For Educational and Institutional Use. This Test Bank Is Licensed for Noncommercial, Educational In- House Or Online Educational

For educational and institutional use. This test bank is licensed for noncommercial, educational in- house or online educational course use only in educational and corporate institutions. Any broadcast, duplication, circulation, public viewing, conference viewing or Internet posting of this product is strictly prohibited. Purchase of the product constitutes an agreement to these terms. In return for the licensed use, the Licensee hereby releases, and waives any and all claims and/or liabilities that may arise against ASRT as a result of the product and its licensing. SECTIONAL ANATOMY ESSENTIALS MODULE 4 — THE SPINE

1. Sectional Anatomy Essentials: Module 4 – The Spine Welcome to Module 4 of Sectional Anatomy Essentials – The Spine. This module was written by Michael A. Manders, B.S., R.R.A., R.T.(R), and Jeffrey D. Houston, MD.

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3. Objectives After completing this module, you will be able to: Name the components of the spine. Identify the features of the vertebrae. Describe the differences between cervical, lumbar and thoracic vertebrae. Identify the major curves found in the vertebral column. Locate the major ligaments found in the spinal column. Identify the major muscle groups supporting the spine. Describe the major components of the spinal cord.

4. Introduction Throughout the Sectional Anatomy Essentials series, we display most of the cross-sectional anatomy using multidetector computed tomography (CT) and magnetic resonance (MR) images to illustrate the three-dimensional relationship of the structures. Because you can easily lose your frame of reference when viewing cross-sectional images, the location of the featured slice on many slides will be displayed on adjacent localizer images of the other 2 planes, like the image shown here.

5. Vertebral Column The vertebral column, also known as the spine or backbone, is located in the posterior aspect of the neck and torso. It typically is made up of 33 vertebrae, 9 of which are fused to each other. The remaining 24 vertebrae are separated by intervertebral discs, structures that are responsible for absorbing some of the shock the body experiences throughout life.

The spine comprises 4 sections. From superior to inferior, they are the cervical, thoracic, lumbar and sacral/coccygeal sections. The spinal column performs many different functions: It provides strength and support, facilitates movement, offers protection and shock absorption, and maintains blood supply.

6. Vertebral Body The typical vertebra is divided into anterior and posterior elements. The anterior element, called the vertebral body, makes up the majority of each vertebra. The superior and inferior borders of each vertebral body have a raised outer rim with a rough, flattened recess that provides an attachment point

for the intervertebral discs. Multiple tiny foramina are located anteriorly and posteriorly for the passage of the vertebral vessels.

7. Vertebral Arch The posterior elements, also called the vertebral arch, are formed by multiple osseous segments that extend posteriorly and create a semicircular shape. The most anterior of these segments are the paired pedicles. The pedicles extend posterolaterally from the superior posterolateral border of each vertebral body in the cervical, thoracic and lumbar spine.

8. Pedicles The pedicles are labeled on slice 22 of the CT teaching scan. Note their location in relation to the vertebral arch.

9. Intervertebral Foramina Small recesses above, and more prominently, below each pedicle are called vertebral notches. When the spine is in anatomic alignment, the inferior notches of a superiorly situated vertebra combine with the superior notches of an inferiorly adjoining vertebra to form the intervertebral foramina. The intervertebral foramina allow the nerves and vessels to pass to and from the spinal column.

10. Laminae Paired laminae extend posteromedially from the posterior tip of each pedicle. Each lamina extends medially until it reaches midline, where it connects with the contralateral lamina to complete an osseous ring, called the vertebral foramen.

11. Vertebral Canal When the spine is in anatomical alignment, the vertebral foramina form a tunnel called the vertebral canal. The spinal cord extends through the center of this canal. The vertebral canal is visible on this slide, which is slice 5 of the teaching scan.

12. Osseous Processes Arising from the vertebral arch are 7 osseous processes, or projections. There are 4 articular processes, 2 transverse processes and 1 spinous process. These projections serve as attachment points for muscles and form articulating surfaces for adjacent vertebrae, giving the spine its pillar-like strength.

13. Articular Processes Each vertebra contains 2 superior and 2 inferior articular processes. The articular processes are located above and below the junction of the pedicle and lamina. Each articular process is coated with hyaline cartilage and articulates with the opposite articular process of the adjacent vertebra. In other words, the superior articular process forms a joint with the inferior articular process of the vertebra above it.

