Spinal Nerves and Plexuses

INTRODUCTION

FIGURE C–1

Motor and sensory levels of the spinal cord.

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FIGURE C–2

The cervical plexus.

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FIGURE C–3

The brachial plexus.

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FIGURE C–4

Segmental innervation of the right upper extremity, anterior view.

Two illustrations depict an anterior view of segmental innervation of the right upper extremity. The dermatomes of the upper extremity are- C4 – Over the acromioclavicular joint. C5 – On the lateral (radial) side of the antecubital fossa, just proximally to the elbow. C6 – On the dorsal surface of the proximal phalanx of the thumb. C7 – On the dorsal surface of the proximal phalanx of the middle finger. C8 – On the dorsal surface of the proximal phalanx of the little finger. T1 – On the medial (ulnar) side of the antecubital fossa, just proximally to the medial epicondyle of the humerus. The myotoms of the arm include- T2 – At the apex of the axilla. C3: neck lateral flexion C4: shoulder elevation C5: shoulder abduction C6: elbow flexion C7: elbow extension/wrist flexion/finger extension C8: finger flexion T1: finger abduction

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FIGURE C–5

Segmental innervation of the right upper extremity, posterior view.

Two illustrations depict an posterior view of segmental innervation of the right upper extremity. The dermatomes of the upper extremity are- C4 – Over the acromioclavicular joint. C5 – On the lateral (radial) side of the antecubital fossa, just proximally to the elbow. C6 – On the dorsal surface of the proximal phalanx of the thumb. C7 – On the dorsal surface of the proximal phalanx of the middle finger. C8 – On the dorsal surface of the proximal phalanx of the little finger. T1 – On the medial (ulnar) side of the antecubital fossa, just proximally to the medial epicondyle of the humerus. The myotomes of the arm include- T2 – At the apex of the axilla. C3: neck lateral flexion C4: shoulder elevation C5: shoulder abduction C6: elbow flexion C7: elbow extension/wrist flexion/finger extension C8: finger flexion T1: finger abduction

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FIGURE C–6

Musculocutaneous (C5, 6) and axillary (C5, 6) nerves.

Two illustrations depict the course and sensory distribution of musculocutaneous and axillary nerves. The first illustration shows the anatomical course of musculocutaneus and axillary nerves. The musculocutaneous nerve arises from the lateral cord of the brachial plexus, opposite the lower border of the pectoralis major, its fibers being derived from C5, C6. It follows the course of the third part of the axillary artery, that is distal to the pectoralis minor, laterally and enters the frontal aspect of the arm where it penetrates the coracobrachialis muscle. It then passes downwards and laterally between the biceps brachii above and the brachialis muscles below, to the lateral side of the arm; at 2 cm above the elbow it pierces the deep fascia lateral to the tendon of the biceps brachii and is continues into the forearm as the lateral cutaneous nerve of the forearm. The axillary nerve originates from the brachial plexus, upper trunk, posterior division, posterior cord, at the level of the axilla and carries nerve fibers from C5 and C6. It winds from anterior to posterior side around the neck of the humerus, along with the posterior humeral circumflex artery, through the quadrangular space bounded above by the teres minor, below by the teres major, medially by the long head of the triceps brachii, and laterally by the surgical neck of the humerus, and divides into an anterior, a posterior, and a collateral branch to the long head of the triceps brachii branch. The second illustration shows sensory distribution of musculocutaneous and axillary nerves. The musulocutaneous nerve gives rise to the lateral cutaneous nerve of forearm, which innervates the lateral aspect of the forearm. The aillar nerve gives rise to upper lateral cutaneous nerve of arm, which innervates the skin over the lower deltoid.

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FIGURE C–7

The radial nerve (C6–8; T1).

