Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Spinal cord
Spinal nerves

General Information. Each body metamere corresponds to a pair of Spinal Nerves. The number of nerves varies in different animals, as the number of segments making up their body is not the same. Each nerve divides into dorsal and ventral branches, containing both motor and sensory fibers.

In fish, the dorsal branch innervates the dorsal musculature, and the ventral branch innervates the ventral musculature. In terrestrial vertebrates, the dorsal branch supplies Muscles derived from the dorsal musculature of fish, while the ventral branch supplies those developed from the ventral musculature, and consequently, the limb muscles. In addition, a communicating branch extends from the ventral branch to the autonomic nerve chain. Sensory nerves arising from both branches innervate the Skin (dermatomes) of the segments. Motor nerves supply the Muscle segments (myotomes).

Paired limbs are innervated by the ventral branches of spinal nerves associated with the metameres from which the limbs developed.

These nerves form plexuses that develop gradually during phylogeny as limb Structure becomes more complex. Fish have a common cervicobrachial plexus; in amphibians, its division begins, eventually leading to the complete Separation of the Cervical plexus from the Brachial Plexus. At the same time, the first three Anterior branches of the cervical plexus separate from it and, moving into the Skull, form the Hypoglossal nerve. Elongation of the neck causes the brachial plexus to shift further caudally. In mammals, it is composed of the anterior Branches of the last four cervical segments and the I thoracic segment.

Fish have a common Lumbosacral plexus; in amphibians, its division begins, and in mammals, it completely differentiates into separate lumbar and sacral plexuses. Reduction, as well as Atrophy of the limbs, leads to the loss of these plexuses.

Humans have 31 pairs of spinal nerves, each arising from the Spinal Cord by two roots: anterior and posterior. In the region of the intervertebral foramen, there is an enlargement on the posterior ROOT—the spinal (sensory) ganglion. Distal to this, both roots unite to form a mixed spinal nerve, consisting of motor efferent and sensory afferent fibers. The nerve may also contain axons of lateral horn Cells belonging to the Autonomic Nervous system.

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Fig. 3.7. Successive stages of embryonic spinal cord development:

A — 5,5 weeks; B — 6,5 weeks; C — 8 weeks; D — newborn; 1 — alar plate; 2 — ependymal layer; 3 — basal plate; 4 — mantle layer; 5 — sulcus limitans; 6 — marginal zone; 7 — spinal ganglion; 8 — posterior horn, 9 — lateral horn, 10 — anterior horn of Gray matter; 11 — anterior funiculus; 12 — posterior funiculus

Spinal nerves are named after the corresponding regions of THE Vertebral Column. In all regions, the nerves are numbered According to the vertebra lying above them. The exception is the cervical region, where the nerve is numbered according to the vertebra lying below it. Thus, the I cervical nerve emerges between the Occipital bone and the atlas, and the VIII cervical nerve between the VII cervical and I thoracic vertebrae. There are 12 pairs of nerves in the thoracic region, with the first emerging between the I and II thoracic vertebrae; 5 in the lumbar region; 5 in the sacral region; and 1 pair of nerves in the coccygeal region.

Upon exiting the intervertebral foramen, the spinal nerve divides into terminal branches (Fig. 3.8 and Atl.). Two of them are long—the posterior and anterior branches, and two are short—the meningeal and communicating branches.

The posterior (dorsal) rami (Atl. Fig. 102) retain their segmental pattern of distribution in all Regions of the body (Atl. Fig. 159). Upon exiting the intervertebral or posterior sacral foramen, each of them divides into medial and lateral branches, which innervate the corresponding segments of musculature of dorsal origin (deep back muscles), as well as the skin of the occiput, back, lower back, and gluteal region.

