• Eldan Kapur, MD
• Chi Wong, MS
• Admir Hadzic, MD

        INTRODUCTION

It is often said that the practice of regional anesthesia is the practice of applied anatomy. Indeed, the practice of regional anesthesia is inconceivable without a sound knowledge of the basic anatomic facts that pertain to the individual anesthesia techniques. However, just as surgeons rely on surgical anatomy or pathologists rely on pathologic anatomy, the anatomic Information necessary for the practice of regional anesthesia must be specific to this application. In the past, many new nerve block techniques and “me-too” approaches were devised by academicians merely relying on idealized anatomic diagrams and schematics, rather then on functional anatomy. Ultimately, many of these techniques have only introduced unnecessary confusion in the field and been of negligible relevance to clinical practice. Indeed, once the anatomic layers and tissues sheets are dissected, the fully exposed nerve structures are almost irrelevant to the practice of regional anesthesia. This is because accurate placement of the needle and the spread of the local anesthetic after an injection depends on the interplay between neurologic structures and the neighboring tissues where local anesthetic pools and ac- cumulates, rather than on the mere anatomic organization of the nerves and plexuses. However, much research by regional anesthesiologists has been done in the past 10-15 years on this subject, and many myths of the past have been dispelled. The reader should note that specific anatomic discussions pertaining to individual regional anesthesia techniques are detailed in their respective chapters. The purpose of this chapter is to provide a generalized and rather concise overview of anatomy relevant to the practice of regional anesthesia. The reader is referred to Figure 3-1 for an easier orientation of the body planes discussed throughout the book.

        ANATOMY OF PERIPHERAL NERVES

All peripheral nerves are similar in structure. The neuron is the basic functional neuronal unit responsible for the conduction of nerve impulses. Neurons are the longest cells in the body, many reaching a meter in length. Most neurons are incapable of dividing under normal circumstances and have a very limited ability to repair themselves after injury. A typical neuron consists of a cell body (soma) that contains a large nucleus. The cell body is attached to several branching processes, called dendrites, and a single axon. Dendrites receive incoming messages; axons conduct outgoing messages. Axons vary in length, and there is one only per neuron. In peripheral nerves, axons are very long and slender. They are also called nerve fibers. The peripheral nerve (PN) is composed of three parts: (1) somatosensory or afferent neurons, (2) motor or efferent neurons, and (3) autonomic neurons.

        Individual nerve fibers bind together, somewhat like individual wires in an electric cable (Figure 3-2). In a peripheral nerve, individual axons are enveloped in a loose connective tissue, the endoneurium. The endoneurium is a delicate layer of connective tissue around each nerve that is embedded within the perineurium. Small groups of axons are closely associated within a bundle called a nerve fascicle, which imparts mechanical strength to the peripheral nerve. In surgical procedures, the perineurium holds sutures without tearing. In addition to its mechanical strength, the perineurium functions as a diffusion barrier to the fascicle, isolating the endoneural space around the axon from the surrounding tissue.¹ This barrier helps to preserve the ionic milieu of the axon and functions as a blood-nerve barrier. Several fascicles together form fascicular bundles within an extensive multilaminated perineurium. The perineurium surrounds each fasciculus and splits with it at each branching point. The fascicular bundles in turn, collectively form the peripheral nerve that is embedded in loose connective tissue called the epineurium. The epineurium surrounds the entire nerve and holds it loosely to the connective tissue through which it runs. This layer also sends septa into the nerve that divide the nerve fibers into bundles (fasciculi or funiculi) of varying sizes.

Figure 3-2. Organizaron of the peripheral nerve.

        Of note, the fascicular bundles are not continuous throughout the peripheral nerve. They divide and anastomose with one another as frequently as every few millimeters.¹ However, the axons within a small set of adjacent bundles redistribute themselves so that the axons remain in approximately the same quadrant of the nerve for several centimeters. This arrangement is a practical concern to the surgeons trying to repair a severed nerve. If the cut is clean, it may be possible to suture individual fascicular bundles together. In such a scenario, the probability is good that the distal segment of nerves synapsing with the muscles will be sutured to the central stump of motor axons; the same is true for sensory axons. In such cases, good functional recovery is possible. If a short segment of the nerve is missing, however, the fascicles in the various quadrants of the stump may no longer correspond with one another, good axial alignment may not be possible, and functional recovery is greatly compromised or improbable.¹ This arrangement of the peripheral nerve helps explain why intraneural injections result in disastrous consequences as opposed to clean needle nerve cuts which tend to heal much more readily and regularly.

