Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Nervous System
Peripheral Nervous System
Spinal Nerves

The spinal nerves (31 pairs) arise from the nerve roots originating from the Spinal Cord. There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal spinal nerves. The spinal nerves correspond to the segments of the spinal cord (Fig. 96).

Each spinal nerve is formed by two roots: an anterior (efferent/motor) ROOT and a posterior (afferent/sensory) root, which unite within the intervertebral foramen. Adjacent to the posterior root is the Cytology/practical/71.html">Sensory spinal ganglion, which contains The Cell bodies of large pseudounipolar sensory Neurons. The long processes (dendrites) of these Cells extend to the periphery, where they terminate in receptors. The axons of these sensory cells enter the corresponding posterior horn of the spinal cord via the posterior root.

The fibers of the anterior and posterior roots form mixed spinal nerves containing both sensory (afferent) and motor (efferent) fibers. Spinal nerves arising at the level of the eighth cervical, all thoracic, and the upper two lumbar segments of the spinal cord also contain autonomic (sympathetic) fibers. These fibers are the processes of cells located in the lateral horns, which exit the spinal cord as part of the anterior roots.

Immediately upon exiting the intervertebral foramen, each spinal nerve divides into branches: anterior, posterior, and meningeal. In addition to these three branches present in all spinal nerves, the thoracic and upper lumbar regions feature a fourth, communicating branch (ramus communicans) that connects to the Sympathetic trunk. Within this branch, autonomic fibers run to the sympathetic trunk; these are processes of cells located in the lateral horns of the spinal cord from the eighth cervical or first thoracic segment to the second lumbar segment. The meningeal branch loops back through the intervertebral foramen into the vertebral canal to innervate the Meninges of the spinal cord. The posterior branches curve sharply backward to the Muscles and Skin of the posterior neck, back, lumbar region, and buttocks. The thicker anterior branches run forward, with their fibers innervating the skin and Muscles of the neck, chest, abdomen, and upper and lower limbs. In the cervical, lumbar, and sacral regions, the anterior branches exchange fibers to form plexuses—cervical, brachial, lumbar, and sacral—from which peripheral nerves arise. In the thoracic region, the Anterior branches of the thoracic spinal nerves do not exchange fibers; they run within the walls of the chest and abdomen as intercostal nerves.

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Fig. 96. The Peripheral Nervous system:

1 — Brain, 2 — spinal cord, 3 — Cervical plexus, 4 — Brachial Plexus, 5 — intercostal nerves, 6 — radial nerve, 7 — Lumbar plexus, 8 — Sacral Plexus, 9 — sciatic nerve, 10 — tibial nerve, 11 — common fibular (peroneal) nerve, 12 — superficial fibular (peroneal) nerve, 13 — deep fibular (peroneal) nerve, 14 — saphenous nerve, 15 — obturator nerve, 16 — femoral nerve, 17 — ulnar nerve, 18 — median nerve, 19 — sympathetic trunk

Cervical plexus. The cervical plexus is formed by the anterior rami of the upper four cervical spinal nerves. It is located between the Deep Muscles of the Neck. The Branches of the cervical plexus emerge from behind the posterior border of the sternocleidomastoid Muscle. These include muscular branches to adjacent muscles, the great auricular nerve, the lesser occipital nerve, the supraclavicular nerves, the transverse cervical nerve, and the phrenic nerve. The muscular branches, by uniting with the Hypoglossal nerve (cranial nerve XII), form the ansa cervicalis, which innervates the Infrahyoid muscles. The NERVES OF THE cervical plexus innervate the deep muscles of the neck, the skin of the auricle and external acoustic meatus, the lateral part of the occiput, the anterior Regions of the neck, and the supraclavicular and infraclavicular regions. The long phrenic nerve descends into the thoracic cavity, passes between the Pericardium and mediastinal Pleura, and branches in the Diaphragm. The phrenic nerve innervates the pericardium, mediastinal pleura, as well as the diaphragmatic Peritoneum and the peritoneal ligaments of the Liver.

Brachial plexus. The brachial plexus is formed by the anterior rami of the lower four cervical (V — VIII) and, in part, the first thoracic spinal nerves. The plexus is located between the anterior and middle scalene muscles, from where it descends behind the clavicle into the axilla as three cords surrounding the axillary artery. The plexus is divided into supraclavicular and infraclavicular parts. Short Branches of the brachial plexus arise from the supraclavicular part, innervating some of the Neck Muscles, the muscles and skin of the shoulder girdle, and the shoulder joint.

The supraclavicular branches of the plexus include the dorsal scapular nerve, which runs to the muscles of the back; the suprascapular nerve, which goes to the supraspinatus and infraspinatus muscles; the subscapular nerve, branching within the subscapularis muscle; the pectoral nerves, innervating the pectoralis Major and minor muscles; the long thoracic nerve, descending to the serratus anterior muscle; the thoracodorsal nerve, running to the latissimus dorsi muscle; and the axillary nerve, which branches in the deltoid muscle, the shoulder Joint Capsule, and the skin of the shoulder.

From the infraclavicular part of the brachial plexus, represented by three thick cords, arise the long branches (nerves) that supply the skin, muscles, and JOINTS OF THE upper limb.

The Long branches of the brachial plexus include the medial cutaneous nerve of the arm, the medial cutaneous nerve of the forearm, and other large nerves.

The musculocutaneous nerve supplies the anterior muscles of the arm (biceps brachii, coracobrachialis, and brachialis), as well as the skin on the lateral side of the forearm.