Fig. 3.8. Spinal nerve:

1 — posterior root; 2 — anterior root; 3 — white and 4 — gray communicating rami; 5 — sympathetic chain; 6 — anterior branch; 7 — muscular branches; 8 — spinal nerve; 9 — spinal cord; 10 — body of thoracic vertebra; 11— muscles; 12 — Sternum; 13 — anterior and 14 — lateral cutaneous branches; 15 — cutaneous and 16 — muscular PARTS OF THE posterior branch (17)

The anterior (ventral) rami differ from the posterior ones in that their segmental distribution in the body is preserved only in the thoracic region, where they are called intercostal nerves. In all other regions of the body, the anterior rami join with each other in loops to form the cervical, brachial, lumbar, and sacral plexuses.

The meningeal branch immediately returns to the vertebral canal and innervates the Meninges of the spinal cord.

The communicating branch usually arises from the anterior branch and runs to the corresponding ganglion of the Sympathetic trunk. The communicating branch contains both efferent fibers from the lateral horn Cells of the spinal cord and afferent fibers from the Internal Organs.

The intercostal nerves (nn. intercostales) are the anterior rami of the thoracic spinal nerves. Consisting of 12 pairs, they run segmentally in the intercostal spaces, below the intercostal Arteries (Fig. 3.9). The lower six nerves, having reached the anterior end of the intercostal spaces, continue into the anterior abdominal wall. Upon reaching the rectus abdominis muscle, the nerves pierce it and emerge under the skin as the anterior cutaneous branch. In addition, all intercostal nerves give off a lateral cutaneous branch.

Fig. 3.9. Intercostal nerves (external intercostal and abdominal oblique muscles have been removed):

1 and 12 — I and VI intercostal nerves; 2 and 3 — anterior cutaneous branches of II and VIII intercostal nerves; 4 — rectus sheath (opened along the course of the nerves); 5 — rectus abdominis muscle; 6 — anterior cutaneous branches of the XII intercostal nerve (10); 7 and 8 — NERVES OF THE Lumbar plexus; 9 — transversus abdominis muscle; II — muscular branches; 13 — lateral cutaneous branch of the IV intercostal nerve

Thus, the intercostal nerves innervate the skin of the chest and abdomen, as well as the following muscles: the external and internal intercostals, transversus thoracis, levatores costarum, posterior serratus, abdominal obliques (external and internal), transversus abdominis, rectus abdominis, and pyramidalis—that is, all muscles of ventral origin located on the trunk.

The cervical plexus (plexus cervicalis) is formed by the anterior rami of the upper four cervical nerves and a branch from the fifth. It lies on the Deep Muscles of the Neck, lateral to the transverse processes of the vertebrae, and is arranged into three loops from which cutaneous and muscular nerves arise (Atl. Fig. 103).

The cutaneous nerves emerge from behind the posterior border of the sternocleidomastoid muscle (Atl. Figs. 104, 105) and course upward, forward, and downward, innervating the skin of the occiput, auricle, neck, and upper chest.

Muscular nerves supply with their branches the Neck Muscles of ventral origin: the deep muscles lying on the spine, the scalene muscles, and the anterior muscles located below the Hyoid bone.

The phrenic nerve (n. phrenicus), arising from the plexus, enters the thoracic cavity, passes anterior to the root of the lung, and reaches the Diaphragm near its central tendon. The Innervation of the diaphragm by the cervical plexus is explained by the embryonic development of this muscle in the neck region.

The brachial plexus (plexus brachialis) is formed by the anterior rami of the lower four cervical nerves and a branch from the first thoracic nerve (I). It descends behind the clavicle into the axillary fossa (Atl. Fig. 103). The plexus is divided into supraclavicular and infraclavicular parts.

The supraclavicular part gives off A number of relatively short nerves that supply the Muscles of the Upper Limb located on the trunk and scapula: the rhomboid, levator scapulae, serratus anterior, pectoralis Major and minor, teres major, latissimus dorsi, subscapularis, supraspinatus, and infraspinatus.

In the axillary fossa, the infraclavicular part forms three large trunks: superior, inferior, and middle. The long nerves of the upper limb arise from them. The superior trunk gives rise to the musculocutaneous nerve and one of the roots of the median nerve. From the inferior trunk, In addition to the root of the median nerve, arise the cutaneous nerves of the arm and forearm, and the ulnar nerve. From the middle trunk arise the radial and axillary nerves.

