Dissector Answers - Deep Back & Spinal Cord

Learning Objectives:

Upon completion of this session, the student will be able to:

1. Identify and give the function of the significant parts of a typical vertebra and associated ligaments.
2. Identify the specialized vertebrae.
3. Describe the spine, its curvatures, and vertebral column movements.
4. Identify the coverings and the supporting structures of the spinal cord. Give the point of the termination of the spinal cord and the dural sac.
5. Identify the terminal specialties of the cord, their relation to lumbar puncture, the nerve rootlets, and blood supply.
6. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.
7. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord.
8. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution.
9. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation.
10. Define and explain the significance of dermatomes.
11. Explain the difference between superficial and deep (true) back muscles.

Learning Objectives and Explanations:

1. Identify and give the function of the significant parts of a typical vertebra and associated ligaments. (W&B 339-344, N 154A, 154B, 155, TG 1-02, 1-03A, 1-03D, 1-05A, 1-05B, 1-05D, 1-08A, 1-08B, 1-10, 1-11A, 1-11B) )

• Vertebral body: the largest part of the vertebra, shaped like a short cylinder. Supports the spine and entire body.
• Vertebral arch: the ring of bone formed by the paired pedicles and paired laminae of the vertebrae. It connects the body with the transverse processes.
• Pedicle: part of the arch. This short, strong process extends posteriorly from the posterolateral surface of the vertebral body. It is paired and connects the body with the transverse process.
• Intervertebral notch: notches on the superior and inferior surface of the vertebral pedicle. The superior intervertebral notch of one vertebra combined with the inferior intervertebral notch of the adjacent vertebra forms the intervertebral foramen.
• Intervertebral foramen: opening between the pedicles of adjacent vertebra. It is the opening for the passage of the spinal nerves out from the spinal cord.
• Lamina: part of the arch. This broad flat plate of bone is located between the transverse process and the spinous process of the vertebra. It is paired. It gives attachment to the ligamenta flava, which span the intervals between the laminae of adjacent vertebrae.
• Transverse process: lateral process that extends from the junction of the pedicle and the lamina of the vertebra. It is the site for muscle attachment and rib articulation.
• Spinous process: posterior mid-line process arising from the junction of the two laminae of the vertebrae. The spinous processes are important sites of muscle attachment.
• Articular processes: processes that project inferiorly and superiorly from the junction of the lamina and pedicle of the vertebra. There are two superior and two inferior processes on each vertebra, and they articulate with adjacent vertebrae through synovial joints. Clinicians refer to these as "facet joints."
• Vertebral canal: the opening formed by the combination of the body and the vertebral arch. It contains the spinal cord, meninges, epidural fat, and the internal vertebral plexus of veins.
• Anterior longitudinal ligament: ligament that runs from superior to inferior along the anterior surface of the body of the vertebrae. It helps attach the bodies of the vertebrae to each other.
• Posterior longitudinal ligament: ligament that goes from superior to inferior along the posterior surfaces of all vertebral bodies, joining them together. It is broader at the discs and narrower at the bodies, giving it a scalloped edge. It is located in the vertebral canal but it is NOT penetrated by the needle during spinal tap.
• Ligamenta flava: ligaments formed predominantly by elastic fibers which join the laminae of adjacent vertebrae. These are paired and are penetrated by a needle during a spinal tap. (Latin, flavus = yellow)
• Intervertebral disc: fibrocartilaginous disc between adjacent vertebral bodies which are important shock absorbers and give the spine flexibility. The discs are composed of two parts:
  • Anulus fibrosus: outer fibrous rim. (Latin, anulus = ring)
  • Nucleus pulposus: pulpy mass located inside the anulus.

2. Identify the specialized vertebrae. (W&B 343, N 17, 18, TG 1-03A, 1-03C, 1-08A)

• Atlas (C1): has no vertebral body, only anterior and posterior arches, and it articulates with the odontoid process of C2. (Think of Atlas supporting the weight of the world on his shoulders.)
• Axis (C2): dens (odontoid process) projects superiorly from its body and articulates with the anterior arch of the atlas. There is no rotation of the skull with respect to the atlas, so the first moveable joint in the spinal column is the C1-C2 joint - the "axis" of rotation if you will.
• Vertebra prominens (C7): also called "spina prominens". Although the two terms are used interchangeably, one is most often referring to the spinous process of C7. It is long and non-bifid, and is palpable through the skin at the nape of the neck.

