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49

chapter

2Back1. INTRODUCTION

2. SURFACE ANATOMY

3. VERTEBRAL COLUMN

4. MUSCLES OF THE BACK

5. SPINAL CORD

6. EMBRYOLOGY

CHALLENGE YOURSELF

QUESTIONS

1. INTRODUCTION

Te back forms the axis (central line) of the humanbody and consists of the vertebral column, spinalcord, supporting muscles, and associated tissues

(skin, connective tissues, vasculature, and nerves).A hallmark of human anatomy is the concept ofsegmentation, and the back is a prime example.Segmentation and bilateral symmetry of theback will become obvious as you study the verte-bral column, the distribution of the spinal nerves,the muscles of the back, and its vascular supply.Functionally, the back is involved in three primarytasks, as follows:

Support. Te vertebral column forms theaxis of the body and is critical for uprightposture (standing or sitting), as a support forthe head, as an attachment point and bracefor movements of the upper limbs, and as asupport for transferring the weight of thetrunk to the lower limbs.

Protection. Te vertebral column protectsthe spinal cord and proximal portions of thespinal nerves before they distribute through-out the body.

Movements.Muscles of the back function inmovements of the head and upper limbs andin support and movements of the vertebralcolumn.

2. SURFACE ANATOMY

Figure 2-1 shows key surface landmarks of theback, including the following bony landmarks:

Vertebrae prominens:the spinous processof the C7 vertebra, usually the most promi-nent process in the midline at the posteriorbase of the neck.

Scapula:part of the pectoral girdle that sup-ports the upper limb; note its spine, inferiorangle, and medial border.

Iliac crests: felt best when you place yourhands on your hips. An imaginary horizon-

tal line connecting the iliac crests passesthrough the spinous process of the vertebraL4 and the intervertebral disc of L4-L5, pro-viding a useful landmark for lumbar punc-ture or epidural block (see Clinical Focus2-11).

Posterior superior iliac spines:an imagi-nary horizontal line connecting these twopoints passes through the spinous process ofS2 (second sacral segment).

3. VERTEBRAL COLUMN

Te vertebral column (spine) forms the centralaxis of the human body, highlighting the segmen-tal nature of all vertebrates, and usually is com-posed of 33 vertebrae distributed as follows(Fig. 2-2):

Cervical: seven total; first two called theatlas (C1) and axis (C2).

Toracic: 12 total; each articulates with apair of ribs.

Lumbar: five total; large vertebrae for

support of the bodys weight.

Sacral: five fused vertebrae for stability inthe transfer of weight from the trunk to thelower limbs.

Coccyx: four total; Co1 often is not fused,but Co2-Co4 are fused (a remnant of embry-onic tail).

Te actual number of vertebrae can vary, espe-cially the number of coccygeal vertebrae.


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50 Chapter 2 Back

FIGURE 2-1 Key Bony and Muscular Landmarks of the Back. (FromAtlas of human anatomy,ed 6, Plate 152.)

Trapezius m.

Spine of scapula

Infraspinatus m.

Teresmajor m.

Latissimus dorsi m.

Thoracolumbar fascia

Iliac crest

Posterior superior iliac spineSacrum

Spinous process of T12 vertebra

Inferior angle of scapula

Medial border of scapula

Deltoid m.

Spinous process of C7 vertebra

Ligamentum nuchae

External occipital protuberance

FIGURE 2-2 Vertebral Column. (FromAtlas of human anatomy,ed 6, Plate 153.)

Posterior viewLeft lateral view

C1

C2 Cervical

vertebrae

Cervicalcurvature

Thoraciccurvature

Lumbarcurvature

Sacralcurvature

Thoracicvertebrae

T1C7T1

Atlas (C1)Axis (C2)

T12

L1

Lumbarvertebrae

L5

L5

Sacrum(S1-5)

Coccyx

Level Corresponding structureC2-3 Mandible

C3 Hyoid bone

C4-5 Thyroid cartilage

C6 Cricoid cartilage

C7 Vertebra prominens

T3 Spine of scapula

T8 Level that inferior vena cava pierces diaphragm

T10 Xiphisternal junction

T10 Level that esophagus pierces diaphragm

T12 Level that aorta pierces diaphragm

L1 End of spinal cord (conus medullaris)

L3 Subcostal plane

L3-4 Umbilicus

L4 Bifurcation of aorta

L4 Iliac crests

S2 End of dural sac


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54 Chapter 2 Back

Thoracic and Lumbar Vertebrae

Te thoracic spine is composed of 12 thoracicvertebrae (Fig. 2-5and Table 2-2). Te 12 pairs ofribs articulate with the thoracic vertebrae. Tisregion of the spine is more rigid and inflexiblethan the cervical region.

Te lumbar spine is composed of five lumbarvertebrae (see Figs. 2-3 and 2-5 and Table 2-2).Te lumbar vertebrae are comparatively large forbearing the weight of the trunk and are fairlymobile, but not nearly as mobile as the cervicalvertebrae.

Sacrum and Coccyx

Te sacrum is composed of five fused vertebraethat form a single, wedge-shaped bone (Fig. 2-5andTable 2-2). Te sacrum provides support for thepelvis. Te coccyx is a remnant of the embryonictail and usually consists of four vertebrae, with the

last three often fused into a single bone.Te coccyxlacks vertebral arches and has no vertebral canal.Te features and number of vertebrae can vary,

and clinicians must always be aware of subtle dif-ferences, especially on radiographic imaging, thatmay be variants within a normal range.

FIGURE 2-5 Representative Vertebrae. (FromAtlas of human anatomy,ed 6, Plates 154 and 157.)

Vertebralforamen

Superior vertebral notch

Superiorcostal facet

Superiorcostal facet

Superior articularprocess and facet

Transversecostal facet

Transverseprocess

Transversecostal facet

T6 vertebra: superior view

T6 vertebra: lateral view

BodyPedicle

Lamina

Spinous process

PedicleInferiorarticularprocess

Inferiorvertebral notch

Inferiorcostalfacet

Vertebral bodyVertebralforamen

Transverseprocess

Superiorarticularprocess

Lamina

Spinous process

L2 vertebra: superior view

Thoracic and lumbar vertebrae

Sacrum and coccyx vertebrae

Pedicle

Ala (lateral part)

Lumbosacral articular surface Superior articular process

Sacral canalAla (wing)

Promontory

Sacral part ofpelvic brim

Facets of superior articular processes

Anterior (pelvic)sacral foramina

Dorsal surfacePelvic surface

Median sagittal section

Anterior inferior view Posterior superior view

Coccygeal cornu (horn)

Sacral cornu (horn)

Sacral hiatus

Pelvicsurface

Dorsalsurface

Transverseridges

Auricular surface

Coccyx

Sacral tuberosity

Lateral sacral crest

Median sacral crest

Sacral hiatus

Posteriorsacralforamina

Lateral view radiograph of thelower spine (with vertebralbodies numbered)

L5

S1

S2

Intervertebraldisc space

Intervertebral foramen

Inferior vertebralnotch of L2 vertebra

Pedicle of L3 vertebra

Spinous processof L3 vertebra

Superior vertebralnotch of L3 vertebra

Inferior articular

process of L3 vertebraSuperior articularprocess of L4 vertebra

L1

L2

L3

L4

T12


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56 Chapter 2 Back

Joints and Ligaments

of Craniovertebral Spine

Te craniovertebral joints include the atlanto-occipital (atlas and occipital bone of the skull)and atlanto-axial(atlas and axis) joints. Both aresynovial joints that provide a relatively wide range

of motion compared with other joints of the ver-tebral column. Te atlanto-occipital joint permitsone to nod the head up and down (flexion andextension), whereas the atlanto-axial joint is apivot joint that permits one to rotate the headfrom side to side, as if to indicate no (Fig. 2-6and Table 2-3).

