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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
Respective spinalnerves of eachregion*
Stabilizes spine duringlocal movements
Rotatores Transverse processesof cervical, thoracic,and lumbar regions
Lamina and transverseprocess or spineabove, spanning oneor two segments
Respective spinalnerves of eachregion*
Stabilize, extend, androtate spine
*Dorsal rami of spinal nerves.
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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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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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