These joints are called the zygoapophyseal, or facet, joints. Like most joints, the facet joints can break down with age, causing irritation and osseous overgrowth, a common condition known as facet arthropathy.

14. Transverse Processes The paired transverse processes also arise from the junction of the pedicle and lamina, between the superior and inferior articular processes. Each transverse process extends laterally and acts as an attachment point for muscles and ligaments of the spine, which we’ll discuss later.

15. Spinous Process The spinous process arises from the junction of the laminae. The spinous process extends posteriorly and also serves as an attachment point for muscles and ligaments. Spinous processes are typically bifid in the midcervical spine, meaning that the tips of the process are split and splayed laterally.

16. Intervertebral Discs The intervertebral discs are located between the vertebral bodies. Each intervertebral disc is made up of 3 parts: the nucleus pulposus, the annulus fibrosus and the paired vertebral endplates. The intervertebral disc provides cushioning, acts as a ligament to hold the vertebral bodies together and works as a joint to provide movement to the spine.

17. Intervertebral Discs Each individual component of the disc serves a different function that allows the disc to perform its tasks. The nucleus pulposus is a jelly-like substance that absorbs the shocks the spine is subjected to on a daily basis. The annulus fibrosus surrounds the nucleus pulposus and helps evenly distribute pressure across the nucleus pulposus. The superior and inferior vertebral endplates consist of cartilage and anchor the disc to the vertebral body.

18. Intervertebral Discs A good analogy of how an intervertebral disc works is to compare it to a waterbed filled with gel, where the gel represents the nucleus pulposus and the rubber covering of the mattress represents the annulus fibrosus. Depending on where pressure is applied to the bed, the surrounding areas expand to accommodate the force. This same expansion takes place in the disc and keeps the vertebral bodies from touching each other.

Sometimes the disc is subjected to so much pressure that the annulus fibrosus can no longer contain the nucleus pulposus. At this point, the annulus fibrosus can partially tear, and the nucleus pulposus squeezes into the surrounding annulus fibrosus tissue, a condition known as an annular tear. A herniated disc occurs if the tear continues to the edge of the disc and any part of the disc projects into the vertebral canal. These conditions can compress the contents of the spinal canal and produce neurological symptoms. For example, sciatica occurs when the spinal cord is compressed in the lower spine and pinches a lower extremity nerve, causing leg pain, numbness or weakness.

19. Vacuum Disc Phenomenon Another commonly observed abnormality of the intervertebral disc is vacuum disc phenomenon. This condition occurs when the nucleus pulposus dries out and cracks, and nitrogen extracted from surrounding fluid builds up within the cracks. On computed tomography (CT), this condition appears as a

linear area of low attenuation between vertebral bodies, whereas on magnetic resonance (MR) imaging, it remains linear but appears as a signal void.

20. Spinal Curves There are 2 types of normally occurring curves of the spine: lordotic and kyphotic. A lordotic curve is a curvature with anterior convexity and posterior concavity. The cervical and lumbar spines have a natural, mild lordotic curve.

A kyphotic curve is the opposite, with anterior concavity and posterior convexity. A normal, mild kyphotic curve can be found in the thoracic and sacral spines. When the kyphotic curve becomes excessive, the condition is called kyphosis. A patient with kyphosis has a hunchback appearance.

Although kyphosis can affect anyone, it is particularly common in elderly women, secondary to a loss of bone density that occurs after menopause. This loss increases the risk for anterior compression fractures of the vertebral bodies, which leads to an increase in the degree of kyphotic curve of the spine.

21. Pathologic Misalignments Although the primary purpose of this module is to discuss normal cross-sectional anatomy, we should mention a common pathologic misalignment of the spine: scoliosis. The spine normally runs midline in the coronal plane through the body. Scoliosis occurs when there is lateral curvature of the spinal column.

Although we won’t discuss the different causes of scoliosis, it’s important to know the names of the scoliosis curves. A lateral curve with the convexity on the patient’s right side and the concavity on the patient’s left is called a dextroconvex curve of the spine. A levoconvex curve is the opposite, with the convexity on the patient’s left side and the concavity on the patient’s right. A rotoscoliotic curve implies that, in addition to a scoliotic curve, the vertebral body is rotated in the axial plane.