Joints and Ligaments of

Vertebral Arches and Bodies

Te joints of the vertebral arches (zygapophysialjoints) occur between the superior and inferior

articular processes (facets) of adjacent vertebraeand allow for some gliding or sliding movement(Fig. 2-7and Table 2-4). Tese joints slope inferi-orly in the cervical spine (facilitate flexion andextension), are more vertically oriented in the tho-racic region (limit flexion and extension but allowfor rotation), and are interlocking in the lumbarspine (but do allow flexion and extension, butnot to the degree present in the cervical spine).Corresponding ligaments connect the spinousprocesses, laminae, and bodies of adjacent verte-brae (see Tables 2-2 and 2-3). Strong anterior

TABLE 2-3 Key Features of Atlanto-occipital and Atlanto-axial Joints

LIGAMENT ATTACHMENT COMMENT

Atlanto-occipital (Biaxial Condyloid Synovial) Joint

Articularcapsule

Surrounds facetsand occipitalcondyles

Allows flexionand extension

Anterior andposteriormembranes

Anterior andposterior archesof C1 to foramenmagnum

Limit movementof joint

Atlanto-axial (Uniaxial Synovial) Joint

Tectorialmembrane

Axis body tomargin offoramen magnum

Is continuationof posteriorlongitudinalligament

Apical Dens to occipitalbone

Is very small

Alar Dens to occipitalcondyles

Limits rotation

Cruciate Dens to lateralmasses

Resembles across; allowsrotation

TABLE 2-4 Features of the Zygapophysialand Intervertebral Joints

LIGAMENT ATTACHMENT COMMENT

Zygapophysial (Plane Synovial) Joints

Articularcapsule

Surrounds facets Allows glidingmotion.

C5-C6 is mostmobile.

L4-L5 permits

most flexion.

Intervertebral (Secondary Cartilaginous

[Symphyses]) Joints

Anteriorlongitudinal(AL)

Anterior bodiesand intervertebraldiscs

Is strong andpreventshyperextension

Posteriorlongitudinal(PL)

Posterior bodiesand intervertebraldiscs

Is weaker thanAL andpreventshyperflexion

Ligamentaflava

Connect adjacentlaminae of

vertebrae

Limit flexionand are moreelastic

Interspinous Connect spines Are weakSupraspinous Connect spinous

tipsAre stronger

and limitflexion

Ligamentumnuchae

C7 to occipitalbone

Is cervicalextension ofsupraspinousligament and isstrong

Intertransverse Connecttransverseprocesses

Are weakligaments

Intervertebraldiscs

Between adjacentbodies

Are secured byAL and PLligaments

and posterior longitudinal ligaments run alongmost of the length of the vertebral column. Ofthese two ligaments, the anterior longitudinalligament is stronger and prevents hyperextension(see Table 2-4).Te joints of the vertebral bodies (interverte-

bral joints) occur between the adjacent vertebralbodies (see Fig. 2-7and Table 2-4). Te interver-tebral joints are lined by a thin layer of hyalinecartilage with an intervening intervertebral disc(except between first two cervical vertebrae).Tese stable, weight-bearing joints also absorbpressure because the intervertebral disc is betweenthe bodies. Intervertebral discs are composedof a central nuclear zone of collagen and hydratedproteoglycans called the nucleus pulposus,which is surrounded by concentric lamellae ofcollagen fibers that compose the anulus fibrosus.

Te inner gelatinous nucleus pulposus (remnantof embryonic notochord) is hydrated and acts


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Chapter 2 Back 57 2

FIGURE 2-6 Craniovertebral Joints and Ligaments. (FromAtlas of human anatomy,ed 6, Plate 23; radiograph from Major

N:A practical approach to radiology,Philadelphia, 2006, Saunders-Elsevier.)

Capsule of lateralatlanto-axial joint

Atlas (C1)

Axis (C2)

Tectorial membrane

Posterior longitudinal lig.

Alar lig.

Atlas (C1)

Axis (C2)

Superior longitudinal bandTransverse lig. of atlas

Inferior longitudinal band

Cruciate lig.

Alar lig.Synovial cavities

Transverse lig. of atlas

Normal open-mouth view of the dens of C2(arrowhead) and the lateral masses of C1(arrows).

Dens

Upper part of vertebral canal with spinous processes and parts ofvertebral arches removed to expose ligaments on posterior vertebralbodies: posterior view

Median atlanto-axial joint: superior view

Principal part of tectorial membrane removed to expose deeper lig.: posterior view

Deeper (accessory) part of tectorial membrane

Capsule ofatlanto-occipital joint

Capsule of zygapophysialjoint (C2C3)

Apical lig. of densAlar lig.

Atlas (C1)

Axis (C2)

Cruciate lig. removed to show deepest ligs.: posterior view

FIGURE 2-7 Joints of Vertebral Arches and Bodies. (FromAtlas of human anatomy,ed 6, Plate 159.)

Left lateral view(partially sectioned in median plane)

Inferior articularprocess

Superior articularprocess

Ligamentumflavum

Interspinous lig.

Supraspinous lig.

Capsule ofzygapophysial

joint (partiallyopened)

Anteriorlongitudinallig.

Posteriorlongitudinallig.

Lumbarvertebralbody

Intervertebral

disc

Interarticular lig.

of head of rib

Radiate lig. ofhead of rib

Inferior costalfacet (for head of ribone number higher)

Superior costalfacet (for head ofrib of same number)

Left lateral view

Superiorcostotransverse lig.

Transverse costal facet (for tubercleof rib of same number as vertebra)

Intertransverse lig.


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Chapter 2 Back 63 2

Movements of the Spine

Te essential movements of the spine areflexion, extension, lateral flexion (lateral bending),and rotation (Fig. 2-8). Te greatest freedomof movement occurs in the cervical and lumbar

spine, with the neck having the greatest rangeof motion. Flexion is greatest in the cervicalregion, and extension is greatest in the lumbarregion. Te thoracic region is relatively stable, asis the sacrum.

Again, the atlanto-occipital joint permitsflexion and extension (e.g., nodding in acknowl-edgment), and the atlanto-axial joint allows side-to-side movements (rotation; e.g., indicating no).Tis is accomplished by a uniaxial synovial jointbetween the dens of the axis and its articulationwith the anterior arch of the atlas. Te dens

functions as a pivot that permits the atlas andattached occipital bone of the skull to rotate onthe axis. Alar ligaments limit this side-to-sidemovement so that rotation of the atlanto-axialjoint occurs with the skull and atlas rotating as asingle unit on the axis (see Fig. 2-6).

Movements of the spine are a function of thefollowing features:

Size and compressibility of the interverte-bral discs

Tightness of the joint capsules

Orientation of the articular facets (zyg-apophysial joints)

Muscle and ligament function

Articulations with the thoracic cage

Limitations imposed by the adjacent tissuesand increasing age

FIGURE 2-8 Movements of the Spine.

Lateral flexion

Rotation

FlexionExtension


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Chapter 2 Back 65 2

FIGURE 2-9 Arteries and Veins of the Spine. (FromAtlas of human anatomy,ed 6, Plates 168 and 169.)

Posterior spinal aa.

Anterior spinal a.Anterior segmental medullary a.

Posterior radicular a.

Spinal branch

Dorsal branch of posterior intercostal a.

Posterior intercostal a.

Thoracic (descending) aorta

Arteries of the spine: Section through thoracic level: anterosuperior view

Anterior radicular a.

Anteriorexternal

vertebralvenousplexus

Anteriorinternal

vertebral

(epidural)venousplexus

Anterior spinal v.

Basivertebral v.Anterior internal vertebral(epidural) venous plexus

Intervertebral v.