The median nerve, which runs along the arm alongside the brachial artery and Veins, continues to the forearm and hand. In the forearm, this nerve gives off branches to the anterior muscles of the forearm (except for the flexor carpi ulnaris and part of the flexor digitorum profundus), and then passes through the carpal tunnel to the hand. In the hand, the median nerve innervates the muscles of the thenar eminence (except for the adductor pollicis and part of the flexor pollicis brevis), the two lateral lumbrical muscles, and the skin of the thumb, index, middle, and half of the ring finger.

The ulnar nerve runs along the medial side of the arm, where, like the median nerve, it gives off no branches. In the forearm, it runs alongside the ulnar artery, innervates the flexor carpi ulnaris and part of the flexor digitorum profundus, and then enters the hand. In the hand, the ulnar nerve supplies branches to the muscles of the hypothenar eminence, all interossei, the two medial lumbricals, the adductor pollicis, and the deep HEAD of the flexor pollicis brevis. The ulnar nerve also innervates the skin on the palmar side of the little finger and the medial half of the ring finger; on the dorsum of the hand, it supplies the skin of two and a half fingers, including the little finger.

In the arm, the radial nerve runs with the deep artery of the arm (profunda brachii) in the radial groove on the posterior surface of the bone, where it gives off branches to the triceps brachii muscle and the skin of the posterior arm. Upon reaching the forearm, the radial nerve innervates all the muscles and skin of the posterior forearm (extensors), as well as the skin on the dorsum of two and a half fingers, starting from the thumb.

The anterior rami of the thoracic spinal nerves do not form plexuses; instead, they run into the intercostal spaces as intercostal nerves. The intercostal nerves innervate the skin and muscles of the chest (intercostal and others). The lower six intercostal nerves innervate the skin and muscles of the anterior abdominal wall.

Lumbar plexus. The lumbar plexus is situated within the psoas major muscle and is formed by the anterior rami of the first three lumbar nerves and PARTS OF THE twelfth thoracic and fourth lumbar nerves. Muscular branches arise from the lumbar plexus to supply the psoas major, psoas minor, and quadratus lumborum muscles. The long branches of the lumbar plexus include the following nerves. The iliohypogastric and ilioinguinal nerves innervate the lower parts of the anterior abdominal wall, supplying both its skin and muscles. The ilioinguinal nerve also gives off branches to the skin of the external genitalia. The genitofemoral nerve descends along the anterior surface of the psoas major muscle. One of its branches supplies the skin of the thigh below the inguinal fold, while the other innervates the Spermatic Cord and Tunics of the Testes in men, and the round ligament of the Uterus and skin of the external genitalia in women.

The lateral femoral cutaneous nerve runs downward across the anterior surface of the iliacus muscle. It then enters the thigh medial to the anterior superior iliac spine, supplying the skin on its lateral aspect.

The obturator nerve runs along the wall of the lesser pelvis to the obturator canal, passing through it to supply the adductor muscles of the thigh and the overlying skin.

The femoral nerve, the largest branch of the lumbar plexus, descends and enters the thigh through the muscular lacuna beneath the inguinal ligament. This nerve innervates the muscles of the anterior compartment of the thigh (quadriceps femoris and sartorius) and the overlying skin. The long branch of the femoral nerve, the saphenous nerve, runs downward alongside the great saphenous vein, giving off branches to the skin of the anteromedial aspect of the knee joint, leg, and FOOT.

Sacral plexus. The sacral plexus is formed by the anterior rami of the fifth lumbar, all sacral, and coccygeal nerves, as well as (partially) fibers of the fourth lumbar spinal nerve. The sacral plexus is located on the anterior surface of the sacrum. Branches arising from the sacral plexus supply the pelvic muscles (piriformis, gemelli, obturator internus), the quadratus femoris muscle, the gluteal nerves (superior and inferior) to the gluteal muscles, the pudendal nerve running to the skin and muscles of the Perineum and external genitalia, as well as long branches: the posterior cutaneous nerve of the thigh and the sciatic nerve.

The posterior cutaneous nerve of the thigh exits the pelvic cavity along with the sciatic, inferior gluteal, and pudendal nerves through the infrapiriform foramen, innervating the skin of the posterior thigh and gluteal region.

The sciatic nerve, the largest nerve in The Human Body, emerges onto the posterior aspect of the thigh, where it gives off branches to the muscles (biceps femoris, semitendinosus, and semimembranosus). In the popliteal fossa, the sciatic nerve divides into its large terminal branches: the tibial nerve and the common fibular (peroneal) nerve.

The tibial nerve, after giving off the medial sural cutaneous nerve (to the skin of the medial aspect of the leg), descends between the superficial and deep muscles (within the cruropopliteal canal) and innervates all the posterior Muscles of the Leg. The tibial nerve then curves behind the medial malleolus and enters the SOLE OF THE foot, where it divides into the medial and lateral plantar nerves, which innervate the muscles and skin of this region.

The common fibular nerve runs laterally from the popliteal fossa, giving off the lateral sural cutaneous nerve (to the skin of the posterolateral calf), and divides into the superficial and deep fibular nerves at the level of the head of the Fibula. The superficial fibular nerve innervates the fibularis longus and brevis muscles and the skin of the dorsum of the foot (except for the skin of the first interdigital space). The deep fibular nerve travels with the anterior tibial artery between the muscles of the anterior compartment of the leg, innervating these muscles. In the foot, the deep fibular nerve innervates the extensor digitorum brevis and the skin of the first interdigital space.

The medial and lateral sural cutaneous nerves (branches of the tibial and common fibular nerves) join on the posterior surface of the calf to form the sural nerve, which innervates the skin of the lateral border of the foot.