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24. Vertebrae Types As we mentioned previously, the spinal column typically is divided into 4 or 5 sections. Moving from the neck to the pelvis, the sections are the cervical, thoracic, lumbar, sacral and coccygeal spine. Some texts include the coccyx with the sacrum.

Each vertebra is named with a letter representing the section of the spinal column it’s located in followed by a number. For example, the third cervical vertebra is called C3, the eleventh thoracic vertebra is T11, the second lumbar vertebra is L2 and the first sacral vertebra is S1. This numbering convention is not typically applied to the coccyx.

Let’s discuss the characteristics of each of these vertebral components.

25. Cervical Vertebrae The cervical spine typically consists of 7 vertebrae and extends from the base of the skull to about the level of the top of the lungs. Cervical vertebrae 3 through 6, or C3 through C6, have a typical appearance, with each vertebra having a body, pedicles, laminae, transverse processes, articular processes and a vertebral foramen; however, C1, C2 and C7 are unique.

C3 through C6 each have a spinous process that is typically bifid, meaning that it splits in two at the tip, like a snake’s tongue. C1 through C6 also have paired transverse foramina, one located within each transverse process. Each transverse foramen allows for the passage of a vertebral artery from its origin in the upper thorax into the head.

26. Cervical Vertebrae In contrast, C1, C2 and C7 are different than the regular cervical vertebrae. C1, also known as the atlas, is an osseous ring that supports the head. It is named for Atlas, the Titan who held up the celestial sphere in Greek mythology. The atlas has no body and no spinous process, but instead has large superior articular processes that articulate with the occipital condyles of the skull.

The front portion of the ring is called the anterior arch and the rear portion is called the posterior arch. The paired lateral masses, the most bulky parts of the atlas, are located on the sides of the vertebra between the superior and inferior articular processes.

27. Cervical Vertebrae C2, called the axis, also has a peculiar appearance. It takes its name from the Latin for “axle,” because it forms a pivot upon which the atlas and head rotate. The odontoid process, also known as the dens, arises from the superior border of the body. The odontoid process is a dense bony projection that extends superiorly and articulates with the posterior border of the anterior arch of the atlas. The superior articular facets of the axis, which articulate with the inferior articular facets of the atlas, are located on each side of the odontoid process. These 3 articulations allow the skull to rotate. Another notable detail of C2 is its diminutive spinous process.

28. Cervical Vertebrae The last unique cervical vertebra is C7, also called the vertebral prominens because of its prominent spinous process. Typically, this spinous process is not bifid like the other cervical vertebrae. Although rare, C7 may articulate with a pair of abnormal small, or hypoplastic, ribs called cervical ribs.

29. Thoracic Vertebrae The cervical spine articulates inferiorly with the thoracic spine. There are normally 12 thoracic vertebrae. The thoracic vertebrae have a typical vertebral appearance with 2 differences: the costovertebral and costotransverse facets. Each thoracic vertebra articulates with a pair of ribs at 2 places per rib. The head of each rib articulates with the vertebral body along the posterolateral border at the costovertebral facet. The tubercle of each rib articulates with the anterolateral border of each transverse process at the costotransverse facet.

30. Lumbar Vertebrae There are typically 5 lumbar vertebrae. The vertebrae of the lumbar spine generally don’t have unique characteristics. However, the lumbar vertebrae differ from the cervical vertebrae in that they don’t have transverse foramina. They differ from the thoracic vertebrae in that they don’t have facets to accommodate the ribs.

The lumbar vertebrae are the largest vertebral bodies, gradually increasing in size from superior to inferior, with L5 being the largest vertebra of the movable portion of the spine. The full weight of the body is transferred from L5 to S1.

31. The Sacrum The sacrum is a triangular-shaped bone located inferior to the lumbar spine and in between the right and left iliac bones, which we discuss in another module. The sacrum is unique in that is looks like a bat when viewed in the axial plane. It usually is made up of 5 bones, which normally fuse by age 26.

The sacrum consists of a body, paired alae (the “wings” of the sacrum), a sacral canal and paired articular processes. The blocks of bone located lateral to the sacral foramina form the lateral masses of the sacrum. Lateral to each sacral ala are the iliac crests of the pelvis. The joints between the alae and the iliac crests are called the sacroiliac joints. A prominent lip of bone projecting anterosuperiorly from S1 is termed the sacral promontory.