Anteriorsegmentalmedullary/radicular v.

Posteriorsegmentalmedullary/radicular v.Posterior internal vertebral

(epidural) venous plexus

Posterior spinal v.

Posterior externalvertebral venousplexus

Posterior internalvertebral (epidural)venous plexus

Intervertebral v.

4. MUSCLES OF THE BACK

Although the spine is the axis of the human bodyand courses down the bodys midline, dividing itinto approximately equal right and left halves, it is

not midway between the anterior and posteriorhalves of the body. In fact, most of the bodys weightlies anterior to the more posteriorly aligned verte-bral column. Consequently, to support the bodyand spine, most of the muscles associated with thespine attach to its lateral and posterior processes,assisting the spine in maintaining an upright pos-ture that offsets the uneven weight distribution.Te muscles of the back are divided into two

major groups, as follows:

Extrinsic back muscles, involved in move-ments of the upper limb and withrespiration.

Intrinsic back muscles, involved in move-ments of the spine and maintenance of

posture.

Extrinsic Back Muscles

Te extrinsic muscles of the back are consideredextrinsic because embryologically they arisefrom hypaxial myotomes (see Fig. 2-22). Teextrinsic back muscles are divided into thefollowing two functional groups (Fig. 2-10 andTable 2-5):


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66 Chapter 2 Back

TABLE 2-5 Muscles of the Back

MUSCLE

PROXIMAL

ATTACHMENT

(ORIGIN)

DISTAL

ATTACHMENT

(INSERTION) INNERVATION MAIN ACTIONS

Extrinsic Back Muscles

Trapezius Superior nuchal line,

external occipitalprotuberance, nuchalligament, andspinous processes ofC7-T12

Lateral third of clavicle,

acromion, and spineof scapula

Accessory nerve

(cranial nerve XI)and C3-C4

Elevates, retracts, and

rotates scapula; lowerfibers depress scapula

Latissimus dorsi Spinous processes ofT7-L5, sacrum,thoracolumbar fascia,iliac crest, and lastthree ribs

Humerus(intertuberculargroove)

Toracodorsalnerve (C6-C8)

Extends, adducts, andmedially rotateshumerus

Levator scapulae Transverse processesof C1-C4

Superior angle ofscapula

C3-C4 and dorsalscapular (C5)nerve

Elevates scapula and tiltsglenoid cavity inferiorly

Rhomboid minorand major

Minor:nuchalligament and spinousprocesses of C7-T1

Major:spinousprocesses of T2-T5

Medial border ofscapula

Dorsal scapularnerve (C4-C5)

Retract scapula, rotate itto depress glenoidcavity, and fix scapula

to thoracic wall

Serratus posteriorsuperior

Ligamentum nuchaeand spinousprocesses of C7-T3

Superior border ribs2-4

T1-T4 ventral rami Elevates ribs

Serratus posteriorinferior

Spinous processes ofT11-L3

Inferior border ribs9-12

T9-T12 ventralrami

Depresses ribs

Intrinsic Back Muscles

Splenius capitis Nuchal l igament,spinous processes ofC7-T3

Mastoid process oftemporal bone andlateral third ofsuperior nuchal line

Middle cervicalnerves*

Bilaterally: extends headUnilaterally: laterally

bends (flexes) androtates face to sameside

Splenius cervicis Spinous processes ofT3-T6

Transverse processes ofC1-C3

Lower cervicalnerves*

Bilaterally: extends neckUnilaterally: laterally

bends (flexes) and

rotates neck towardsame side

Erector spinae Posterior sacrum, iliaccrest, sacrospinousligament,supraspinousligament, andspinous processes oflower lumbar andsacral vertebrae

Iliocostalis:angles oflower ribs and cervicaltransverse processes

Longissimus:betweentubercles and angles ofribs, transverseprocesses of thoracicand cervical vertebrae,mastoid process

Spinalis:spinousprocesses of upperthoracic andmidcervical vertebrae

Respective spinalnerves of eachregion*

Extends and laterallybends vertebral columnand head

Semispinalis Transverse processes

of C4-T12

Spinous processes of

cervical and thoracicregions

Respective spinal

nerves of eachregion*

Extends head, neck, and

thorax and rotatesthem to opposite sideMultifidi Sacrum, ilium, and

transverse processesof T1-T12 andarticular processes ofC4-C7

Spinous processes ofvertebrae above,spanning two to foursegments


Stabilizes spine duringlocal movements

Rotatores Transverse processesof cervical, thoracic,and lumbar regions

Lamina and transverseprocess or spineabove, spanning oneor two segments


Stabilize, extend, androtate spine

*Dorsal rami of spinal nerves.


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Chapter 2 Back 67 2

FIGURE 2-10 Extrinsic Muscles of the Back. (FromAtlas of human anatomy,ed 6, Plate 171.)

Serratus posterior inferior m.

Latissimus dorsi m.

Serratus posterior superior m.

Rhomboid major m. (cut)

Trapezius m.

Spinous process of C7 vertebra

Thoracolumbar fascia

Iliac crest

Spinous process of T12 vertebra

Splenius capitis m.

Splenius cervicis m.

Levator scapulae m.

Rhomboid minor m. (cut)

12th rib

Erector spinae m.Note:On the right side, thetrapezius, latissimus dorsi, andrhomboid muscles were removedto show the intermediate muscles.

Spine of scapula

Superficial muscles, involved in move-ments of the upper limb (trapezius, latissi-mus dorsi, levator scapulae, two rhomboids),attach the pectoral girdle (clavicle, scapula,humerus) to the axial skeleton (skull, ribs,spine).

Intermediate muscles, thin accessorymuscles of respiration (serratus posteriorsuperior and inferior) that assist with move-ments of the rib cage, lie deep to the super-ficial muscles, and extend from the spine tothe ribs.

Intrinsic Back Muscles

Te intrinsic back muscles are the true musclesof the back because they develop from epaxial

myotomes, function in movements of thespine, and help maintain posture. Te intrinsicmuscles are enclosed within a deep fascial layerthat extends in the midline from the medial crestof the sacrum to the nuchal ligament and skull,and that spreads laterally to the transverse pro-cesses and angles of the ribs. In the thoracic andlumbar regions, the deep fascia makes up a dis-tinct sheath known as the thoracolumbar fascia(Figs. 2-10 and 2-11).

In the lumbar region, this fascial sheath has thefollowing three layers (see also Fig. 4-31):

Posterior layer,extending from the lumbarand sacral spinous processes laterally overthe surface of the erector spinae muscles.

Middle layer, extending from the lumbartransverse processes to the iliac crest inferi-orly and to the 12th rib superiorly.

Anterior layer, covering the quadratuslumborum muscle of the posterior abdomi-nal wall and extending to the lumbar trans-verse processes, iliac crest, and superiorly,forming the lateral arcuate ligament forattachment of the abdominal diaphragm.

Te intrinsic back muscles also are among the

few muscles of the body that are innervated bydorsal rami of a spinal nerve. From superficial todeep, the intrinsic muscles include the followingthree layers (Fig. 2-11and Table 2-5):

Superficial layer, including the spleniusmuscles that occupy the lateral and poste-rior neck (spinotransversales muscles).

Intermediate layer, including the erectorspinae muscles that mainly extend the spine.


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68 Chapter 2 Back

FIGURE 2-11 Intrinsic Muscles of the Back. (FromAtlas of human anatomy,ed 6, Plates 172 and 173.)

Semispinalis thoracis m.

Multifidi mm.

Multifidi mm. (cut)

LongusBrevis Rotatores thoracis mm.

Brevis

Longus

Rotatores cervicis mm.

Serratus posterior inferior m.

Multifidi mm.

Semispinalis capitis m.