        The connective tissue of a nerve is tough, compared with the nerve fibers themselves. The connective tissue of a nerve permits a certain amount of stretch without damage to the nerve fibers. The nerve fibers are somewhat “wavy,” and when they are stretched, the connective tissue around them is also stretched—giving it some protection. This feature, per- haps, plays a “safety” role in nerve blockade by allowing the nerves to be “pushed” rather than pierced by the advancing needle during nerve localization. For this reason, it is prudent to avoid stretching the nerves and nerve plexuses during nerve blockade (eg, in axillary brachial plexus blocks and some approaches to the sciatic block).

        Nerves receive blood from the adjacent blood vessels running along their course. These feeding branches to larger nerves are macroscopic in size and irregularly arranged, forming anastomoses to become longitudinally running vessel(s) that supply the nerve and give off subsidiary branches. Although the connective tissue sheath enveloping nerves serves to protect the nerves from stretching, it is also believed that neuronal injury after nerve blockade may be due, at least partly, to the pressure or stretch within connective sheaths that do not stretch well and the consequent interference with the vascular supply to the nerve.

Figure 3-3. Anatomy of the spinal nerve.

Communication Between the Central & Peripheral Nervous Systems

The functional boundary between the central (CNS) and the peripheral nervous system (PNS) lies at the junction where oligodenodrocytes meet Schwann cells along the axons that form the cranial and spinal nerve. The PNS, as opposed to the CNS, is not surrounded by bone and is therefore accessible for neural blockade, but it is also quite susceptible to physical injury. The CNS communicates with the body through spinal nerves. Spinal nerves have both sensory and motor components (Figure 3-3). The sensory fibers arise from neurons in the dorsal root ganglia. Fibers enter the dorsolateral aspect of the spinal cord to form the dorsal root. The motor fibers arise from neurons in the ventral horn of the spinal cord. The fibers pass through the ventrolateral aspect of the spinal cord and form the ventral root. The dorsal and ventral roots converge in the intervertebral foramen to form a spinal nerve. After passing through the intervertebral foramen, the spinal nerve divides into dorsal and ventral rami. The dorsal ramus innervates muscle, bones, joints, and the skin of the back. The ventral ramus innervates muscle, bones, joints, and the skin of the anterior neck, thorax, abdomen, pelvis, and the extremities.

Spinal Nerves

There are 31 pairs of spinal nerves. The spinal nerves are enumerated by region: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Spinal nerves pass through the vertebral column at the intervertebral foramina. The first cervical nerve (Cl) passes superior to the Cl vertebra (atlas). The second cervical nerve (C2) passes between the Cl (atlas) and C2 (axis) vertebrae. This pattern continues down the cervical spine. A shift in pattern occurs at the C8 nerve because there is no C8 vertebra. The C8 nerve passes between the C7 and T1 vertebrae. The T1 nerve passes between the Ti and T2 vertebrae. This pattern continúes down the through the remainder of the spine. The vertebral arch of the fifth sacral and first coccygeal vertebrae is rudimentary. Because of this, the vertebral canal opens inferiorly at the sacral hiatus. The fifth sacral and first coccygeal nerves pass through the sacral hiatus. Because the inferior end of the spinal cord (conus medullaris) in adults is located at the L1 to L2 vertebral level, roots of spinal nerves must descend through the vertebral canal before exiting the vertebral column through the appropriate intervertebral foramen. Collectively, these roots are called the cauda equina (Figure 3-4).

        Outside the vertebral column, ventral rami from different spinal levels coalesce to form intricate networks called plexuses. From the plexuses, nerves extend into the neck, the arms, and the legs.^1,2

        DERMATOMES, MYOTOMES, & OSTEOTOMES

Dermatomal, myotomal, and osteotomal innervation are often emphasized in regional anesthesiology texts as important for the application of nerve blocks. However, it is more practical to think in terms of which block techniques provide adequate analgesia and anesthesia for specific surgical procedures, rather than trying to match nerves and spinal segments to the dermatomal, myotomal, and osteotomal territory.

Figure 3-4. Cauda equina.

Nevertheless, their description is of didactic importance in regional anesthesia and is briefly presented here.