The musculocutaneous nerve (n. musculocutaneus), usually piercing the coracobrachialis muscle, innervates the flexors located on the arm: the coracobrachialis, brachialis, and biceps brachii muscles (Fig. 3.10, A). Emerging onto the forearm, the nerve supplies the skin of its lateral surface.

The median nerve (n. medianus) begins with two roots: lateral and medial (Fig. 3.10, B). In the arm, the nerve accompanies the brachial artery without giving off branches. In the forearm, it lies between the superficial and deep flexors of the fingers. Through the carpal tunnel, the median nerve emerges onto the palm, where it divides into three branches that innervate the fingers. Thus, the median nerve innervates the pronators, almost all flexors located on the forearm, some Muscles of the hand, as well as the skin of the palm and fingers.

Fig. 3.10. Infraclavicular part of the brachial plexus (diagram):

A — musculocutaneous, B — median, C — ulnar, and D — radial nerves; 1 — clavicle; 2 — scapula; 3 — ulna and 4 — radius; 5 and 6 — axillary artery and vein; 7 — cutaneous branches; 8 — branches to the Anterior Group of arm muscles; 9 — branches to the Superficial layer of the anterior group of forearm muscles and 10 — branches to its deep layer; 11 — branch to the skin and muscles of the hand; 12 — branch to the flexor carpi ulnaris; 13 — branches to the deep and 17 — to the superficial layer of the Posterior group of forearm muscles; 14 — axillary nerve; 15 — branch to the deltoid muscle; 16 — branches to the triceps muscle

The ulnar nerve (n. ulnaris), like the median, gives off no branches in the arm (Fig. 3.10, C). Entering the forearm between the olecranon and the medial epicondyle of the humerus, it innervates the flexor carpi ulnaris. Near the pisiform bone, the nerve divides into terminal branches that innervate almost all the muscles of the hand and its skin (Fig. 3.11).

Fig. 3.11. Nerves of the hand (palmar view):

1 — flexor digitorum superficialis; 2 — median nerve with its muscular branches (4); 3 — transverse ligament (cut); 5 — palmar digital nerves; 6 — communicating branch between the median and ulnar nerves; 7 — superficial and 8 — deep branches of the ulnar nerve (9); 10 — pisiform bone

The radial nerve (n. radialis) is the thickest nerve in the brachial plexus (Fig. 3.10, D). Initially, it lies behind the brachial artery, then enters the triceps brachii muscle, innervating the muscle itself, the skin overlying it, and the skin of the posterior surface of the forearm. Winding spirally around the humerus, the nerve enters the cubital fossa, deep within which it divides into deep and superficial branches. The deep branch supplies all muscles of the posterior group of the forearm. The superficial branch runs along with the radial artery, distributing to the skin of the hand.

The axillary nerve (n. axillaris) innervates the deltoid and teres minor muscles, the shoulder joint, and the skin of the shoulder (Fig. 3.10, D).

The lumbar plexus (plexus lumbalis) is formed by the anterior rami of the upper three lumbar nerves and branches from the IV lumbar and XII Thoracic nerves (Fig. 3.12) and lies within the psoas major muscle.

Short nerves of the plexus innervate the iliopsoas muscle and the quadratus lumborum muscle.

The long nerves (except for the obturator nerve) emerge from under the lateral border of the psoas major muscle. The upper two innervate the lower part of the anterior abdominal wall, as well as the skin of the external genitalia. The remaining long nerves are represented by the lateral femoral cutaneous nerve, the femoral nerve, and the obturator nerve.

The lateral femoral cutaneous nerve (n. cutaneus femoris lateralis), piercing the abdominal wall, emerges onto the thigh at the level of the anterior superior iliac spine. Here, this nerve innervates the skin of the lateral side of the thigh (Fig. 3.13).