The superficial and intermediate (erecter spinae) layers of the intrinsic back muscles

The deep (transversospinal) layer of the intrinsic back muscles

Erector spinae m.

Iliocostalis m.

Longissimus m.

Spinalis m.

Serratus posterior superior m.

Splenius capitis and splenius cervicis mm.

Superior nuchal line of skull

Posterior tubercle of atlas (C1)Longissimus capitis m.

Thoracolumbar fascia (cut edge)

Iliocostalis lumborum m.

Longissimus thoracis m.

Spinalis thoracis m.

Iliocostalis thoracis m.

Iliocostalis cervicis m.

Longissimus cervicis m.

Spinalis cervicis m.

Lateral intertransversarius m.

Interspinalis lumborum m.

Note:Deep dissection shown on right side.

Thoracolumbar fascia (anterior layer)

Thoracolumbar fascia (posterior layer) (cut)

BrevisLongus Levatores costarum mm.

Levator costae m.

Interspinalis cervicis m.


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Chapter 2 Back 69 2

Deep layer, including the transversospina-les muscles that fill the spaces between thetransverse processes and spinous processes.

Te intermediate, or erector spinae, layer ofmuscles is the largest group of the intrinsic back

muscles and is important for maintaining postureand extending the spine.Tese muscles are dividedinto three major groups, as follows (Fig. 2-11):

Iliocostalis, most laterally located and asso-ciated with attachments to the ribs and cer-vical transverse processes.

Longissimus, intermediate and largestcolumn of the erector spinae muscles.

Spinalis, most medially located and small-est of the erector spinae group, with attach-ments to the vertebral spinous processes.

Tese three groups are further subdivided intoregional divisionslumborum, thoracis, cervicis,and capitisbased on their attachments as oneproceeds superiorly (Fig. 2-11).Te transversospinales (transversospinal)

muscles (deep layer) are often simply called theparavertebral muscles because they form a solidmass of muscle tissue interposed and runningobliquely between the transverse and spinous pro-cesses (Fig. 2-11). Te transversospinal musclescomprise the following three groups:

Semispinalis group:thoracis, cervicis, andcapitis muscles; the most superficial trans-versospinal muscles, found in the thoracicand cervical regions superior to the occipitalbone.

Multifidus group:found deep to the semi-

spinalis group and in all spinal regions, butmost prominent in the lumbar region.

Rotatores group: deepest transverso-spinal muscles; present in all spinal regions,but most prominent in the thoracicregion.

Deep to the transversospinal muscles lies arelatively small set of segmental muscles that assistin elevating the ribs (levatores costarum) and sta-bilizing adjacent vertebrae while larger musclegroups act on the spine (interspinales, intertrans-

versarii) (Fig. 2-11).

Suboccipital Muscles

In the back of the neck, deep to the trapezius,splenius, and semispinalis muscles, several smallmuscles that move the head are attached to theskull, the atlas, and the axis (Fig. 2-12and Table2-6).Tese muscles are the suboccipital muscles,innervated by the suboccipital nerve (dorsal ramusof C1) and forming a (suboccipital) triangle withthe following muscle boundaries:

FIGURE 2-12 Suboccipital Triangle and Associated Musculature. (FromAtlas of human anatomy,ed 6, Plate 175.)

Rectus capitis posterior minor m.

Rectus capitis posterior major m.

Obliquus capitis superior m.

Obliquus capitis inferior m.

Greater occipital n. (dorsal ramus of C2 spinal n.)

Occipital a.

Lesser occipital n. (cervical plexus C2, C3) Greater occipital n. (dorsal ramus of C2 spinal n.)

Posterior arch of atlas (C1 vertebra)

Suboccipital n. (dorsal ramus of C1 spinal n.)

Vertebral a.

3rd (least) occipital n. (dorsal ramus of C3 spinal n.)

3rd (least) occipital n.(dorsal ramus of C3 spinal n.)

Semispinalis capitis and

splenius capitis mm.in posterior triangle of neck


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70 Chapter 2 Back

TABLE 2-6 Suboccipital Muscles

MUSCLE

PROXIMAL

ATTACHMENT

(ORIGIN)

DISTAL

ATTACHMENT

(INSERTION) INNERVATION MAIN ACTIONS

Rectus capitisposterior major

Spine of axis Lateral inferior nuchalline

Suboccipital nerve (C1) Extends head androtates to same side

Rectus capitisposterior minor

Tubercle of posteriorarch of atlas

Median inferior nuchalline

Suboccipital nerve (C1) Extends head

Obliquus capitissuperior

Atlas transverse process Occipital bone Suboccipital nerve (C1) Extends head andbends it laterally

Obliquus capitisinferior

Spine of axis Atlas transverseprocess

Suboccipital nerve (C1) Rotates head tosame side

FIGURE 2-13 Spinal Cord and Nerves In Situ. (FromAtlas of human anatomy,ed 6, Plate 160.)

C1 spinal n.

C8 spinal n.

Intercostal nn.

The 31 spinal segmentsand associated pairs ofspinal nerves are regionallyarranged as follow:

Key nerve plexuses include:

8 cervical pairs

Sacral: L4S4

T12 spinal n.

Conus medullaris

L1 spinal n.

Cauda equina

L5 spinal n.

Internal terminal filum (pial part)

Termination of dural sac

External terminal filum (dural part)

Cervical plexus

Brachial plexus

Lumbar plexus

T12 vertebra

Sacral plexus

Sciatic n.

Coccyx

12 thoracic pairs

5 lumbar pairs

5 sacral pairs

1 coccygeal pair

Cervical: C14

Brachial: C5T1

Lumbar: L14


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Chapter 2 Back 71 2

Rectus capitis posterior major

Obliquus capitis superior (superioroblique muscle of head)

Obliquus capitis inferior(inferior obliquemuscle of head)

Deep within the suboccipital triangle, thever-tebral artery,a branch of the subclavian artery inthe lower anterior neck, passes through the trans-verse foramen of the atlas and loops medially toenter the foramen magnum of the skull to supplythe brainstem.Te first three pairs of spinal nervesare also found in this region (Fig. 2-12).

5. SPINAL CORD

Te spinal cord is a direct continuation of themedulla oblongata, extending below the foramen

magnum at the base of the skull and passingthrough the vertebral (spinal) canal formed by thearticulated vertebrae (Fig. 2-13).Te spinal cord has a slightly larger diameter in

the cervical and lumbar regions, primarily becauseof increased numbers of neurons and axons inthese regions for innervation of the many musclesin the upper and lower limbs. Te spinal cord endsas a tapered region called the conus medullaris,which is situated at about the L1-L2 vertebral level(or L3 in neonate). From this point inferiorly, thenerve rootlets course to their respective levels andform a bundle called the cauda equina (horsestail). Te spinal cord is anchored inferiorly by theterminal filum,which is attached to the coccyx.Te terminal filum is a pial extension that picks upa layer of dura mater after passing through thedural sac (L2 vertebral level) before attaching tothe coccyx (see Spinal Meninges). Features of thespinal cord include the following:

Te 31 pairs of spinal nerves that comprise8 cervical, 12 thoracic, 5 lumbar, and 5 sacralpairs and 1 coccygeal pair.

Each spinal nerve is formed by a dorsal (pos-

terior) and a ventral (anterior) root.

Motor neurons reside in the spinal cord graymatter (anterior horn).

Sensory neurons reside in the spinal dorsalroot ganglia.

Ventral rami of spinal nerves often convergeto form plexuses(mixed networks of nerveaxons; cervical, brachial, lumbar, sacral)or segmental thoracic nerves (intercostalnerves and the subcostal nerve).

