Human Anatomy and Physiology - I. V. Gaivoronsky 2011

Functional Anatomy of the Peripheral Nervous System
Spinal Nerves

According to the number of Spinal Cord segments, there are 31 pairs of Spinal Nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each nerve is connected to the spinal cord via anterior and posterior roots. The posterior ROOT is sensory, while the anterior root contains motor and sympathetic fibers. Practically at the junction of the roots lies the sensory ganglion (dorsal root ganglion) of the spinal nerve, located in the intervertebral foramen. It represents a cluster of sensory neuron Cell bodies.

At a short distance from their point of formation, the spinal nerves divide into branches (Fig. 15.5). The meningeal branch consists of sensory and sympathetic fibers. Immediately after branching off from the spinal nerve, it returns to the intervertebral foramen to innervate the Meninges of the spinal cord and their Blood Vessels. The posterior and anterior branches are mixed in fiber composition (sensory, motor, and sympathetic). They innervate the Skin and Muscles of the Trunk and limbs: the posterior branches supply the back, PARTS OF THE neck, and the posterior pelvic region; the anterior branches supply the anterior neck, trunk, and limbs. The anterior branches contribute to The formation of the cervical, lumbar, and sacral plexuses. Additionally, a white communicating branch arises from the 8th cervical to the 3rd lumbar spinal segments, ending in the Sympathetic trunk ganglia. From these ganglia, a gray communicating branch returns to the trunk of each spinal nerve. It consists of sympathetic fibers that subsequently distribute into the meningeal, anterior, and posterior branches.

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Fig. 15.5. Branches of the spinal nerve:

1 — posterior root of the spinal nerve; 2 — sensory ganglion of the spinal nerve; 3 — meningeal branch; 4 — posterior branch; 5 — anterior branch; 6 — communicating branch; 7 — sympathetic trunk ganglion; 8 — anterior root of the spinal nerve

The Cervical plexus, plexus cervicalis, is formed by the Anterior branches of the upper four cervical spinal nerves, which interconnect via arched loops. It has connections with the accessory and hypoglossal nerves, as well as the sympathetic trunk.

Based on fiber composition, the branches of the cervical plexus are classified as motor, sensory, and mixed. Muscular (motor) branches innervate the deep Neck Muscles, the Infrahyoid muscles, and the geniohyoid Muscle. Sensory branches (transverse cervical, supraclavicular, lesser occipital, and great auricular nerves) supply the skin of the supraclavicular region, neck, and earlobe (see Fig. 15.3). A mixed branch of the cervical plexus is the phrenic nerve, n. phrenicus, which descends into the thoracic cavity, innervates the Diaphragm, and gives off sensory branches to the Pericardium, Pleura, and diaphragmatic Peritoneum.

The Brachial Plexus, plexus brachialis, is formed by the anterior branches of the 5th–8th cervical and partially the 1st thoracic spinal nerves. In the supraclavicular region, it is represented by three cords surrounding the axillary artery: medial, lateral, and posterior (Fig. 15.6).

The brachial plexus is divided into supraclavicular and infraclavicular parts. Branches of the supraclavicular part arise from the three aforementioned cords and innervate the muscles of the shoulder girdle, chest, and back: the dorsal scapular nerve innervates the levator scapulae and rhomboid muscles; the suprascapular nerve passes through the scapular notch to the supraspinatus and infraspinatus muscles; the subscapular nerve supplies the subscapularis and teres major muscles; the long thoracic nerve supplies the serratus anterior; the pectoral nerves supply the pectoral muscles; and the thoracodorsal nerve supplies the latissimus dorsi.

Fig. 15.6. Brachial plexus:

1 — medial cord; 2 — medial cutaneous nerve of the forearm; 3 — lateral root of the median nerve; 4 — medial root of the median nerve; 5, 8 — median nerve; 6 — musculocutaneous nerve; 7 — ulnar nerve; 9 — muscular branches of the musculocutaneous nerve; 10 — brachial artery; 11 — lateral cutaneous nerve of the forearm; 12 — posterior cutaneous nerve of the forearm; 13 — radial nerve; 14 — posterior cutaneous nerve of the arm; 15 — branches of the axillary nerve; 16 — lateral cord; 17 — axillary artery

The infraclavicular part is represented by the continuation of the three aforementioned cords. In the arm, the medial cord gives rise to the medial cutaneous nerve of the arm, the medial cutaneous nerve of the forearm, the ulnar nerve, and the medial root of the median nerve. The musculocutaneous nerve and the lateral root of the median nerve emerge from the lateral cord. The axillary and radial nerves originate from the posterior cord.