Two illustrations depict the course and sensory distribution of the radial nerve. The radial nerve originates from the brachial plexus, carrying fibers from ventral roots of spinal nerves C5, C6, C7, C8 & T1. The radial nerve and its branches provide motor innervation to the dorsal arm muscles including the triceps brachii and the anconeus and the extrinsic extensors of the wrists and hands. From the brachial plexus, it travels behind the third part of the axillary artery. In the arm, it runs behind the brachial artery and then enters the lower triangular space to reach the radial sulcus of back of humerus. It travel downwards together with profunda brachii artery, between the lateral and medial heads of triceps brachii until it reaches the lateral side the arm at 5 cm below the deltoid tuberosity where it pierces the lateral intermuscular septum to reach the anterior compartment of the arm. It, then, descends down to cross the lateral epicondyle of the humerus where the nerve terminates by branching itself into superficial and deep branch which continues into cubital fossa and then into the forearm. Radial nerve gives out muscular branches to supply the long head, medial head, and lateral head of triceps brachii muscles before and during its course in the radial sulcus. After it emerges out from the radial sulcus, it supplies the brachialis, brachioradialis and extensor carpi radialis longus. Above the radial sulcus, the radial nerve gives off posterior cutaneous nerve of the arm which supplies the skin at the back of the arm. In the radial sulcus, it gives off lower lateral cutaneous nerve of the arm and posterior cutaneous nerve of the forearm. The radial nerve also gives articular branches to supply the elbow joint. In the forearm, it is divides into a superficial branch, primarily sensory, and a deep branch, primarily motor. The deep branch becomes the posterior interosseous nerve, and a superficial branch, goes on to innervate the dorsum of the hand. It also provides cutaneous sensory innervation to most of the back of the hand, except for the back of the little finger and adjacent half of the ring finger, which are innervated by the ulnar nerve, as shown in the second illustration.

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FIGURE C–8

The median nerve (C6–8; T1).

Two illustrations depict the course and sensory distribution of the median nerve. The median nerve originates from the lateral and medial cords of the brachial plexus, and has contributions from ventral roots of C5-C7, lateral cord and C8 and T1, medial cord. It enters the arm from the axilla at the inferior margin of the teres major muscle, and passes vertically down and courses lateral to the brachial artery between biceps brachii above and brachialis below. At first, it is lateral to the artery and lies anterior to the elbow joint; it then crosses anteriorly to run medial to the artery in the distal arm and into the cubital fossa. Inside the cubital fossa, it passes medial to the brachial artery. The median nerve gives off an articular branch to the elbow joint. A branch to pronator teres muscle arise from the median nerve immediately above the elbow joint. In the forearm, the median nerve continues in the cubital fossa medial to the brachial artery and passes between the two heads of the pronator teres, deep to the bicipital aponeurosis and superficial the brachialis muscle. It crosses the ulnar artery while being separated by the deep head of the pronator teres. It then travels between the flexor digitorum superficialis (above) and flexor digitorum profundus (below). The median nerve is accompanied by the median artery, a branch of anterior interosseous artery, during this course. About 5 cm above the flexor retinaculum, it emerges between the flexor digitorum superficialis medially and the flexor carpi radialis laterally into the hand. The main trunk of the median nerve innervates the superficial and deep groups of the muscles in the anterior compartment of the forearm with the exception of flexor carpi ulnaris. The median nerve does this by giving off two branches as it courses through the forearm: Muscular branches are given off in the cubital fossa to supply flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. The anterior interosseous branch is given off in the upper part of the forearm, courses with the anterior interosseous artery and innervates flexor pollicis longus and the lateral half of flexor digitorum profundus (the ulnar half is supplied by ulnar nerve, as is the flexor carpi ulnaris muscle). It ends with its innervation of pronator quadratus. In addition to its supply to muscles, this nerve also supplies the distal radioulnar joint and wrist joint. The median nerve also gives off sensory branches in the forearm, as shown in the second illustration. The palmar cutaneous branch of the median nerve arises at the distal part of the forearm. It supplies sensory innervation to the thenar eminence of the palm and the central palm. The median nerve enters the hand through the carpal tunnel, deep to the flexor retinaculum along with the tendons of flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus. From there, it is divided into recurrent muscular branch and digital cutaneous branch. The muscular branch supplies the thenar muscles. The proper palmar digital branch gives out three digital branches to the lateral one and a half digits, two digital branches to the thumb, one digital branch to the lateral side of the index finger. The digital branch to the index finger also supplies the first lumbrical. The common palmar digital branch divides further into two branches. Both the medial and lateral branches supply the second and third interdigital clefts with adjoining index, middle, and lateral half of ring finger. The lateral branch also supplies the second lumbrical.