Dorsal rami of spinal nerves are small andinnervate the intrinsic back muscles andthe suboccipital region (epaxial muscles ofembryo) and a narrow band of skin abovethe intrinsic muscles.

Typical Spinal NerveTe typical scheme for a somatic(innervates skinand skeletal muscle) peripheral nerve shows amotor neuron in the spinal cord anterior horn(gray matter) sending a myelinated axon througha ventral (anterior) root and into a peripheralnerve, which ends at a neuromuscular junctionon a skeletal muscle (Fig. 2-14). Likewise, a nerveending in the skin sends a sensory axon towardthe spinal cord in a peripheral nerve. (Sensoryaxons also arise from the muscle spindles andjoints and are similarly conveyed back to the spinal

cord.) Tus, each peripheral nerve contains hun-dreds or thousands of motor and sensory axons.Te sensory neuron is a pseudounipolar neuronthat resides in a dorsal root ganglion(a ganglionin the periphery is a collection of neurons, just asa nucleus is in the brain) and sends its centralaxon into the posterior horn (gray matter) of thespinal cord. At each level of the spinal cord, thegray matter is visible as a butterfly-shaped centralcollection of neurons, exhibiting a posterior andan anterior horn (Fig. 2-14).Te spinal cord gives rise to 31 pairs of spinal

nerves, which then form two major branches(rami), as follows:

Dorsal ramus:a small ramus that coursesdorsally to the back and conveys motor andsensory information to and from the skinand the intrinsic back muscles and suboc-cipital skeletal muscles.

Ventral ramus: a much larger ramus thatcourses laterally and ventrally and inner-vates all the remaining skin and skeletalmuscles of the neck, limbs, and trunk.

Once nerve fibers (sensory or motor) arebeyond, or peripheral to, the spinal cord proper,the fibers then reside in nerves of the peripheralnervous system (PNS). Components of the PNSinclude the following (see Nervous System,Chapter 1):

Somatic nervous system: sensory andmotor fibers to skin, skeletal muscle, andjoints (Fig. 2-15, left side).


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72 Chapter 2 Back

FIGURE 2-14 Typical Spinal Nerve.

Sensory neuron cell bodyDorsal root ganglion

Dorsal root

Posterior horn

Anterior horn

Motor neuron cell body

Motor neuron

Neuromuscular junction

MuscleSkin

Sensory neuron

Ventralroot

Peripheral n.

Axon

Myelin sheath

Schematic of a typical peripheral nerve showing the somatic axons (autonomic axons not shown)

Segment of the spinal cord showing the dorsal and ventral roots, membranes removed: anterior view (greatly magnified)

Gray matter

White matter

Filaments of dorsal root

Dorsal root of spinal n.

Filaments of ventral root

Spinal sensory(dorsal root) ganglion

Dorsal ramusof spinal n.

Ventral ramusof spinal n.

Ventral rootof spinal n.

Spinal n.

Gray and white rami communicantes

Autonomic nervous system (ANS):sensory and motor fibers to all smoothmuscle (including viscera and vasculature),cardiac muscle (heart), and glands (Fig.2-15, right side).

Enteric nervous system: plexuses andganglia of the gastrointestinal tract thatregulate bowel secretion, absorption, and

motility (originally, considered part of theANS); linked to the ANS for optimal regula-tion (see Fig. 1-26).

Tus, each peripheral nerve arising fromthe spinal cord contains hundreds or thou-sands of three types of axons (Fig. 2-15, leftand right sides):


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Chapter 2 Back 73 2

FIGURE 2-15 Structural Anatomy of a Toracic Spinal Nerve.

Sensory

Motor

Preganglionicsympathetic

PostganglionicsympatheticDorsal root

Ventral root

Sensory neuron of abdominal viscera Neuroeffector junctions on smooth m., cardiac m.,secretory glands, metabolic cells, immune cells

Note:For simplicity, the left side of the figure only shows the somaticcomponents while the right side only shows the sympathetic efferent components.

Preganglionic sympathetic neurons passing

to synapse in another sympathetic chain ganglion

Vascular smoothm., sweatglands, andarrector pilimm. in skin

White ramus communicans

Gray ramus communicans

Sympathetic chain ganglion

Splanchnic n.

Collateralsympatheticganglion

Dorsalramus

Paciniancorpuscle

Ventral ramus

Freeendings

Skeletal m.

Skeletal m.

Dorsal rootganglion

Sympathetic chain

Intermediolateral cell column

Somatic efferent(motor) axons to skeletalmuscle

Afferent (sensory) axons from the skin,skeletal muscle, and joints or viscera

Postganglionic sympathetic efferentaxons to smooth muscle (vascular smoothmuscle and arrector pili muscles in theskin) and glands (sweat and sebaceousskin glands)

Each of the 31 pairs of spinal nerves exits thespinal cord and passes through an opening in thevertebral column to gain access to the periphery.Te C1 nerve pair passes between the skull and

the atlas, with subsequent cervical nerve pairsexiting the intervertebral foramen above the ver-tebra of the same number; C2 nerve exits via theintervertebral foramen superior to the C2 verte-bra, and so on, until one reaches the C8 nerve,which then exits the intervertebral foramen abovethe T1 vertebra. All the remaining thoracic,lumbar, and sacral nerves exit via the interverte-bral foramen below the vertebra of the samenumber (Fig. 2-16).

As it divides into its small dorsal ramus andlarger ventral ramus, the spinal nerve also gives offseveral small recurrent meningeal branches thatreenter the intervertebral foramen and innervatethe dura mater, intervertebral discs, ligaments,and blood vessels associated with the spinal cordand vertebral column (see Fig. 2-18).

Dermatomes

Te region of skin innervated by the somaticsensory nerve axons associated with a singledorsal root ganglion at a single spinal cord level iscalled a dermatome.(Likewise, over the antero-lateral head, the skin is innervated by one of thethree divisions of the trigeminal cranial nerve, as

discussed later.)Te neurons that give rise to thesesensory fibers are pseudounipolar neurons thatreside in the single dorsal root ganglion associatedwith the specific spinal cord level. (Note that foreach level, we are speaking of a pair of nerves,roots, and ganglia, with 31 pairs of spinal nerves,one pair for each spinal cord level.)Te first cervi-cal spinal cord level, C1, does possess sensoryfibers, but these provide minimal if any contribu-tion to the skin, so at the top of the head the


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74 Chapter 2 Back

FIGURE 2-16 Relationship of Spinal Nerves to

Vertebrae. (FromAtlas of human anatomy,ed 6, Plate 161.)

C1 spinal n. exitsabove C1 vertebra

Cervicalenlargement

Baseof skull

C8 spinal n. exitsbelow C7 vertebra(there are 8 cervicalnn. but only 7cervical vertebrae)

Lumbarenlargement

Conus medullaris(termination ofspinal cord)

Internal terminalfilum (pial part)

Externalterminal filum(dural part) Termination of dural sac

Cauda equina

Coccygeal n.

Cervical nn.

Thoracic nn.

Lumbar nn.

Sacral and coccygeal nn.

C1

C2

C3C4

C5

C6

C7T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

L1

C1

C2

C3C4

C5

C6

C7

C8

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

L1

L2L2

L3

L5

L3

L4

L4

L5

Sacrum

S1S2

S3

S4S5

FIGURE 2-17 Distribution of dermatomes. (FromAtlas

of human anatomy,ed 6, Plate 162.)

Schematic demarcationof dermatomes (accordingto Keegan and Garrett)shown as distinct segments.There is actually consider-able overlap between anytwo adjacent dermatomes.