3. Describe the spine, its curvatures and vertebral column movement. (W&B 344-345, N 153, TG 1-02)

• The spine extends from the skull to the apex of the coccyx and forms the skeleton of the neck, and back and the main part of the axial skeleton. It consists of 33 vertebrae arranged in five regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal. (One professor suggests thinking of times of the day that you eat, i.e. 7am, 12pm, and 5pm.)
• Motion only occurs among 24 vertebrae: 7 cervical, 12 thoracic, and 5 lumbar. The sacral and coccygeal vertebrae are usually fused. (Latin/Greek, coccyx = cuckoo's beak. Latin, os sacrum = holy bone)
• Possible movements of the spinal column:
  • Flexion (touching your toes)
  • Extension (bending backwards)
  • Lateral bending
  • Rotation (either of head and neck only, or of the entire torso)
• Four curvatures of the spine are apparent in adults:
  • Cervical: secondary curvature - concave posteriorly
  • Thoracic: primary curvature - concave anteriorly
  • Lumbar: secondary curvature - concave posteriorly
  • Sacral: primary curvature - concave anteriorly

4. Identify the coverings and the supporting structures of the spinal cord. Give the point of the termination of the spinal cord and the dural sac. (W&B 350-353, N 160, 161, 169, TG 1-18, 1-19, 1-20)

• Dura mater: outermost covering membrane. This is tough, pierced by spinal nerves, and anchored inferiorly to coccyx as the filum terminale (externum). It extends through the intervertebral foramina and along the dorsal and ventral nerve roots to a point distal to the spinal ganglia, forming dural root sleeves. The dural sac proper ends at the level of S2, and is anchored inferiorly to the coccyx by the coccygeal ligament, or filum terminale (externum). (Latin, dura mater = tough mother)
• Arachnoid mater: delicate membrane made of fibrous and elastic tissue that lines the dural sac and the dural root sleeves. The arachnoid mater encloses the CSF-filled subarachnoid space containing the spinal cord, spinal nerve roots, and spinal ganglia. It is held against the dura but separable from it. In a lumbar spinal puncture, the arachnoid is pierced. NO subdural space exists between dura and arachnoid, it is only a POTENTIAL space.
• Subarachnoid space: contains cerebrospinal fluid. Below L2 it contains the cauda equina.
• Pia mater: innermost covering membrane of the spinal cord. The pia mater closely follows entire spinal cord and directly covers the roots of spinal nerves and the spinal blood vessels. Inferior to the conus medullaris, the pia continues as the filum terminale. (Latin, pia mater = delicate mother)
  • Denticulate ligaments: specializations of pia mater. These ligaments form a longitudinal shelf separating dorsal and ventral rootlets. Also, by extending laterally from cord and attaching to the dura by way of 21 pairs of denticulations, they suspend the spinal cord in the subarachnoid space.

5. Identify the terminal specialties of the cord, their relation to lumbar puncture, the nerve rootlets, and blood supply. (W&B 350-353, N 160, 161, TG 1-11, 1-18, 1-20)

• Conus medullaris: located at L2. This is the tapered termination of the spinal cord proper.
• Filum terminale: continuation of the pia mater inferior to the conus medullaris. It descends all the way to the coccyx, anchoring the spinal cord.
• Cauda equina: the collection of nerve roots coming from the end of the spinal cord, within the lumbar cistern, traveling to the vertebral foramina inferior to the conus medullaris. (Latin, cauda equina = horse's tail)
• A lumbar puncture is done either above or below L4 in the region of the cauda equina.
• Blood supply follows along with the nerve rootlets themselves.

6. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.

• Herniation (N 161, 162, TG 1-11, 1-18, 1-20): Focal protrusion of disc material secondary to rupture of anulus fibrosus. This most commonly occurs in the lumbar region, and can result in compression of spinal nerves (which usually leads to the clinical presentation).
• Dislocation of vertebrae: Like any other dislocation, this is a case of the bone slipping out of place, most commonly in the cervical region. The spinal canal is relatively small and these injuries very often result in damage to the spinal cord.
• Fracture of the dens: Due to the relative strength of the ligaments in the region compared to the dens, stress often results in its fracture. In effect, it is "the first thing to go".
• Cord injuries: The spinal cord can be injured in various ways, all of which are extremely serious and can result in significant impairment of function.

7. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord. (W&B 353-355, N 169, 170)

A cross-section of the spinal cord will show a butterfly-shaped area of gray matter surrounded by white matter. The butterfly has four "horns", two dorsal and two ventral. Sensory neurons originate in the dorsal horns and make up the dorsal roots, but their cell bodies are in the dorsal root ganglia, outside of the spinal cord. Motor neurons have their cell bodies within the spinal cord, in the ventral horn. (The ventral horn is "fatter", since those cell bodies take up space!) They unite to form the ventral roots. Distal to the dorsal root ganglia, the roots unite, forming the spinal nerve. The nerve quickly splits into two branches, or ventral and dorsal rami, which carry both sensory and motor fibers. (Latin, ramus = branch)

8. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution. (W&B 9-10, N 169, 170, TG 1-17)

• Parts (proximal to distal):
  • Roots: dorsal and ventral. Roots are made of several rootlets
  • Spinal nerve: where the two roots come together briefly
  • Primary rami: dorsal and ventral. These are the first branches off the spinal nerve.
  • Cutaneous branches: posterior, lateral and anterior.
• Functional components:
  • Sensory: Afferent, either somatic or visceral.
  • Motor: Efferent, either somatic or visceral.
    • Somatic motor: to voluntary muscle.
    • Visceral motor (autonomic): to involuntary muscle and glands.
  • White ramus communicans: connection between sympathetic trunk and ventral primary ramus. These are only found between T1 and L2. (More on these later.)
  • Gray ramus communicans: connection between sympathetic trunk and ventral primary ramus. Found in all spinal nerves (31 pairs). (More on these later.)

9. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation. (W&B 364-368, N 180, 258, TG 1-17)

Both the nerves and the arteries are segmented along vertical axis of the thorax. The intercostal nerves come from the spinal nerves. The arteries are mostly segmental branches of the aorta. The veins, arteries, and nerves run in a groove under each rib with the veins most superior and the nerves most inferior (VAN). Each also gives off a smaller collateral branch that runs on top of the rib below.

10. Define and explain the significance of dermatomes. (W&B 123, 581 (limbs), N 164, TG 1-23A, 1-23B)

One dermatome is the region innervated by one spinal nerve. They typically overlap, with one nerve covering its own dermatome as well as half of the one above and below it.

11. Explain the difference between superficial and true back muscles. (W&B 89-93,335-338, N 174, 179, TG 1-12, 1-13, 1-14, 1-15)

• Superficial back muscles control upper limb movements as well as aid in respiration. They mostly receive their nerve supply from the ventral rami of cervical nerves. These include the:
  • Trapezius: innervated by the accessory nerve (CN XI).
  • Latissimus dorsi: innervated by thoracodorsal nerve, made from branches of ventral primary rami of C7 & C8.
  • Rhomboideus major & minor: innervated by dorsal scapular nerve, a branch of the VPR of C5.
  • Levator scapulae: innervated by dorsal scapular nerve and branches of the VPR's of C3 & C4.
  • Serratus posterior (superior and inferior): innervated by intercostal nerves, first four and last four respectively. (Latin, serratus = to saw)
• Deep or true back muscles specifically act on the vertebral column, producing its movements and maintaining posture. They are innervated by dorsal rami of spinal nerves. The fascia covering them constitutes the thoracolumbar fascia.

Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless. The Vertebral Column - General Characteristics of a Vertebra - The Cervical Vertebrae - The Thoracic Vertebrae -The Lumbar Vertebrae - The Sacral and Coccygeal Vertebrae - The Vertebral Column as a Whole - The Deep Muscles of the Back - The Spinal Cord - The Meninges of the Brain and Spinal Cord - The Spinal Nerves

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Questions and Answers:

12. What is the coccygeal ligament?

At the level of termination of the dura sac, S2, the dura continues below as the coccygeal ligament (or filum terminale externum) to attach to the coccyx. (N 160, TG 1-20)

13. What do the lateral continuities of the dura mater cover at and distal to the intervertebral foramen?

They cover the dorsal root ganglia and spinal nerves, eventually blending with the epineurium of the spinal nerves and their dorsal and ventral primary rami. (N 173, TG 1-19)

14. What does the subarachnoid space usually contain?

The subarachnoid space usually contains cerebrospinal fluid (CSF). (N 169A, 169B, TG 1-19, 1-20)

15. With what is the subarachnoid space continuous?

The subarachnoid space surrounding the spinal cord is continuous with the subarachnoid space surrounding the brain. This means that it is also continuous with the ventricular system within the brain, where CSF is produced. (N 109, TG 7-50)