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FIGURE C–9

The ulnar nerve (C8, T1).

Two illustrations depict the course and sensory distribution of the ulnar nerve, and another diagram shows the claw-hand deformity. The first illustration depicts the course of ulnar nerve. It originates from the C8-T1 nerve roots, occasionally carries C7 fibres which arise from the lateral cord, which then form part of the medial cord of the brachial plexus, and descends medial to the brachial artery, up until the insertion point of coracobrachialis muscle. Then, it pierces the medial intermuscular septum and enters the posterior compartment of the arm, accompanied by superior ulnar collateral vessels. It runs at the posteromedial aspects of the humerus, passing behind the medial epicondyle, in the cubital tunnel at the elbow. In the forearm, the ulnar nerve enters the anterior flexor compartment of the forearm between the two heads of flexor carpi ulnaris, lie along the lateral border of the flexor carpi ulnaris. The ulnar nerve runs between the flexor digitorum superficialis laterally and flexor digitorum profundus medially. Near the wrist, it courses superficial to the flexor retinaculum of hand, but is covered by volar carpal ligament to enter the hand. In the forearm it gives the following branches- Muscular branches of ulnar nerve - supplies one and a half muscles that is flexor carpi ulnaris and the medial half of flexor digitorum profundus, Palmar branch of ulnar nerve - arises from the middle part of the forearm and supplies the skin over the hypothenar eminence; Dorsal branch of ulnar nerve - arises from 7.5 cm above the wrist, winds backwards to supplies the skin of the proximal part of the ulnar one and half fingers and the adjoining area between the fingers; articular branches are given off to the elbow joint. In h hand, the ulnar nerve enters the palm of the hand via the Guyon’s canal, superficial to the flexor retinaculum and lateral to the pisiform bone, and gives the following branches- Superficial branch of ulnar nerve - supplies the palmaris brevis and gives digital branches to the medial one and a half fingers Deep branch of ulnar nerve - It accompanies the deep branch of the ulnar artery, passes backwards between the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi, supplying all the three muscles, and lying on the hook of hamate bone. It then turns laterally, supplying the 3rd and 4th lumbricals and all the palmar interossei muscles and dorsal interossei of the hand, and terminates by supplying the adductor pollicis; and Articular branches to the wrist.

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FIGURE C–10

The lumbar plexus.

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FIGURE C–11

The femoral (L2–4) and obturator (L2–4) nerves.