Along the posterior midline border of the sacrum is the sacral crest, the fused, rudimentary spinous processes of the sacral vertebrae. The laminae of the fifth sacral vertebra typically fail to unite, producing a defect in the posterior wall of the sacral canal, which is called the sacral hiatus. The sacral cornua are the rounded processes that project inferiorly from the fifth sacral vertebra and articulate with the cornua of the coccyx.

32. The Sacrum Watch this animation to see all of the sacrum structures we just discussed.

33. Sacroiliac Joints There are 2 sacroiliac joints: 1 on the right and 1 on the left. These joints are held together by strong ligaments and have a small amount of movement. The articulating surfaces of the sacrum and ilium form an uneven topography about the time that we begin to walk. These ridges and depressions work in a manner similar to a zipper and with the help of the ligaments lock the sacrum in place between the left and right ilium. The primary function of the sacroiliac joints is shock absorption.

Conditions important to the sacroiliac joints are the seronegative spondyloarthropathies, which typically first involve these joints. These autoimmune diseases affect the axial skeleton but are not associated with the presence of an autoantibody called rheumatoid factor, meaning that the diseases follow a different pathological mechanism than typical rheumatoid arthritis. The most common of the seronegative spondyloarthropathies is ankylosing spondylitis, but others include reactive arthritis or Reiter syndrome, enteropathic spondylitis, psoriatic arthritis and isolated acute anterior uveitis.

34. The Coccyx The final and most inferior portion of the spine is called the coccyx. The coccyx is commonly referred to as the tailbone because it’s thought to be the remnant of a vestigial tail, meaning a piece of anatomy that no longer serves a particular purpose. However, the coccyx has not lost all of its usefulness because it is the attachment point for many ligaments and muscles.

The coccyx typically is formed from 4 vertebrae. Whereas the appearance of the first 3 coccygeal vertebrae have the basic vertebral format of a body with articular and transverse processes, the fourth segment is normally an osseous stub arising from the inferior coccyx.

35. Developmental Variations Incidental developmental variations of the spine are relatively common and typically involve the junctional vertebrae. A transitional vertebra has characteristics of 2 different vertebral categories, such as the hypoplastic ribs arising from C7. At the thoracolumbar junction, features of the thoracic spine and the lumbar spine can be seen in a single vertebra. For instance, a transitional vertebra may have a hypoplastic rib on the right and a transverse process on the left. Bilateral hypoplastic ribs may mimic the transverse processes of the lumbar spine. At the lumbosacral junction, the most inferior lumbar vertebra can fuse with the sacrum, a process called sacralization, or the superior sacral vertebra fail to fuse with the rest of the sacrum, a condition termed lumbarization.

It’s important to be aware of variant spinal anatomy, and you should never assume that every individual has exactly 12 thoracic vertebrae and 5 lumbar vertebrae. On cross-sectional imaging, it can be difficult, or frankly impossible, to accurately identify or characterize variant spinal anatomy because the entire spine is rarely included in the field of view. Therefore, you should note any suggestion of variant anatomy and then complete spine radiographs can be obtained for further characterization if indicated. Incorrect assumptions regarding the number of vertebrae can lead to erroneous fluoroscopic localization of a particular vertebra and result in therapeutic procedures or surgery being performed at the wrong level.

36. Spina Bifida Spina bifida is a congenital malformation with a wide spectrum of severity. There are several types of spina bifida that are caused by malformation of the spinal canal. The most common and least severe form is spina bifida occulta. “Occulta” is the Latin word for “hidden.” Spina bifida occulta is when there is an incomplete closure of the spinal canal, although the opening is not large enough for the spinal cord to exit the canal. This condition is routinely seen on spine radiographs and on cross-sectional imaging studies, particularly at S1, and is generally considered an incidental developmental variant.

At the other end of the spectrum, the most severe form of spina bifida is a myelomeningocele. In this condition, the malformation of the spinal canal allows the spinal cord to protrude outside of the canal itself. This defect can be life threatening and produce serious neurological deficits.

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40. Spinal Ligaments Next, we’ll introduce the different ligaments that connect the vertebrae and give the spine stability, but before we can do that, we need to discuss ligaments in general. A ligament is fibrous connective tissue that is made up of very densely packed collagenous fibers. Ligaments typically connect bone to bone, as opposed to tendons which usually connect muscle to bone. When discussing the spine, we’ll focus on the following specific ligaments: the anterior longitudinal ligament, the posterior longitudinal ligament, the ligamenta flava, the interspinous ligaments, the supraspinous ligament, the ligamentum nuchae, the alar ligaments and the transverse ligament.