16. Where does the filum terminale lie?

The filum terminale (internum) extends inferiorly from the conus medullaris, as part of the cauda equina in the lumbar cistern, to become enclosed within the coccygeal ligament (or filum terminale externum). (N 160A, 160B, 161, TG 1-18, 1-20)

17. Where does the filum terminale attach?

The filum terminale attaches to the coccyx . (N 159, TG 1-18, 1-20)

18. What are the relations of the denticulations and the denticulate ligament to the roots of spinal nerves?

The denticulations separate the dorsal and ventral roots by lying between them. (N 169, TG 1-19)

19. How many denticulations are there?

There are 21 denticulations. (N 169, TG 1-18)

20. Where do the denticulations attach?

They pierce the arachnoid mater to attach to the inner surface of the dura mater. (N 169, TG 1-19)

21. At what vertebral level is the conus medullaris?

The conus medullaris is at L1-2. (N 160A, 160B, 161, TG 1-18, 1-20)

22. Where is the dura mater in relation to the intervertebral foramen?

Within the intervertebral foramen, attached to the periosteum surrounding it. (N 173, TG 1-18, 1-19, 1-20)

23. Examine dorsal and ventral rootlets as they emerge from the cord. How is the segmental pattern created?

Rootlets coalesce as they enter the dural sleeve, lateral to their exit from the cord, thereby forming segmental roots. (N 169)

24. What is a root?

The roots are nerves which leave the gray matter through ventral or dorsal horns and unite to become the spinal nerve. (N 169, TG 1-17, 1-19)

25. What is the functional difference between dorsal and ventral roots?

Ventral roots carry motor fibers, both somatic and visceral. Dorsal roots carry sensory fibers. (N 169, TG 1-17)

26. Trace the roots to the point of union to form the spinal nerve. Are they in separate sheaths before uniting?

No, the spinal nerve roots are usually contained within a common sleeve of dura mater. (N 169, TG 1-17)

27. Where is the dorsal root ganglion?

The dorsal root ganglion is located in the intervertebral foramen on the dorsal root. There is one ganglion per spinal nerve. (N 169, 180, TG 1-17, 1-19)

28. What is the extent of the dura mater, arachnoid mater, and pia mater on the spinal nerves?

The pia mater fuses with the arachnoid mater at the spinal nerve, while the dura mater still provides a covering. (N 169, TG 1-17, 1-19)

29. Note posterior and anterior spinal arteries. What are their sources?

The spinal arteries arise within the skull. Anterior spinal arteries are branches of the vertebral arteries and posterior spinal arteries are branches of the posterior inferior cerebellar arteries. (N 171, 172A172B, TG 1-21, 7-56A, 7-56B)

30. What reinforces the spinal arteries?

Aorta or vertebral aa. --> segmental aa. --> posterior branch --> spinal branches --> radicular branches --> anterior and posterior spinal arteries via anastomoses. (N 171, TG 1-21)

31. What are radicular arteries?

Radicular arteries are branches of spinal branches of segmental arteries that run along the dorsal and ventral spinal nerve roots. (N 171, 172A172B, TG 1-19, 1-20, 1-21)

32. Are all radicular arteries the same?

Most radicular arteries are small, but there is often a rather large radicular artery, the great radicular artery, that may be found in the lower thoracic or upper lumbar levels, usually on the left side. The great radicular artery represents a major blood supply for the lower spinal cord. (N 171, 172A172B, TG 1-19, 1-20, 1-21)

33. What is the source for radicular arteries?

Radicular arteries arise from vertebral arteries in neck, segmental arteries (intercostal & lumbar), and lateral sacral arteries in the pelvis. (N 171, 172A172B, TG 1-19, 1-20, 1-21)

34. Observe arrangement of gray matter and white matter of the cord. What is their significance?

Gray matter is located on the inside of the spinal cord and makes up the "butterfly." It is composed of cell bodies and has dorsal, ventral and lateral horns. White matter, located on the outside of the "butterfly", is made up of myelinated nerve tracks. (W&B 33, N 169)

35. After exposing the posterior longitudinal ligament in cervical and lumbar regions, can you see the intervertebral discs?

You can not see the discs very well. The posterior longitudinal ligament spreads laterally at each disc to attach to it. (N 158, TG 1-11)

36. What is the significance of the attachment of the posterior longitudinal ligament to the discs?

The attachment of the posterior longitudinal ligament to the intervertebral disc helps to reinforce the posterior aspect of the anulus fibrosus. However, because the ligament thins laterally, most disc herniations occur in a posterolateral direction. (N 158, TG 1-11)