Two illustrations depict the course and sensory distribution of femoral and obturator nerves. The first illustrations depicts course of femoral and obturator nerves. Femoral nerve is the major nerve supplying the anterior compartment of the thigh. It is the largest branch of the lumbar plexus, and arises from the dorsal divisions of the ventral rami of L2, L3, and L4. The nerve enters Scarpa’s triangle by passing beneath the inguinal ligament, just lateral to the femoral artery. In the thigh, it lies in a groove between iliacus muscle and psoas major muscles, outside the femoral sheath, and lateral to the femoral artery. After a short course of about 4 cm in the thigh, the nerve is divided into anterior and posterior divisions, separated by lateral femoral circumflex artery. The branches are- Muscular branches- The nerve to the Pectineus arises immediately above the inguinal ligament from the medial side of the femoral nerve, and passes behind the femoral sheath to enter the anterior surface of the muscle; Anterior division supplies the Sartorius muscle- posterior division supplies the rectus femoris, the three vasti, and articularis genus. Cutaneous branches- The anterior division gives anterior cutaneous branches; these are the intermediate femoral cutaneous nerve and the medial femoral cutaneous nerve. The posterior division gives off only one branch, the saphenous nerve. Articular branches- hip joint is supplied by nerve to the rectus femoris. Knee joint is supplied by the nerves to the three vasti. Vascular branches include branches to the femoral artery and its branches. The obturator nerve arises from the ventral divisions of the second, third, and fourth lumbar nerves in the lumbar plexus. It descends through the fibers of psoas major, and emerges from its medial border near the brim of the pelvis; then passes behind the common iliac arteries, and on the lateral side of the internal iliac artery and vein, and runs along the lateral wall of the lesser pelvis, above and in front of the obturator vessels, to the upper part of the obturator foramen. Here it enters the thigh, through the obturator canal, and divides into an anterior and a posterior branch, which are separated at first by some of the fibers of the obturator externus, and lower down by the adductor brevis. The obturator nerve provides sensory innervation of the skin of the medial aspect of the thigh.

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FIGURE C–12

The sacral plexus.

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FIGURE C–13

Segmental innervation of the right lower extremity, anterior view. Note the similarity between dermatomes (on left) and myotomes (on right).

Two illustrations depict an anterior view of segmental innervation of the right lower extremity. The two illustrations depict dermatomes and myotomes on the anterior aspect of right lower limb. The dermatomes, along the preaxial border from above downwards, are T12, L1, 2,3 and 4. The middle three toes, the adjoining area of the dorsum of the foot and the lateral side of the leg are supplied by segment L5. Along the postaxial border from below upwards, there are dermatomes S1, 2,3. Myotomes include- L1 Psoas major, L2- Psoas major, iliacus, sartorius, gracilis, pectineus, adductor longus, adductor brevis; L3- Quadriceps, adductor, longus, brevis, magnus; L4- quadriceps, tensor fasciae latae, adductor mangus, obturator exturnus, tibialis anterior, tibialis anterior, tibialis posterior. L5- gluteus medius, gluteus minimus, obturator internus, semimembranosus, semitendinosus, extensor hallucis longus, extensor digiotorum longus, peroneus tertius, popliteus, S1- gluteus maximus, obturator internus, piriformis, biceps femoris, semitendinosus, gastrocnemius, soleus, peronei, extensor digitorum brevis.

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FIGURE C–14

Segmental innervation of the right lower extremity, posterior view.

Two illustrations depict an posterior view of segmental innervation of the right lower extremity. The two illustrations depict dermatomes and myotomes on the posterior aspect of right lower limb. The dermatomes, along the preaxial border from above downwards, are T12, L1, 2,3 and 4. The middle three toes, the adjoining area of the dorsum of the foot and the lateral side of the leg are supplied by segment L5. Along the postaxial border from below upwards, there are dermatomes S1, 2,3. Myotomes include- L1 Psoas major, L2- Psoas major, iliacus, sartorius, gracilis, pectineus, adductor longus, adductor brevis; L3- Quadriceps, adductor, longus, brevis, magnus; L4- quadriceps, tensor fasciae latae, adductor mangus, obturator exturnus, tibialis anterior, tibialis anterior, tibialis posterior. L5- gluteus medius, gluteus minimus, obturator internus, semimembranosus, semitendinosus, extensor hallucis longus, extensor digiotorum longus, peroneus tertius, popliteus, S1- gluteus maximus, obturator internus, piriformis, biceps femoris, semitendinosus, gastrocnemius, soleus, peronei, extensor digitorum brevis.

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FIGURE C–15

The sciatic nerve (L4, 5; S1–3).