The medial cutaneous nerve of the arm, n. cutaneus brachii medialis, contains sensory and sympathetic fibers. It innervates the skin of the corresponding region and the axilla.

The medial cutaneous nerve of the forearm, n. cutaneus antebrachii medialis, has a similar fiber composition and innervates the skin of the anterior and medial surfaces of the forearm.

The ulnar nerve, n. ulnaris, is a mixed nerve that accompanies the brachial artery and Veins OF THE same name in the forearm (Fig. 15.7). It innervates the capsule of the elbow joint, the flexor carpi ulnaris, and the medial part of the flexor digitorum profundus. In the hand, its branches supply the skin of the dorsal surface of digits V, IV, and the ulnar side of digit III, and on the palmar surface — digit V and the ulnar side of digit IV (superficial branch), as well as all hypothenar muscles, all interossei, the 3rd and 4th lumbricals, and some thenar muscles (deep branch).

The median nerve, n. medianus, is a mixed nerve formed by the medial and lateral roots from the corresponding cords of the brachial plexus. In the arm, it runs within the neurovascular bundle and gives off no branches. It participates in the Innervation of the elbow joint and supplies branches to most muscles of the anterior forearm group. Its largest branch (the anterior interosseous nerve) innervates the wrist joint and the Deep Muscles of the anterior forearm. It then enters the hand and innervates most of the thenar muscles; digits I and II lumbricals via its deep branch; and the skin of the palmar hand over digits I, II, III, and the lateral half of digit IV via its superficial branch.

The musculocutaneous nerve, n. musculocutaneus, is a mixed nerve originating from the lateral cord of the brachial plexus. It innervates the flexor muscles of the arm and the skin of the lateral forearm.

The axillary nerve, n. axillaris, is a mixed nerve originating from the posterior cord of the brachial plexus. It passes through the quadrangular space, wrapping around the surgical neck of the humerus. It gives off muscular branches to the deltoid and teres minor muscles, and innervates the shoulder Joint Capsule and the skin of the upper posterior-lateral region of the arm.

Fig. 15.7. NERVES OF THE forearm and hand:

1 — brachial artery; 2 — ulnar nerve; 3 — ulnar artery; 4 — deep branch of the ulnar nerve; 5 — superficial branch of the ulnar nerve; 6 — median nerve; 7 — superficial branch of the radial nerve; 8 — radial artery; 9 — pronator teres; 10 — deep branch of the radial nerve; 11 — radial nerve

The radial nerve, n. radialis, is a mixed nerve originating from the posterior cord of the brachial plexus. In the arm, it initially runs alongside the brachial artery and then accompanies the deep brachial artery in the musculospiral (radial) groove. In the arm, it divides into motor branches supplying the extensor muscles and sensory branches to the skin of the posterior-lateral arm. In the forearm, its branches innervate all extensor muscles and the brachioradialis, providing sensory innervation to the skin of the posterior forearm. In the hand, it gives off no motor branches but innervates the skin of the dorsal hand corresponding to digits I, II, and the lateral half of digit III.

Thus, the brachial plexus provides innervation to the muscles of the shoulder girdle, the skin and Muscles of the Free Upper Limb, and certain muscles of the chest and back.

The anterior branches of the thoracic spinal nerves run within the intercostal spaces and are called intercostal nerves, nn. intercostales. The anterior branch of the 12th thoracic nerve lies below the rib and is termed the subcostal nerve. The intercostal nerves are part of the intercostal neurovascular bundle. The upper six intercostal nerves reach the Sternum, and their terminal branches ramify in the skin of the anterior chest wall. The lower five intercostal nerves and the subcostal nerve extend onto the anterior abdominal wall. These nerves provide sensory innervation to the chest and abdominal skin, and divide into motor branches supplying the intrinsic chest muscles and Abdominal muscles. In women, the 3rd and 4th intercostal nerves innervate the mammary gland.