41. Spinal Ligaments The anterior longitudinal ligament is located along the anterior border of the spine and extends the entire length of the spine. The posterior longitudinal ligament runs within the spinal canal along the posterior borders of the vertebral bodies and extends from C2 to the sacrum. Each ligamentum flavum runs within the spinal canal from the inferior border of each lamina to the superior border of the adjacent lamina. They extend anteriorly as far as each articular process and unite posteriorly with each lamina, with gaps to allow for the passage of vessels and nerves. Ligamenta flava run from C2 superiorly to S1 inferiorly.

The interspinous ligaments attach each spinous process to the adjacent spinous process. They are found just posterior to the ligamentum flavum and anterior to the supraspinous ligament. The supraspinous ligament runs from C7 to the sacrum and connects the tips of the spinous processes of those levels. Attached to the superior border of the C7 spinous process is a similar ligament, the ligamentum nuchae or nuchal ligament, that extends to the external occipital protuberance. This ligament also attaches to the tips of the cervical spinous processes.

42. Spinal Ligaments As you may have noticed, many of these ligaments only extend to the C2 level superiorly. Two ligaments involving C1 and C2 are important to know: the alar ligaments and the transverse ligament. The alar ligaments extend bilaterally from the lateral borders of the odontoid process to the medial border of the occipital condyles. The alar ligaments keep the skull aligned with the cervical spine when rotating the head. The transverse ligament extends from the medial borders of the lateral masses, across the anterior arch of the atlas and posterior to the odontoid process. It maintains articulation between the odontoid process and the anterior arch.

43. Spinal Muscle Groups Several large and small muscles help support and move the spine. Although various muscles are involved in these functions, 3 primary muscle groups that attach to the spine are responsible for respiration and

movement. From deepest to superficial, these muscles are the transversospinal, erector spinae and superficial groups.

44. Transversospinal Muscle Group The transversospinal muscle group is a deep muscle group that is partly responsible for the rotation and extension of the vertebral column. It’s so named because the muscle fibers run from the transverse processes to the spinous processes of the vertebrae. The transversospinal muscle group can be divided into 3 parts: the semispinalis, the multifidus and the rotatores.

45. Transversospinal Muscle Group As its name implies, the semispinalis muscle group arises from about half of the spine. It is made up of 3 layers of muscles that are named according to their superior attachments: semispinalis thoracis, semispinalis cervicis and semispinalis capitis. These small, long muscles extend from the transverse processes of the vertebrae to the spinous processes of the vertebrae above them and are responsible for extension and rotation of the spine.

One exception is the semispinalis capitis, which does not insert into the spinous processes, but rather into the superior and inferior nuchal lines of the occipital bone. The semispinalis capitis rotates and extends the head.

46. Transversospinal Muscle Group The multifidus muscle is the largest of the transversospinal group and runs from the sacrum to the axis. Its name comes from Latin “multus,” meaning “many,” and “findo,” meaning “cleave.” The muscle is divided into many bundles, or fasciculi. It can be found in the groove between the spinous and transverse processes on each side of the spine. The multifidus extends and rotates the vertebral column.

47. Transversospinal Muscle Group The rotatores muscles of the transversospinal group run the length of the spine but are best developed in the thoracic spine. They can be found deep to the multifidus muscle. They extend from the upper border of the thoracic transverse processes upward to insert into the lamina of the superior adjacent vertebra. They also are responsible for the extension and rotation of the spine.

The psoas major muscles, which are part of the iliopsoas muscle group, also are located bilateral to the spine. Because these muscles are only partially responsible for spinal flexion, we’ll discuss them in another module.

48. Erector Spinae Muscles Working our way outward, we now come to the erector spinae muscles, which are found superficial to the transversospinal muscle group. The erector spinae muscle group forms the bulk of the palpable paraspinous muscle mass and is responsible for rotation, extension and lateral flexion of the vertebral column. The erector spinae muscles are typically divided into 3 columns: the iliocostalis, the longissimus and the spinalis layers.