Fig. 3.12. Lumbar and sacral plexuses (muscles removed on the left):

1 — XII intercostal nerve; 2, 3 and 5 — nerves of the lumbar plexus to the body walls; 4 — lateral femoral cutaneous nerve; 6 — femoral nerve; 7 — obturator nerve; 8 — sciatic nerve; 9 — sympathetic trunk

The femoral nerve (n. femoralis) is the thickest nerve in the lumbar plexus (Fig. 3.13). Emerging onto the thigh under the inguinal ligament (along with the iliopsoas muscle), it immediately divides into terminal muscular and cutaneous branches. The muscular branches innervate the sartorius muscle and all heads of the quadriceps, while the cutaneous branches supply the skin of the anterior surface of the thigh. The longest cutaneous branch — the saphenous nerve — accompanies the great saphenous vein, innervating the skin of the Medial surface of the leg and FOOT.

The obturator nerve (n. obturatorius) passes into the lesser pelvis, along the wall of which it reaches the obturator foramen and exits through it to the medial side of the thigh (Fig. 3.13, A). It innervates the skin of the medial surface of the thigh, the hip joint, and all adductor muscles.

Fig. 3.13. Long branches of the lumbar and sacral plexuses (diagram):

A — anterior view; B — posterior view; 1 — lateral femoral cutaneous nerve; 2 — ilium; 3 — femoral nerve; 4 — obturator nerve and 5 — its branches to the medial thigh muscles; 6 — branches of the femoral nerve to the quadriceps muscle; 7 — common peroneal nerve; 8 — muscular branches of the deep peroneal nerve; 9 — muscular branches of the superficial peroneal nerve; 10 — branch to the dorsum of the foot; 11 — posterior femoral cutaneous nerve; 12 — sciatic nerve; 13 — piriformis muscle; 14 — branches to the Posterior Thigh Muscles; 15 — branches of the tibial nerve to the posterior leg muscles; 16 — medial malleolus; 17 — branches to the plantar muscles; 18 — medial sural cutaneous nerve; 19 — sural nerve; 20 — saphenous nerve

The Sacral Plexus (plexus sacralis) is formed by the anterior rami of the V lumbar, three sacral nerves, and branches from the IV lumbar nerve.

Within the lesser pelvis, the sacral plexus forms prominent loops On the surface of the piriformis muscle (Fig. 3.12). Short and long nerves arise from the plexus.

The short nerves branch into the hip rotators, the gluteal muscles, the tensor fasciae latae, the skin and muscles of the Perineum, and the skin of the external genitalia.

The long nerves are represented by the sciatic nerve and the posterior femoral cutaneous nerve.

The posterior femoral cutaneous nerve (n. cutaneus femoris posterior), exiting the pelvis through the greater sciatic foramen, supplies the skin of the posterior thigh and the popliteal region with its branches (Fig. 3.13).

The sciatic nerve (n. ischiadicus), formed by all the anterior rami of the sacral plexus, is the largest nerve in The Human Body (Fig. 3.13). It exits the pelvis along with the posterior femoral cutaneous nerve and gives off branches mainly to the posterior thigh muscles. Before reaching the popliteal fossa, the sciatic nerve divides into the tibial (thick) and common peroneal nerves.

In the leg, the tibial nerve (n. tibialis) passes between the superficial and deep layers of the flexor muscles, innervating them and the knee joint; here, it gives off the long medial sural cutaneous nerve (Fig. 3.13). The latter, joining with the lateral sural cutaneous nerve (a branch of the common peroneal nerve), innervates the skin of the posterior leg (forming the sural nerve). Passing behind the medial malleolus, the tibial nerve enters the SOLE OF THE foot, where it innervates the ankle joint, all the muscles of the foot, and the skin of the sole and toes.

The common peroneal nerve (n. peroneus communis) winds around the lateral side of the popliteal fossa and, at the level of the fibular HEAD, divides into the superficial and deep peroneal nerves (Fig. 3.13).

The superficial peroneal nerve innervates the peroneal muscles, as well as the skin of the foot and toes.

The deep peroneal nerve supplies the anterior compartment of the leg muscles.

Review Questions

1. WHAT IS A spinal nerve, and what branches does it form?

2. Which regions of the body are innervated by the nerves arising from the cervical, brachial, lumbar, and sacral plexuses?