Two illustrations depict the course of the sciatic nerve. The sciatic nerve is formed from L4 to S3 segments of the sacral plexus, a collection of nerve fibres that emerge from the sacral part of the spinal cord. The fibres unite to form a single nerve in front of the piriformis muscle. The nerve passes beneath piriformis and through the greater sciatic foramen, quitting the pelvis. From here, it travels down the posterior thigh to the popliteal fossa. The nerve travels in the posterior compartment of the thigh behind or superficial to the adductor magnus muscle, and is itself in front of or deep to the long head of the biceps femoris muscle. At the popliteal fossa, the nerve divides into its two branches- tibial nerve, which travels down the posterior compartment of the leg into the foot, and the common peroneal nerve, which travels down the anterior and lateral compartments of the leg into the foot.

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FIGURE C–16

The common peroneal nerve (L4, 5; S1, 2).

Two illustrations depict the course and sensory distribution of the common peroneal nerve. The common peroneal nerve is the smaller terminal branch of the sciatic nerve, with root values of L4, L5, S1, and S2. It arises from the superior angle of the popliteal fossa and extends to the lateral angle of the popliteal fossa, along the medial border of the biceps femoris. It then winds around the neck of the fibula to pierce the fibularis longus and divides into terminal branches of superficial fibular nerve and deep fibular nerve. Before its division, the common fibular nerve gives off several branches in the popliteal fossa; these are- Cutaneous branches- Lateral sural cutaneous nerve, that supplies the skin of upper two-thirds of the lateral side of leg; Sural communicating nerve, that runs on the posterolateral aspect of the calf and joins the sural nerve Articular branches- Superior lateral genicular nerve, that accompanies artery of the same name and lies above the lateral femoral condyle; Inferior lateral genicular nerve, that accompanies artery of the same name and lies just above the head of the fibula; the recurrent genicular nerve, that arises from the point of division of the common fibular nerve; then ascends anterior to the knee joint together with the anterior recurrent tibial artery to supply the knee joint and the tibialis anterior muscle. Motor branches- only one motor branch that arises directly from common fibular nerve, the nerve to the short head of the biceps femoris muscle.

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FIGURE C–17

The tibial nerve (L4, 5; S1–3).

Three illustrations depict the course and sensory distribution of the tibial nerve. The tibial nerve is a branch of the sciatic nerve, and arises at the apex of the popliteal fossa. It travels through the popliteal fossa, giving off branches to muscles in the superficial posterior compartment of the leg. It also gives rise to branches that contribute towards the sural nerve, which innervates the posterolateral aspect of the leg. The tibial nerve continues its course down the leg, posterior to the tibia. During its descent, it supplies the deep muscles of the posterior leg. At the foot, the nerve passes posteriorly and inferiorly to the medial malleolus, through a structure known as the tarsal tunnel. This tunnel is covered superiorly by the flexor retinaculum. Within this tunnel, branches arise from the tibial nerve to supply cutaneous innervation to the heel. Immediately distal to the tarsal tunnel, the tibial nerve terminates by dividing into sensory branches, which innervate the sole of the foot. The tibial nerve innervates all the muscles in the posterior compartment of the leg. In the popliteal fossa, the tibial nerve gives off cutaneous branches. These combine with branches from the common fibular nerve to form the sural nerve. This sensory nerve innervates the skin of the posterolateral side of the leg and the lateral side of the foot. The tibial nerve also supplies all the sole of the foot via three branches- Medial calcaneal branches: These arise within the tarsal tunnel, and innervate the skin over the heel. Medial plantar nerve: Innervates the plantar surface of the medial three and a half digits, and the associated sole area. Lateral plantar nerve: Innervates the plantar surface of the lateral one and a half digits, and the associated sole area.

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FIGURE C–18

The pudendal and coccygeal plexuses.

Appendix C: Spinal Nerves and Plexuses

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INTRODUCTION

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INTRODUCTION

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