C2

C3C4

C5T1T2T3T4

T5T6

T7T8T9

T12

T11

T10

L1

L3

L4

L5

S1

L5L4

C6

T1

C5

C8C7

C6

S2,3

L2

TABLE 2-7 Key Dermatomes as Related

to Body Surface

VERTEBRA(E) BODY SURFACE

C5 ClaviclesC5-C7 Lateral upper limbC6 TumbC7 Middle fingerC8 Little fingerC8-T1 Medial upper limbT4 NippleT10 Umbilicus (navel)T12-L1 Inguinal/groin regionL1-L4 Anterior and inner surfaces of

lower limbsL4 Medial side of big toe; kneeL4-S1 FootS1-S2 Posterior lower limbS2-S4 Perineum

dermatome pattern begins with the C2 derma-tome (Fig. 2-17and Table 2-7).Te dermatomes encircle the body in segmen-

tal fashion, corresponding to the spinal cord levelthat receives sensory input from that segment ofskin. Te sensation conveyed by touching the skinis largely that of pressure and pain. Knowledge ofthe dermatome pattern is useful in localizing spe-cific spinal cord segments and in assessing the


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76 Chapter 2 Back

FIGURE 2-18 Spinal Meninges and Relationship to Spine. (FromAtlas of human anatomy,ed 6, Plates 165 and 166.)

Dura mater

Arachnoidmater

Subarachnoidspace

Pia materoverlyingspinal cord

Filaments ofdorsal root

Denticulate lig. Internal vertebral (epidural) venous plexus

Fat inepidural space

Ventral root

Ventralramus

Dorsalramus

Dura mater

Subarachnoidspace

Arachnoid mater

Pia mater

Spinal sensory(dorsal root)ganglion

Section through thoracic vertebra

Arachnoid Mater

Te fine, weblike arachnoid membrane is avascu-lar and lies directly beneath, but is not attached to,the dura mater. Te arachnoid mater also ends atthe level of the S2 vertebra. Wispy threads of con-nective tissue extend from this layer to the under-lying pia mater and span the subarachnoid space,which is filled with CSF. Te subarachnoid spaceends at the S2 vertebral level.

Pia Mater

Te pia mater is a delicate, transparent inner layerthat intimately covers the spinal cord. At the cervi-cal and thoracic levels, extensions of pia formapproximately 21 pairs of triangular denticulate(having small teeth) ligamentsthat extend later-ally and help to anchor the cord to the dural sac.At the conus medullaris, the pia mater forms the

terminal filum,a single cord of tissue that piercesthe dural sac at the S2 vertebral level, acquires adural covering, and then attaches to the coccyx toanchor the spinal cord inferiorly.

Subarachnoid Space and Choroid Plexus

Cerebrospinal fluid fills the subarachnoid space,which lies between the arachnoid and pia menin-geal layers (Figs. 2-18and 2-19). Tus, CSF circu-lates through the brain ventricles and then gainsaccess to the subarachnoid space through thelateral and median apertures, where it flowsaround and over the brain and spinal cord to themost caudal extent of the dural sac at the S2 ver-tebral level.

Cerebrospinal fluid is secreted by the choroidplexus, and most CSF is absorbed primarilyby the arachnoid granulations (associatedwith superior sagittal dural venous sinus) andsecondarily by small veins on the surface ofthe pia mater throughout the central nervoussystem (Fig. 2-19). With about 500 mL produceddaily, CSF supports and cushions the spinal

cord and brain, fulfills some of the functionsnormally provided by the lymphatic system,and fills the 150-mL volume of the subarachnoidspace.


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78 Chapter 2 Back

Blood Supply to Spinal Cord

Te spinal cord receives blood from spinal arteriesderived from branches of larger arteries that serveeach midline region of the body (Fig. 2-20). Tesemajor arteries include the following:

Vertebral arteries,arising from the subcla-vian arteries in the neck

Ascending cervical arteries,from a branchof the subclavian arteries

Posterior intercostal arteries, from thethoracic aorta

Lumbar arteries, from the abdominalaorta

Lateral sacral arteries,from pelvic internaliliac arteries

A single anterior spinal arteryand two pos-terior spinal arteries, originating intracraniallyfrom the vertebral arteries, run longitudinallyalong the length of the cord and are joined seg-mentally in each region by segmental arteries (Fig.2-20). Te largest of these segmental branches isthe major segmental artery (of Adamkiewicz),

found in the lower thoracic or upper lumbarregion; it is the major blood supply for the lowertwo thirds of the spinal cord. Te dorsal andventral roots are supplied by segmental radicular(medullary) arteries.

Multiple anteriorand posterior spinal veins

run the length of the cord and drain into segmen-tal (medullary) radicular veins (see Fig. 2-9).Radicular veins receive tributaries from theinternal vertebral veins that course within the ver-tebral canal. Radicular veins then drain into seg-mental veins,with the blood ultimately collectingin the following locations:

Superior vena cava

Azygos venous system of the thorax

Inferior vena cava

6. EMBRYOLOGY

Most of the bones inferior to the skull form byendochondral bone formation, that is, from acartilaginous precursor that becomes ossified.Te embryonic development of the musculoskel-etal components of the back represents a classic

FIGURE 2-20 Blood Supply to Spinal Cord. (FromAtlas of human anatomy,ed 6, Plates 167 and 168.)

Basilar a.

Anterior spinal a.

Vertebral a.

Anterior segmental medullary aa.

Cervicalvertebrae

Anterior viewPosterior view

Subclavian a.

Anterior segmental medullary a.

Posterior intercostal a.

Thoracicvertebrae

Lumbar a.

Lumbarvertebrae

Lateral (or medial) sacral aa.Sacrum

Posterior spinal aa.

Vertebral a.

Posterior segmental medullary aa.

Subclavian a.

Posterior segmental medullary aa.

Posterior intercostal aa.

Lumbar aa.

Lateral sacral aa.

Major anterior segmentalmedullary a. (a. of Adamkiewicz)


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Chapter 2 Back 79 2example of segmentation, with each segmentcorresponding to the distribution of peripheralnerves. Tis process begins around the end ofthe third week of embryonic development (day19), during the period called gastrulation (seeChapter 1).

Development of Myotomes,

Dermatomes, and Sclerotomes

Te bones, muscles, and connective tissues of theembryo arise from the following sources:

Primitive streak mesoderm (somites)

Lateral plate mesoderm

Diffuse collections of mesenchyme

As the neural groove invaginates along the pos-

terior midline of the embryonic disc, it is flankedon either side by masses of mesoderm calledsomites.About 42 to 44 pairs of somites developalong this central axis and subsequently developinto the following (Fig. 2-21):

FIGURE 2-21 Somite Formation and Differentiation.

Neuralgroove

Somite

Mesoderm

Notochord

Ectoderm ofembryonic disc

Cut edgeof amnion

Intraembryoniccoelom

Endoderm(roof of yolk sac)

Neural tube Ectoderm

Dermomyotome

Ectoderm(future

epidermis)

Spinal cord

At 22 days

At 27 days At 30 days

Sclerotomecontributions

Endoderm of gut

Dorsal aortas

Mesoderm

Notochord

Sclerotome

Dermomyotome

to neural arch

to costal process

to vertebralbody (centrum)

Mesoderm

Dorsal aortas

Spinal cord

Aorta

Mesenchymalcontribution tointervertebral disc

Dorsal rootganglion

Mesoderm

Notochord(futurenucleus pulposus)

Myotome

Dermatome(future

dermis)

At 19 daysCross section of human embryos

FIGURE 2-22 Myotome Segmentation into Epimeres and Hypomeres.

Occipital (postotic)myotomes

Cervicalmyotomes

2

2

3

3

4

45678

12

34

56

78

910

11121234512345

1

1

Dorsal(epaxial)columnof epimeres

Thoracicmyotomes

Segmental distribution of myotomes in fetus of 6 weeks

Ventral(hypaxial)columnof hypomeres

Coccygealmyotomes

Sacral myotomesLumbar myotomes

Motor neuroblasts formprimitive axons and enterskeletal m. of body wall.