        A dermatome is an area of the skin supplied by the dorsal (sensory) root of the spinal nerve (Figures 3-5 and 3-6). In the head and trunk, each segment is horizontally disposed, except C1, which does not have a sensory component. The dermatomes of the limbs from the fifth cervical to the first thoracic nerve and from the third lumbar to the second sacral vertebrae extend as a series of bands from the midline of the trunk posteriorly into’the limbs. It should be noted that considerable overlapping occurs between adjacent dermatomes, that is, each segmental nerve overlaps the territories of its neighbors.³

        A myotome is the segmental innervation of skeletal muscle by the ventral (motor) root(s) of the spinal nerve(s). Major myotomes, their function, and corresponding spinal levels are represented in Figure 3-7. The innervation of the bones (osteotome) often does not follow the same segmental pattern as the innervation of the muscles and other soft tissues (Figure 3-8).

        ANATOMY OF PLEXUSES & PERIPHERAL NERVES

Cervical Plexus

The cervical plexus innervates muscles, joints, and skin in the anterior neck (Table 3-1). It is formed by the ventral rami of Cl through C4 (Figures 3-9 and 3-10). The rami form a loop called the ansa cervicalis that sends branches to the infrahyoid muscles. In addition, the rami form nerves that pass directly to several structures in the neck and thorax, including the scalene muscles, diaphragm, clavicular joints, and skin covering the anterior neck.

Ansa Cervicalis

The ventral ramus of Cl attaches to the ventral rami of C2 to C3. The attachment forms a loop called the ansa cervicalis, which sends branches to the infrahyoid muscles. The infrahyoid muscles consist of the omohyoid, sternohyoid, and sternothyroid muscles. They attach to the anterior surface of the hyoid bone or to the thyroid cartilage. Contraction of these muscles moves the hyoid bone or thyroid cartilage downward, effectively opening the laryngeal aditus. This promotes inspi- ration. The Cl component also sends fibers to the thyrohyoid and geniohyoid muscles. Contraction of these muscles moves the anterior hyoid bone superiorly, closing the laryngeal aditus. Closure of the laryngeal aditus is necessary for swallowing to occur safely. This is one of the reasons why high levels of spinal anesthesia result in airway compromise and the risk of aspiration.

Nerves to Scalene Muscles

The ventral rami of C2 to C4 send branches directly to the scalene muscles, which attach between the cervical spine and ribs. When the cervical spine is stabilized, contraction elevates the ribs. This promotes inspiration. Interscalene block may result in block of the scalene muscles in addition to the phrenic block. This is typically asymptomatic in healthy patients but may result in acute respiratory insufficiency in patients with borderline pulmonary function or in those with an exacerbation of asthma or chronic obstructive bronchitis. It is recommended that more distal approaches to a brachial plexus block and smaller injection volumes be used to limit the cephalad extension of the block, as well as shorter acting local anesthetics to avoid prolonged blockade in case of respiratory insufficiency.

Figure 3-5. Dermatomes, anterior.

Figure 3-6. Dermatomes, posterior.

Phrenic Nerve

The phrenic nerve is formed by junction of fibers from C3 to C5, and it innervates the diaphragm. The phrenic nerve descends through the neck on the anterior surface of the anterior scalene muscle, passing through the superior thoracic aperture and descending on the walls of the mediastinum to the diaphragm. In addition to muscular fibers, the phrenic nerve transmits sensory fibers to the superior and inferior surfaces of the diaphragm. All approaches to the block of the brachial plexus above the clavicle result in phrenic blockade (Figure 3-11).

Figure 3-7. Functional innervation ofthe muscles (myotomes): A: Medial and lateral rotation of shoulder and hip. Abduction and adduction of shoulder and hip. B: Flexion and extension of elbow and wrist. C: Pronation and supination of forearm. D: Flexion and extension ofshoulder, hip, and knee. Dorsiflexion and plantar flexion of ankle, lateral views.

