caudomedially. Distally, the cutaneous area of the proximal branch of the dorsal branch of the ulnar overlaps the cutaneous area of the caudal cutaneous antebrachial nerve. This overlap is described when the innervation of the forepaw is described. The caudal cutaneous antebrachial nerve is an excellent site for testing for avulsions of the caudal roots of the spinal nerves contributing to the brachial plexus because of its origins from T-1 and T-2, and seldom from C-8 (Bailey & Kitchell, 1984).

The muscular branches (rami musculares) of the ulnar nerve, which supply the muscles of the antebrachium, are peripheral branches of a short, stout trunk that leaves the caudal side of the ulnar nerve as it passes caudal to the medial epicondyle of the humerus and plunges into the deep surface of the thin, wide ulnar head of the flexor carpi ulnaris (Fig. 17-20). The ulnar nerve, entering the septum between the ulnar and the humeral heads of the flexor carpi ulnaris, sends a branch approximately 1 mm in diameter and 1.5 cm long distally into the caudal border of the humeral head of the deep digital flexor. In the proximal fifth of the antebrachium, as the ulnar nerve curves around the caudal border of the humeral head of the flexor carpi ulnaris, it sends a stout branch into its lateral surface. Throughout the middle third of the antebrachium the ulnar nerve lies on the caudal border of the deep digital flexor, where it is covered by the humeral head of the flexor carpi ulnaris. At approximately the middle of the antebrachium, the small, cutaneous dorsal branch of the ulnar nerve arises. This branch and the palmar branch arise as terminal branches of the ulnar nerve. Both of these branches are distributed to the structures of the forepaw and are described with the nerves of the forepaw.

Both the ulnar and the median nerves supply muscles on the caudomedial side of the forearm. A summary of these muscles and their innervation is shown in Table 17-1.

TABLE 17-1

Innervation of Muscles of the Caudal Side of the Forearm

MEDIAN NERVE ONLY	BOTH ULNAR AND MEDIAN NERVES	ULNAR NERVE ONLY
Pronator quadratus Pronator teres Radial head of deep digital flexor Superficial digital flexor < div class='tao-gold-member'> Only gold members can continue reading. Log In or Register to continue Share this: Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Related Related posts: The Dog and Its Relatives The Integument The Muscular System The Heart and Arteries Stay updated, free articles. Join our Telegram channel Join Tags: Millers Anatomy of the Dog Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Spinal Nerves Full access? Get Clinical Tree Get Clinical Tree app for offline access Get Clinical Tree app for offline access