49. Erector Spinae Muscles The narrow spinalis muscle is the most medial of the 3 columns of erector spinae muscles and runs from L3 to the occipital protuberance of the occipital bone. The bulkier longissimus layer is the middle column of the muscle group and runs from T6 to C2. The iliocostalis, the most lateral column of the muscle group, runs from the sacrum and iliac crests to C4. Although each of these groups extends between the described levels, they do not attach to every level between the points we mentioned. The erector spinae is the primary muscle group responsible for extension of the spine.

50. Superficial Muscle Group This brings us to the superficial muscle group, or the outermost group of spinal muscles. Three primary muscles make up this group: serratus posterior, latissimus dorsi and trapezius. These muscles contribute to the movement of the spine and assist in respiration.

51. Superficial Muscle Group The serratus posterior is the deepest and most lateral of the superficial muscle group and is divided into superior and inferior portions. The serratus posterior superior muscle arises bilaterally from the spinous processes of C7 to T2 and inserts onto the superior borders of the second to fifth ribs. This muscle elevates the ribs to aid inspiration.

The serratus posterior inferior muscles also arise bilaterally from the spinous processes of the T10 to L2 region and extend upward to the inferior borders of the ninth to twelfth ribs. These muscles pull the ribs inferoposteriorly and help forced expiration.

52. Latissimus Dorsi The latissimus dorsi is superficial to the serratus posterior inferior muscles. The latissimus dorsi is a very broad muscle that extends from T6 to T12 and the iliac crest laterally to the intertubercular groove of the humerus. Although the latissimus dorsi is primarily responsible for many movements of the arm, it also has a synergistic role in the extension and lateral flexion of the vertebral column.

53. Trapezius Muscle The trapezius muscle covers the mediosuperior margin of the latissimus dorsi. The trapezius extends from the external occipital protuberance and spinous processes of C7 to T12. The most superior portion of the trapezius extends inferolaterally to attach to the clavicle. The middle of the muscle extends laterally to attach to the acromion. Inferiorly, the muscle travels upward to attach to the spine of the scapula. The trapezius is responsible for certain movements of the arms and shoulders but also for some of the extension of the neck and movement of the vertebral column.

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56. The Spinal Cord

The spinal cord is the main highway for nerve impulses in the body, carrying motor signals from the brain to the body and sensory signals from the body to the brain. It also contains numerous circuits that are responsible for bodily reflexes. Composed of neurons and their supporting cells, the spinal cord runs from the most inferior portion of the brainstem, the medulla oblongata, to about the level of L1 or L2.

57. The Spinal Cord The same 3 layers of tissue that surround the brain also surround and protect the spinal cord: the dura mater, arachnoid mater and pia mater. The dura mater is the outermost and toughest layer, with its name literally meaning “tough mother.” A significant amount of epidural fat can be found in the epidural space between the dura mater and the bony margins of the spinal canal. Anesthetic is administered within the epidural space when a patient has an “epidural,” or epidural anesthesia, for surgery or childbirth.

Deep to the dura mater we find the arachnoid mater, which has a spider-web appearance. Continuing internally is the subarachnoid space, which is filled with cerebrospinal fluid. During a lumbar puncture procedure, this space is accessed with a needle to collect a sample of cerebrospinal fluid. The pia mater, which means “soft mother,” is located internal to the subarachnoid space. The pia mater is tightly connected to the spinal cord.

58. Spinal Cord Enlargements The spinal cord widens at 2 levels to accommodate the innervation of the extremities. The first level extends from approximately C3 to C7 and is called the cervical enlargement. Most of the corresponding nerves form the brachial plexuses, which innervate the upper extremities.

The second level at which the spinal cord widens is called the lumbar, or lumbosacral, enlargement. It extends inferiorly from approximately T11, and tapers into the conus medullaris. Most of the corresponding nerves form the lumbosacral plexuses, which innervate the lower extremities.

59. Conus Medullaris The conus medullaris is the end of the spinal cord and can be found at approximately the level of L1 to L2. At this point, the individual nerve bundles separately travel inferiorly. The nerve bundles look like a horse’s tail and are therefore named the cauda equina. A single fibrous strand continues from the conus medullaris in the cauda equina. This strand, called the filum terminale, attaches to the coccyx and anchors the spinal cord. The denticulate ligaments also secure the spinal cord in place. These ligaments extend laterally from the pia mater through the subarachnoid space, and attach to the arachnoid and dura mater.