Cutaneous Nerves of the Anterior Neck

Cutaneous sensory nerves arise from the cervical plexus, pass around the posterior margin of sternocleidomastoid, and termínate in the scalp and anterior neck. The minor occipital nerve passes to the posterior auricular region of the scalp (Figure 3-12). The major auricular nerve passes to the auricle of the ear and to the region of the face anterior to the tragus. The transverse cervical nerve supplies the anterior neck. A series of supraclavicular nerves innervate the region covering the clavicle. Furthermore, the supraclavicular nerves provide articular branches to the sternoclavicular and acromioclavicular joints.⁴

Brachial Plexus

The brachial plexus innervates muscles, joints, and the skin of the upper extremity (Table 3-2). It is formed by ventral rami of C5 to T1 (Figure 3-13). In the posterior cervical triangle between the anterior and middle scalene muscles, the ventral rami join to form trunks. C5 and C6 join to form the superior trunk. C7 forms the middle trunk. C8 and TI join to form the inferior trunk. All trunks branch into anterior and posterior divisions. All the posterior divisions join to form the posterior cord. The anterior divisions of the superior and middle trunks join to form the lateral cord. The anterior division of the inferior trunk forms the medial cord. Several terminal nerves arise within the posterior cervical triangle. Because they arise superior to the clavicle, they are called supraclavicular branches. The supraclavicular branches include the dorsal scapular nerve, the long thoracic nerve, the suprascapular nerve, and the nerve to subclavius.^5–7

Figure 3-8. Innervation of the major bones (osteotomes).

CN = cranial nerve.

Figure 3-9. Cervical plexus.

Supraclavicular Branches

Dorsal Scapular Nerve

The dorsal scapular nerve arises from the ventral ramus of C5. It follows the levator scapula muscle to the scapula and descends the medial border of the scapula on the deep surface of the rhomboid muscles. In its route, the dorsal scapular nerve innervates the levator scapula and rhomboid muscles.

Long Thoracic Nerve

The long thoracic nerve arises from the ventral rami of C5 to C7. It descends along the anterior surface of the middle scalene to the first rib and then transfers onto the serratus anterior muscle, which it innervates.

Suprascapular Nerve

The suprascapular nerve arises form the superior trunk. It follows the inferior belly of the omohyoid muscle to the scapula, passes through the superior notch into the supraspinatus fossa, where it innervates the supraspinatus muscle, and continúes around the scapular notch (lateral margin of the scapular spine) to the infraspinatus fossa, where it innervates the infraspinatus muscle. In addition to muscle, the suprascapular nerve innervates the glenohumeral joint.

Nerve to Subclavius

The nerve to subclavius arises from the superior trunk. It passes anteriorly a short distance to innervate the subclavius muscle and the sternoclavicular joint.

        The cords of the brachial plexus leave the posterior cervical triangle and enter the axilla through the axillary inlet. The remainder of the terminal branches arise within the axilla from the cords.

Posterior Cord Branches

The posterior cord forms the upper and lower subscapular nerves, thoracodorsal nerve, axillary nerve, and radial nerve.

Figure 3-10. Roots of the cervical plexus.

Figure 3-11. Anatomy of the neck after removal of the sternocleidomastoid muscle (Body is horizontal). Shown are: (1) brachial plexus between the (3) anterior and (4) middle scalene muscles, and the (2) phrenic nerve, (5) carotid artery, and (6) vagus nerve.

Figure 3-12. Sensory innervation of the cervical plexus.

Figure 3-13. Organizaron of the brachial plexus.

Subscapular Nerves

The subscapular nerves are formed by fibers from C5 to C6. The upper subscapular nerve is the first nerve to arise from the posterior cord. It passes onto the anterior surface of the sub- scapularis muscle, which it innervates. The lower subscapular nerve arises more distally. It descends across the anterior surface of the subscapularis muscle to the teres major muscle and innervates both the subscapularis and teres major muscles.

Thoracodorsal Nerve

The thoracodorsal nerve is formed by fibers from C5 to C7. It arises from the posterior cord, usually between the subscapular nerves, and descends across the subscapularis and teres major muscle to the latissimus dorsi muscle. It innervates latissimus dorsi.

Axillary Nerve

The axillary nerve is formed by fibers from C5 to C6 (Box 3-1). It passes from the axilla into the shoulder between the teres major and minor muscles. It innervates the teres minor. The nerve continues posterior to the surgical neck of the humerus to innervate the deltoid muscle. The superior lateral brachial cutaneous branch of the axillary nerve passes around the posterior margin of the deltoid to innervate the skin covering the deltoid. In addition to muscle and skin, the axillary nerve innervates the glenohumeral and acromioclavicular joints. Throughout it coarse, the nerve is associated with the posterior circumflex humeral artery and its branches.