Epaxial mm.

Dorsal ramus

Ventral ramus

Hypaxialmm.(inthoracic andabdominalwall)

Lateralcutaneous n.

Anterior cutaneous n.

Posteriorcutaneous n.

Epaxial mm.Dorsal ramus

Ventral ramusPosterior divisionAnterior division

Hypaxial mm.(extensors of limb)

Hypaxial mm.(flexors of limb)

Hypaxial mm.(flexors of armand shoulder)

A schematic cross section showing the body walland upper limb on the embryos right sideand the embryo body wall only on the left side

Somatic developmentRegion of each trunk myotome also representsterritory of dermatome into which motor andsensory fibers of segmental spinal n. extend.


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80 Chapter 2 Back

Dermomyotomes: divide further to formdermatomes,which become the dermis ofthe skin, and myotomes,which differentiateinto segmental masses of skeletal muscle.

Sclerotomes: medial part of each somitethat, along with the notochord, migrates

around the neural tube and forms the carti-laginous precursors of the axial skeleton.

As in the somites from which they are derived,the myotomes have a segmental distribution. Eachsegment is innervated by a pair of nerves originat-ing from the spinal cord segment. A small dorsalportion of the myotome becomes an epimere(epaxial) mass of skeletal muscle that will form thetrue, intrinsic muscles of the back (e.g., erectorspinae) and are innervated by a dorsal ramus ofthe spinal nerve (Fig. 2-22).

A much larger ventral segment becomes thehypomere (hypaxial) mass of skeletal muscle,which will form the muscles of the trunk wall andlimb muscles, all innervated by a ventral ramus of

the spinal nerve. Adjacent myotome segmentsoften merge so that an individual skeletal musclederived from those myotomes is innervated bymore than one spinal cord segment. For example,the latissimus dorsi muscle is innervated by thethoracodorsal nerve, which is composed of nerves

from spinal cord segments C6-C8.

Vertebral Column Development

Each vertebra first appears as a hyaline cartilagemodel that then ossifies, beginning in a primaryossification center (Fig. 2-23). Ossificationcenters include the following:

Body:forms the vertebral body; importantfor support of body weight.

Costal process:forms the ribs, or in verte-brae without rib articulation, part of the

transverse process; important for move-ment and muscle attachment.

Neural arch: includes the pedicle andlamina, for protection of the spinal cord,

FIGURE 2-23 Ossification of Vertebral Column.

Ossification center

appears at 9th or10th week.

Vestige ofnotochord

Spinousprocess

Lamina

Vertebralforamen

Posteriortubercle

Anteriortubercle

Lamina ossificationcenter appears at 9thor 10th week.

Rib ossificationcenter appears at8th or 9th week.

Ossification centerappears at 9th or10th week.

Superiorarticularprocess

Lamina ossificationcenter appears at 9thor 10th week.

Vestige ofnotochord

Ossification centerappears at 9th or10th week.

Ossification centerappears at 10th week.

Ossification centerappears at 6thmonth (prenatal).

Ossificationcenter appearsat 10th week.

Body

Promontory

Superiorarticular process

Lateralpart (ala)

Median crest

BodyCostal processNeural arch

Fate of body, costal process, and neural arch components of vertebralcolumn, with sites and time of appearance of ossification centers

Cervical vertebra Thoracic vertebra

Lumbar vertebra Sacrum

Transverseprocess

Pedicle

Body

Rib

Body

Sacralcanal


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Chapter 2 Back 81 2development). As the primitive streak recedescaudally, the midline surface ectoderm thickens toform the neural plate,which then invaginates toform the neural groove(Fig. 2-24,A).Te neuralcrest forms at the dorsal aspect of the neuralgroove (Fig. 2-24, B) and fuses in the midline as

the groove sinks below the surface and pinches offto form the neural tube(Fig. 2-24, C). Te neuraltube forms the following:

Neurons of central nervous system (CNS:brain, spinal cord)

Supporting cells of CNS

Somatomotor neurons (innervate skeletalmuscle) of PNS

Presynaptic autonomic neurons of PNS

Te neural crestgives rise to the following (Fig.

2-24,DandE):

Sensory neurons of PNS found in dorsalroot ganglia

and the spinous process, for movement andmuscle attachment.

Te body of the vertebra does not develop froma single sclerotome but rather from the fusion oftwo adjacent sclerotomes (i.e., fusion of caudal

half of sclerotome above with cranial half ofsclerotome below). Te intervertebral foramenthus lies over this fusion and provides the openingfor the exiting of a spinal nerve that will innervatethe myotome at that particular segment.Te notochordinitially is in the central portion

of each vertebral body but disappears. Te noto-chord persists only as the central portion (nucleuspulposus) of each intervertebral disc, surroundedby concentric lamellae of fibrocartilage.

Neurulation and Development

of the Spinal Cord

Neurulation (neural tube formation) beginsconcurrently with gastrulation (formation oftrilaminar embryonic disc during third week of

FIGURE 2-24 Neurulation.

2.0mm

2.3mm

Neural plateof forebrain

Neural groove

Neural folds

Primitive streak

Level ofsection

Level of section

Level ofsection

Ectoderm Future neural crest

Neural plate

Future neural crest Neural grooveNeural fold

Neural plateof forebrain

Fused neural folds

Caudal neuropore

Neural crest

B.Embryo at 21 days(dorsal view)A.Embryo at 20 days(dorsal view)

C.Embryo at 24 days(dorsal view)

2.6mm

Fused neuralfolds

1st occipitalsomite1st cervicalsomite

1st thoracicsomite

Caudalneuropore

Ectoderm

Notochord

Sensory neuron ofdorsal spinal ganglion

Visceral motor neuron

of sympathetic ganglion

Chromaffin cell,suprarenalmedulla cell

MesonephrosGut

DorsalmesenteryCortical

primordiumof suprarenal

gland

Preaorticsympathetic

ganglion

Aorta

Sympathetictrunkganglion

Spinal cord

Dorsalspinalganglion

E.6th weekD.4th week

Neural crestLevel ofsection

Neural crestEctoderm

Neural tube

Neural tube

(spinal cord)Sulcus limitans

The neural tube will form the brain and spinal cord (CNS).

Derivatives of the neural crest includeSensory neurons in the PNSPostsynaptic autonomic neuronsSchwann (neurolemma) cellsAdrenal medulla cellsHead mesenchymeMelanocytes in the skinArachnoid and pia mater of meninges (dura mater from mesoderm)


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82 Chapter 2 Back

Postsynaptic autonomic neurons

Schwann cells of PNS

Adrenal medullary cells

Head mesenchyme and portions of heart

Melanocytes in skin

Arachnoid and pia mater meninges (dura

from mesenchyme)

Te cells in the walls of the neural tube composethe neuroepithelium,which develops into threezones, as follows:

Ependymal zone:inner layer lining centralcanal of spinal cord (also lines ventricles ofbrain).

Mantle: intermediate zone that developsinto gray matter of spinal cord.

Marginal zone: outer layer that becomeswhite matter of spinal cord.

Glial cells are found primarily in the mantle andmarginal zone. Te neural tube is distinguished bya longitudinal groove on each side that forms the

sulcus limitansand divides the tube into a dorsalalar plate and a ventral basal plate (Fig. 2-25).Te dorsal alar plate forms the sensory derivativesof the spinal cord, and the ventral basal plate givesrise to the somatic and autonomic motor neurons,whose axons will leave the spinal cord and passinto the peripheral tissues. Te sensory neuronsof the dorsal root ganglia are formed from neuralcrest cells.

FIGURE 2-25 Alar and Basal Plates of Spinal Cord.