60. Gray and White Matter The spinal cord, like the brain, is made up of gray and white matter. The gray matter extends through the midline of the spinal cord. When viewed axially, it resembles a butterfly, or an H. Surrounding the gray matter is white matter. The anterior portions of the “butterfly wings” are called the ventral horns, and the posterior parts are termed the dorsal horns.

61. Spinal Cord Segments Nerves enter and exit the length of the spinal cord at 31 levels, called segments, aligning with the intervertebral foramina through which they travel. At each segment, there are 2 ventral and 2 dorsal roots, corresponding to and traveling through the ventral and dorsal horns.

The ventral roots contain efferent, or motor, nerve roots. The dorsal roots receive afferent, or sensory, nerve roots and transmit them through the dorsal horns. In the intervertebral foramen, the dorsal root is enlarged and contains the nerve cell bodies of the sensory neurons. This enlargement is called the dorsal root ganglion. Outside the vertebral column, just external to the intervertebral foramina, the ventral and dorsal nerve roots combine to form spinal nerves. The spinal nerves carry both the efferent and afferent nerve roots to different parts of the body.

62. Spinal Nerves The 31 spinal nerves are made up of 8 cervical, 12 thoracic, 5 lumbar, 5 sacral nerves, and 1 coccygeal nerve. An important thing to keep in mind about the spinal nerves is that they don’t always line up in the same fashion with their corresponding numbered vertebral counterparts. You may notice there are 8 cervical nerves, but only 7 cervical vertebrae. The reason for this difference is the way the spinal nerves run. Beginning with the C1 spinal nerve, the nerves exit the spinal canal above each vertebra, with the exception of the C8 spinal nerve, which exits below the C7 vertebra. From T1 inferiorly, the spinal nerves exit below their corresponding numbered vertebrae.

63. Spinal Cord and Vertebrae It’s important to note that because the vertebral column is longer than the spinal cord, spinal cord segments do not directly correspond to vertebral levels. Because of the discrepancy in length between the spinal cord and the vertebral column, the length and obliquity of the dorsal and ventral nerve roots progressively increase at lower vertebral levels. The lumbosacral nerves are the longest, and these nerve roots form the cauda equina as they descend within the spinal canal to reach their respective foramina and exit the vertebral column.

64. Spinal Cord On this sagittal MR image of the lumbar spine, the spinal cord can be seen. Note the location of the spinal cord, conus medullaris and cauda equina as they relate to the lumbar spine.

65. Dorsal Rami As the spinal nerves run laterally, they almost immediately split into ventral and dorsal rami. Each ramus carries both efferent and afferent nerves. The dorsal rami are the smaller of the two, and at most levels they maintain their individuality where they eventually split into medial and lateral branches. They innervate the muscles and skin of the posterior trunk and neck.

66. Ventral Rami Unlike the dorsal rami, the ventral rami only maintain their individuality through the thoracic spine, where they are called intercostal nerves and supply the limbs and anterolateral aspects of the thorax.

Through the cervical, lumbar and sacral segments, they join other ventral rami that serve the same anatomic region. This group is termed a nerve plexus. Within the nerve plexus, the spinal nerves are re- arranged and the nerves leaving the plexus contain fibers from multiple spinal nerves. There are 4 major plexuses: cervical, brachial, lumbar and sacral.

67. Cervical Plexus The cervical plexus is made up of the upper 4 cervical nerves, which are C1 to C4. It can be found in the deep tissues of the neck, underlying the sternocleidomastoid muscle. The cervical plexus innervates the head, neck and shoulders.

68. Brachial Plexus The brachial plexus consists of the lower 4 cervical and first thoracic ventral rami, which are C5 to C8 and T1. Recall that because the cervical spinal nerves exit above their respective bodies and the thoracic spinal nerves exit below their respective bodies, the “extra” nerve between C7 and T1 is called C8. The brachial plexus extends from the base of the neck and through the axilla. It innervates the skin and muscles of the entire upper limb, excluding the trapezius muscle and a patch of skin near the axilla.

69. Lumbar Plexus The lumbar plexus consists of the first 4 lumbar rami, L1 to L4, with additional fibers provided by the subcostal nerve, which represents the ventral ramus of T12. The lumbar plexus is located just lateral to the intervertebral foramina and extends between the superficial and deep layers of the psoas major muscle. It innervates the back, abdomen, groin, thighs, knees and calves.