Radial Nerve

The radial nerve is formed by fibers from C5 to T1 (Box 3-2). It passes from the axilla into the arm through the triangular space. The triangular space is located inferior to the teres major between the long head of triceps brachii and the humerus. The radial nerve innervates the long head of the triceps muscle and sends a posterior brachial cutaneous branch to the skin covering this muscle. It descends along the shaft of the humerus (Figure 3-14) in the spiral groove in association with the deep radial artery. In the spiral groove, the radial nerve innervates the medial and lateral heads of the triceps brachii as well as the anconeus muscles. In addition to innervating these muscles, it sends an inferior lateral brachial cutaneous nerve to the skin covering the posterior arm and a posterior antebrachial cutaneous branch to the skin covering the posterior surface of the forearm. The radial nerve pierces the lateral intermuscular septum and crosses the elbow anterior to the lateral epicondyle between the brachialis and brachioradialis muscles. Here it divides into a superficial and deep branch. The superficial branch descends the forearm on the deep surface of brachioradialis. Proximal to the wrist, it enters the skin providing innervation over the dorsum of the hand onto the thumb, index, middle, and ring fingers to the level of the distal interphalangeal joint. The deep branch pierces the supinator muscle and descends the forearm along the interosseous membrane as the posterior interosseous nerve. En route, it innervates the brachioradialis, extensor carpi radialis longus and brevis, supinator, extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, extensor indicis, extensor pollicis longus and brevis, and abductor pollicis muscles. In addition, it innervates the elbow, radioulnar, and wrist joints.⁸

Box 3–2.

Radial Nerve

Branches from the Lateral Cord

The lateral cord forms the lateral pectoral nerve, musculocutaneous nerve, and part of the median nerve.

Lateral Pectoral Nerve

The lateral pectoral nerve is formed by fibers from C5 to C7. It crosses the axilla deep to the pectoralis minor muscle and penetrates the deep surface of pectoralis major muscle, which it innervates. In addition, it innervates the glenohumeral joint.

Musculocutaneous Nerve

The musculocutaneous nerve is formed by fibers from C5 to C7 (Box 3-3). It pierces the coracobrachialis muscle and descends between the brachialis and biceps brachii muscles (see Figure 3-14). En route, it innervates all of these muscles. At the elbow, the musculocutaneous nerve becomes the lateral antebrachial cutaneous nerve and descends along the superficial surface of the brachioradialis muscle, innervating the skin covering that muscle. In addition to muscle and skin, the musculocutaneous nerve innervates the elbow and proximal radioulnar joints.

Figure 3-14. Cross-sectional anatomy and position of the nerves of the brachial plexus at the upper arm level.

Median Nerve

The median nerve is formed by junction of branches from the lateral and medial cords (Box 3-4). It descends the arm in association with the brachial artery and crosses the cubital fossa medial to the artery (see Figure 3-14). At the elbow, it innervates the pronator teres, flexor carpi radialis, and palmaris longus muscles. It passes into the forearm between the humeral and radial heads of the pronator teres muscle and descends in the space between the flexor digitorum superficialis and profundus muscles. En route, it innervates the flexor digitorum superficialis, the lateral part of flexor digitorum profundus (fibers to the index and middle fingers), the flexor pollicis longus, and the pronator quadratus muscles. In addition, the median nerve sends a palmar cutaneous branch to the skin covering the thenar eminence. At the wrist, the median nerve passes through the carpal tunnel deep to the flexor reti- naculum. In the hand, the median nerve sends branches to the thenar muscles, which are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The median nerve divides into three common palmar digital branches, which innerve the lateral two lumbrical muscles. The common palmar branches divide into proper palmar branches that innervate the skin of the thumb, index, middle, and ring (lateral half) fingers. The innervation covers the palmar surface and the nailbeds. In addition to muscle and skin, the median nerve innervates the elbow and all joints distal to it.^9,10

Box 3–3.

Musculocutaneous Nerve (C5 to C7)

Medial Cord Branches

The medial cord forms the medial pectoral nerve, medial brachial cutaneous nerve, medial antebrachial cutaneous nerve, and ulnar nerve and sends fibers to the median nerve.

Box 3–4.

Median Nerve

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Stimulating Catheters. Ultrasound-Assisted Nerve Blocks in Adults. Cutaneous Blocks for the Upper Extremity. Cervical Plexus Block.