5 1/2weeks (transverse section) Mature (transverse section)

Dorsal alarplate(sensory andcoordinating)

Ventral basalplate (motor)

Sensory

Motor

Central canal

Dorsal graycolumn (horn)

Tracts(whitematter)

Lateral graycolumn (horn)

Ventral graycolumn (horn)

Tracts (white matter)

Motor neuroblastsgrowing out toterminate onmotor end platesof skeletal m.

Neural crest

Spinal cord(thoracic part)

Ependymal layer

Mantle layer

Marginal layer

Differentiation and growth of neurons at 26 days

Sulcuslimitans

Marginallayer

Mantlelayer

Ependymallayer

Centralcanal


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Chapter 2 Back 85 2For each of the following conditions (11-20), selectthe muscle (A-K) most likely responsible.

A. Erector spinaeB. Latissimus dorsiC. Levator scapulaeD.

Obliquus capitisinferiorE. Rectus capitis

posterior majorF. Rhomboid major

G. RotatoresH. SemispinalisI. Serratus posterior

superiorJ. Splenius capitisK. Trapezius

____ 11. A work-related injury results in aweakness against resistance in elevation of thescapula and atrophy of one of the lateral neckmuscles. Te physician suspects damage to acranial nerve.

____ 12. An injury results in significant weak-ness in extension and lateral rotation alongthe entire length of the spine.

____ 13. After an automobile crash, a patientpresents with radiating pain around the shoul-der blades and weakness in elevating the ribson deep breathing.

____ 14. An injury to the back results in aweakened ability to extend and medially rotatethe upper limb.

____ 15. Sharp trauma to the back of the neckdamages the suboccipital nerve, resulting in aweakened ability to extend and rotate thehead to the same side against resistance.

____ 16. Malformation to the craniocervicalportion of the embryonic epaxial (epimere)muscle group that attaches to the ligamentumnuchae results in a weakened ability to extendthe neck bilaterally.

____ 17. Trauma to the lateral neck results ina lesion to the dorsal scapular nerve and aweakened ability to shrug the shoulders.

____ 18. Te loss of innervation to this pairof hypaxial (hypomere) muscles results in abilateral weakened ability to retract the scapu-lae but does not affect the ability to elevate thescapulae.

____ 19. During spinal surgery, these smallintrinsic back muscles must be retracted fromthe lamina and transverse processes of one ortwo vertebral segments.

____ 20. During surgery in the neck, the ver-tebral artery is observed passing just deep tothis muscle prior to the artery entering theforamen magnum.

Answers to ChallengeYourself Questions

1. B. Kyphosis, or humpback (hunchback), is one

of several accentuated spinal curvatures. It is

commonly observed in the thoracic spine.

Halitosis refers to bad breath, and lordosis to

the lumbar curvature, either the normal cur-

vature or an accentuated lordosis similar to

that observed in women during the third tri-

mester of pregnancy. Osmosis is the passage

of a solvent through a semipermeable mem-

brane based on solute concentration, and

scoliosis is an abnormal lateral curvature of

the spine.

2. A. An imaginary line connecting the two iliac

crests demarcates the space between the L3

and L4 spinous processes with patients on

their side and the spine flexed. Lumbar punc-

tures are usually performed between the

L3-L4 or L4-L5 levels to avoid injury to the

spinal cord proper, which usually ends as

the conus medullaris at the L1-L2 vertebral

levels. Below the L2 vertebral level, the nerve

roots comprise the cauda equina, suspended

in the CSF-filled subarachnoid space.

3. D. The nucleus pulposus of the intervertebral

discs usually herniates in a posterolateral

direction, where it can impinge on the nerve

roots passing through the intervertebral

foramen. A disc herniating at the L4-L5 level

usually impinges on the L5 roots, and hernia-

tion at the L5-S1 level involves the S1 roots.

4. A. Hyperextension-hyperflexion (whiplash) of

the cervical spine can occur when the

relaxed neck is thrown backward (hyperex-

tension), tearing the anterior longitudinal

ligament. Hyperflexion is usually limited

when ones chin hits the sternum. Properly

adjusted headrests, if available, can limit the

hyperextension.

5. B. Sensation from the skin is mediated by somatic

afferents (fibers in dorsal root), and the cell

bodies of these sensory neurons (pseudouni-

polar) associated with the T4 dermatome

reside in the T4 dorsal root ganglion.

6. D. Sclerotome-derived mesoderm normally con-

tributes to the formation of the neural arch

(pedicle, lamina, and spinous process), and a

folic acid deficiency in the first trimester of

pregnancy may contribute to this congenital

malformation (spina bifida occulta).


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86 Chapter 2 Back

7. B. The internal vertebral venous plexus (Bat-

sons plexus) resides in the epidural fat sur-

rounding the meningeal-encased spinal cord.

The epidural space lies between the bony ver-

tebral spinal canal and the dura mater.

8. E. The atlanto-axial joint (atlas and axis) func-

tions in the axial rotational movements ofthe head. The cranium and atlas move as a

unit and rotate side to side on the uniaxial

synovial pivot joint between the axis (C2) and

atlas (C1).

9. D. The dorsal root ganglia between T1-L2 contain

sensory neurons for both somatic and visceral

(autonomic) afferent fibers, so both of these

modalities would be compromised. Efferent

(motor) fibers are not associated with the

dorsal root ganglia.

10. E. Of the options, only Schwann cells are derived

from the neural crest. While the arachnoid

and pia mater are derived from neural crest

cells (neither of these choices are options),

the dura mater is from mesoderm.

11. K. The only muscle of this group innervated by

a cranial nerve is the trapezius, by the acces-

sory nerve (CN XI). The other neck muscle

innervated by CN XI is the sternocleidomas-

toid muscle in the lateral neck.

12. A. The major extensors along the entire length

of the spine, also involved in lateral rotation

or bending when unilaterally contracted, are

the erector spinae group of muscles (spinalis,

longissimus, and iliocostalis).

13. I. The only muscles in the list that are associ-ated with the shoulder blades (scapula),

attach to the ribs, and elevate them during

inspiration are the serratus posterior superior

group. These muscles are considered respira-

tory muscles because they assist in respira-

tory movements of the ribs.

14. B. The latissimus dorsi extends and medially

rotates the upper limb at the shoulder and is

the only muscle in this list with these com-

bined actions on the upper limb.

15. E. The suboccipital nerve (dorsal ramus of C1)

innervates the suboccipital muscles in the

posterior neck, and the rectus capitis poste-

rior major is the only one in the list that

extends and rotates the head to the same

side.

16. J. The splenius capitis is the only epaxial (intrin-sic back muscles innervated by dorsal rami of

the spinal nerves) in this list that has signifi-

cant attachment to the ligamentum nuchae

(origin) and exclusively extends the neck

when it contracts bilaterally.

17. C. The levator scapulae is innervated by the

dorsal scapular nerve (C5) and assists the

superior portion of the trapezius in shrugging

the shoulders.

18. F. Hypaxial muscles are innervated by the

ventral rami of spinal nerves, and the rhom-

boid major is a hypaxial muscle that retracts

the scapulae.

19. G. The rotatores muscles are part of the trans-

versospinales group of muscles that largely

fill the spaces between the transverse pro-

cesses and the spinal processes. Specifically,

the rotatores extend between the lamina and

transverse processes and stabilize, extend,

and rotate the spine.

20. E. The vertebral arteries ascend in the neck by

passing through the transverse foramina of

C6 to C1, then loop medially and superiorly to

the posterior arch of the atlas (C1), pass deep

(anterior) to the rectus capitis posterior major

muscle, and enter the foramen magnum to

supply the posterior portion of the brainstem

and brain, and the cerebellum by forming the

basilar artery and its branches.

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