The Lumbar plexus, plexus lumbalis, is formed by the anterior rami of the 12th thoracic and 1st to 4th lumbar spinal nerves. It lies within the substance of the psoas major muscle and on the anterior surface of the quadratus lumborum muscle (Fig. 15.8). Motor branches run from the plexus to supply these muscles. The branches emerging from the lumbar plexus include the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, obturator, and femoral nerves. The iliohypogastric and ilioinguinal nerves provide motor and sensory innervation to the muscles and skin of the abdominal wall, with the latter also supplying the skin of the external genitalia. The genitofemoral nerve supplies the cremaster muscle via its genital branch, while its femoral branch innervates the skin of the thigh just below the inguinal ligament. The lateral femoral cutaneous nerve provides sensory innervation to the skin of the corresponding region of the thigh. The obturator nerve, n. obturatorius, runs in the lesser pelvis adjacent to the obturator artery and vein. Passing through the obturator canal as part of the neurovascular bundle, it innervates the adductor muscles of the medial thigh and the skin of the medial thigh. The femoral nerve, n. femoralis, is the largest branch of the lumbar plexus. It enters the thigh from the lesser pelvis through the muscular lacuna and divides into numerous branches, most of which supply the anterior muscles and skin of the thigh. Its largest branch, the saphenous nerve, travels alongside the great saphenous vein to innervate the skin on the medial side of the knee, leg, and FOOT.

Fig. 15.8. Lumbar plexus:

1 — INFERIOR VENA CAVA; 2 — Esophagus; 3 — aorta; 4 — greater splanchnic nerve; 5 — lesser splanchnic nerve; 6 — subcostal nerve; 7 — iliohypogastric nerve; 8 — ilioinguinal nerve; 9 — lateral femoral cutaneous nerve; 10 — obturator nerve; 11 — muscular branches of femoral nerve; 12 — lumbosacral trunk; 13 — femoral nerve; 14 — sacral part of sympathetic trunk; 15 — Sacral Plexus; 16 — iliopsoas muscle; 17 — femoral branch of genitofemoral nerve; 18 — genital branch of genitofemoral nerve; 19 — psoas major muscle; 20 — psoas minor muscle; 21 — genitofemoral nerve

Fig. 15.9. Nerves of the gluteal region and posterior thigh (posterior view):

1 — superior gluteal nerve; 2 — superior gluteal artery; 3 — posterior femoral cutaneous nerve; 4 — sciatic nerve; 5 — motor branch; 6 — biceps femoris muscle; 7 — tibial nerve; 8 — common fibular nerve; 9 — lateral sural cutaneous nerve; 10 — medial sural cutaneous nerve; 11 — semimembranosus muscle; 12, 14 — inferior cluneal nerves; 13 — inferior gluteal nerve

The sacral plexus, plexus sacralis, is formed by the anterior rami of the 5th lumbar, sacral, and coccygeal spinal nerves. It lies on the anterior surface of the piriformis muscle and exits the lesser pelvis through the greater sciatic foramen.

The branches of the sacral plexus are divided into sensory, motor, and mixed nerves. The motor branches innervate most of the pelvic muscles, the largest being the superior and inferior gluteal nerves. The principal sensory branch is the posterior femoral cutaneous nerve (Fig. 15.9), which supplies the skin of the posterior thigh as well as the skin of the gluteal region via the inferior cluneal nerves.

The superior and middle cluneal nerves originate from the posterior rami of the lumbar and sacral spinal nerves.

The mixed branches are represented by the pudendal and sciatic nerves. The pudendal nerve, n. pudendus, emerges below the piriformis muscle and enters the ischioanal fossa. It supplies the muscles and skin of the Perineum, as well as the skin of the external genitalia.

The largest nerve trunk in the body, the sciatic nerve (n. ischiadicus), originates from the sacral plexus. In the thigh, it runs through the posterior compartment, supplying the hamstring muscles. Within the popliteal fossa, the sciatic nerve divides into the tibial and common fibular nerves. The tibial nerve passes through the adductor canal (popliteal-crural canal, Fig. 15.10) and terminates as the medial and lateral plantar nerves. It gives off motor branches to the posterior leg muscles and provides innervation to the knee and ankle joints. The medial and lateral plantar nerves supply the muscles and skin of the SOLE OF THE foot.

The common fibular nerve enters the superior musculofibrous canal, where it divides into the deep and superficial fibular nerves. The deep fibular nerve innervates the anterior compartment of the leg, the dorsum of the foot, the ankle joint, and the skin of the first web space. The superficial fibular nerve supplies the lateral compartment of the leg and the skin of the dorsum of the foot.

Fig. 15.10. Nerves of the leg (posterior view):

1 — sciatic nerve; 2 — common fibular nerve; 3 — lateral sural cutaneous nerve; 4 — tibial nerve; 5 — medial sural cutaneous nerve

In the popliteal fossa region, the medial and lateral sural cutaneous nerves arise from the tibial and common fibular nerves, respectively; in the lower leg, they unite to form the sural nerve. Together, they provide sensory innervation to the skin of the posterior calf and the lateral border of the foot.