70. Sacral Plexus The sacral plexus consists of the L4 through S4 ventral rami, more specifically the lumbosacral trunk, the anterior portion of the first sacral nerve and portions of the anterior divisions of the second and third sacral nerves. The nerves of the sacral plexus unite to form the sciatic nerve, which is the largest nerve of the body. The sacral plexus innervates the posterior thigh, most of the lower leg, the entire foot and part of the pelvis. It is located in the posterior pelvis, along the anterior surface of the piriformis muscle deep to the internal iliac vessels.

71. Intervertebral Foramina The intervertebral foramina can be seen on the axial CT slice on this slide. Recall that intervertebral foramina allow the nerves and vessels to pass to and from the spinal column. The cervical, brachial, lumbar and sacral plexuses originate from their respective nerve roots which pass through the intervertebral foramina.

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75. Blood Supply The spinal cord, like many structures of the body, needs a blood supply to perform its function. It receives blood from the anterior and paired posterior spinal arteries. The anterior spinal artery runs along the length of the spinal cord within the anterior median fissure. The paired posterior spinal arteries are found along the posterior border of the spinal cord, just lateral to midline of the cord.

76. Spinal Arteries The anterior spinal artery is formed by 2 small branches arising from the vertebral arteries right before they converge to form the basilar artery. It supplies blood to the ventral two-thirds of the spinal cord. The anterior spinal artery receives contributions from other arteries as it descends with the spinal cord, including the great anterior radiculomedullary artery, better known as the artery of Adamkiewicz, which usually arises from a left posterior intercostal artery. This artery is significant because it typically makes a large contribution to the blood supply of the lower spinal cord and is important to recognize if surgery is being performed in this area.

The posterior spinal arteries are formed by small branches arising from either the vertebral or posterior inferior cerebellar arteries. They supply blood to the dorsal one-third of the spinal cord and also receive contributions from other arteries as they descend with the spinal cord.

77. Spinal Arteries Spinal branches diverge from the vertebral, ascending cervical, deep cervical, posterior intercostal, lumbar and lateral sacral arteries. The spinal branches then further split into the anterior and posterior radicular arteries. The anterior and posterior radicular arteries arc around the spinal cord and supply blood to the anterior and posterior spinal arteries.

78. Review Take a moment to look at the axial CT slice of the thoracic spine on this slide. Review the location of the aorta, vertebral body and spinal cord as discussed in the previous slides.

79. Spinal Venous System Finally, we’ll discuss the spinal venous system. Although each individual vein of the spinal cord looks like any other vein, they are arranged in 4 plexuses: 2 internal and 2 external. The anterior and posterior internal venous plexuses travel within the spinal canal. The anterior and posterior external venous plexuses run anterior to the vertebral bodies and posterior to the spinous processes, respectively. The basivertebral veins drain blood from the vertebral bodies into the internal and external venous plexuses. The venous plexuses continue superiorly to drain blood into the vertebral veins at about the level of the foramen magnum.

80. Knowledge Check

81. Knowledge Check

82. Conclusion

This concludes Sectional Anatomy Essentials: Module 4 – The Spine. You should now be able to: Name the components of the spine. Identify the features of the vertebrae. Describe the differences between cervical, lumbar, and thoracic vertebrae. Identify the major curves found in the vertebral column. Locate the major ligaments found in the spinal column. Identify the major muscle groups supporting the spine. Describe the major components of the spinal cord.

83. References Kelley L, Petersen C. Sectional Anatomy for Imaging Professionals. 3rd ed. St. Louis, MO: Mosby; 2012.

Madden M. Introduction to Sectional Anatomy. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.

El-Khoury GY, Bergman RA, Montgomery WJ, et al. Sectional Anatomy by MRI and CT. 3rd ed. Philadelphia, PA: Churchill Livingstone Elsevier; 2007.

Govind C, Bhavin J. Cross Sectional Anatomy CT & MRI. New Delhi, India: Jaypee Brothers Medical Publishers Ltd; 2012.

Moeller TB, Reif E. Atlas of Sectional Anatomy: The Musculoskeletal System. Stuttgart, Germany: Georg Thieme Verlag; 2009.

Wolf-Heidegger G, Kopf-Maier P. The Color Atlas of Human Anatomy. New York, NY: Sterling Publishing Co; 2006.